Nanomaterials represent today’s cutting edge in the
development of novel advanced materials, which
promise tailor-made functionality for unique appli-
cations in all important industrial sectors. Nanoma-
terials can be clusters of atoms, grains 100 nm in size,
fibers that are less than 100 nm in diameter, films that
are less than 100 nm in thickness, nanoholes, and com-
posites that are a combination of these. In other words,

it implies that the microstructures (crystallites, crystal
boundaries) are nanoscale [1]. Nanomaterials include
atom clusters, nanoparticles, nanotubes, nanorods,
nanowires, nanobelts, nanofilms, compact nano-
structured bulk materials, and nanoporous materials
[2]. Materials in nanosize range exhibit fundament-
ally new properties and functionalities such as surface
effects, dimensionality effects, quanta effects, and
quanta tunnel effects, etc.

2006 by Taylor & Francis Group, LLC.

Due to their unique optical, electronic, acoustic,

magnetic, thermal characteristics and advantages as
catalysts, etc., the applications of nanomaterials can
be found in many fields and some are listed as below
[3–6]:

Pharmaceuticals, healthcare, and life sciences: New
nanostructured drugs, gene and drug delivery sys-
tems; biocompatible replacements for body parts
and fluids, self-diagnostics for use in the home,
sensors for labs-on-a-chip, material for bone and
tissue regeneration; blood replacement

Manufacturing: Improvements for precision en-
gineering; new processes, and tools to manipu-
late matter at the atomic level; nanopowders
that are sintered into bulk materials with special
properties that may include sensors to detect
incipient failures and actuators to repair prob-
lems; chemical–mechanical polishing with nano-
particles, self-assembling of structures from
molecules; bioinspired materials and biostruc-
tures

Automotive and aeronautics industries: Nanopar-
ticle-reinforced tires; external painting; non-
flammable plastics; self-repairing coatings and
textiles

Electronics and communications: Media-recording
devices using nanolayers and dots, wireless
technology; dramatically more capable electronic
circuits

Chemicals and materials: Catalysts; superhard
and tough-drill bits and cutting tools; ‘‘smart’’
magnetic fluids for vacuum seals and lubricants

Energy technologies: New types of batteries, ar-
tificial photosynthesis for clean energy, quan-
tum well solar cells, safe storage of hydrogen
for use as a clean fuel, energy savings from
using lighter materials and smaller circuits

Space exploration: Lightweight space vehicles,
economic energy generation and management,
ultrasmall and capable robotic systems

Environment: New membranes that can select-
ively filter contaminants; nanostructured traps
for removing pollutants from industrial efflu-
ents, characterization of the effects of nanos-
tructures in the environment

National security: Detectors and detoxifiers of
chemical and biological agents, hard nanostruc-
tured coatings and materials, camouflage mater-
ials, light and self-repairing textiles, miniaturized
surveillance systems

Research done on nanotechnology is found

throughout the literature. For example, when a drug
is administered as nanoparticles, the absorption and

oral bioavailability of them, e.g., heparin [7], enala-
prilat [8], tobramycin [9], and antitubercular drugs
[10], were significantly enhanced in comparison to
oral-free drugs. Chastellain et al. [11] synthesize the
magnetic nanoparticles with specific shape and precise
size with tailored surface chemistry and topography
for biochemical purposes, i.e., the precise delivery of
drugs using magnetic nanoparticles to the exact tissue
by applying the external magnetic fields. Pandey et al.
[12] investigated the process that the drug was encap-
sulated in nanoparticles of a synthetic polymer. The
results emphasize the power of nanotechnology to
make the concept of enhancement in oral bioavailabil-
ity of azole antifungal drugs come to reality.

Since 1976, over 80,000 nanotechnology-related

patents have been issued by the United States Patents
and Trademarks Office (USPTO) [13]. However,
among them, the United States leads the biomaterials
such as tissue engineering and advanced controlled
release as well as semiconductor technology. Europe
is in a strong position in molecular sensors and diag-
nostics. In Asia, Japan leads in ceramics and magnetic
materials. China and Korea also are amongst the
most rapidly developing nations with strong R&D
activities in nanotechnology in recent years [1].

The preparation of nanomaterials can be classified

into three main approaches according to the states of
the reactants used: liquid-phase, solid-phase, and gas-
phase method [14,15]. One of the most popular
methods in both laboratory and industry at present
is the liquid-phase method. In the preparation of
nanoparticles by the liquid-phase method, drying is
an indispensable unit operation. Nanoparticles tend
to agglomerate and properties of nanoparticles are
adversely affected if we do not choose appropriate
drying methods. So to assure that nanoparticles are
well dispersed during drying is a vital requirement in
the preparation of such products.

Rabani et al. [16] have presented an important

finding that the drying process may mediate the
self-assembly of nanoparticles since the systems may
exhibit complex transitory structures [17] when the
equilibrium fluctuations are mundane. They used a
numerical model to show how the choices of solvent,
nanoparticle size, and thermodynamic state affect the
final morphologies. Since drying plays a role in nano-
material processing, Pakowski [18] investigated the
phenomenon of nanomaterials drying, such as drying
stress and deformation of structure during drying, as
well as the diffusion among the nanopores, etc. Wang
et al. [6] presented and discussed more about the typical
drying methods used in nanomaterials processing.

The preparation of gels includes mainly two steps:

synthesizing ramiform- and continuous-structured
gels in a liquid medium by the sol–gel process, and

2006 by Taylor & Francis Group, LLC.

then removing solvents from the gel pores [19]. Be-
cause gels are prone to deform or crack during drying,
one of the difficulties in preparing aerogels is to pre-
serve the microporous-structured gels. So rigorous-
controlling conditions are demanded during drying
and a minor mistake may result in failure of the
whole preparation process.

30.2 RESEARCH IN DRYING

OF NANOMATERIALS

Compared with drying of conventional materials,
drying of nanoporous materials is really a complex
task, because we must remove the solvents carefully
without damaging the porous microstructure of gels.
In the case of nanoparticles, agglomeration among
particles must be prevented. The theoretical research
on the drying of nanomaterials focuses mainly on the
mechanism for cracking of porous-structured gels,
agglomeration of nanoparticles, and solvent evapor-
ation during drying.

30.2.1 S

OLVENT

VAPORATION

ECHANISM

ANOMATERIAL

RYING

It is generally believed that solvent evaporation in-
volves three periods, namely a constant rate drying, a
first falling rate, and a second falling rate period [20].

In the constant rate-drying period, drying rate is

independent of time and thickness of the gel. Addition-
ally, the contraction (or shrinkage) rate of gel volume
is equal to the solvent evaporation rate. Therefore,
pores of gels are always filled with solvents in this
period. In the initial period when contraction of gel
volume takes place, the skeleton of the gel is very soft
and capillary tension acting on the gel body is also very
low. As the gel skeleton contracts and tends to become
harder, its resistance ability against shrinkage is also
strengthened while capillary tension increases simul-
taneously. In the constant rate-drying period, liquid
flux from the pores interior to surface is equal to the
liquid-evaporating rate. Liquid transfer occurs mainly
by flow; diffusion is responsible for only a small fraction
of the total flow.

Along with liquid solvent evaporation, the gel

body shrinks and its skeleton strength also increases
until it is strong enough to withstand compressive
stress. When the contraction rate of gel is no longer
equal to solvent evaporation rate, the solid surface is
exposed and the first falling rate-drying period begins.
At the critical point where the constant rate-drying
period ends and the first falling rate-drying period
begins, the radius of curved liquid is equal to the
size of the gel pores, if contact angle is 08, capillary
tension reaches maximal value, and the gel skeleton

does not contract anymore. Below the critical point,
the evaporation rate starts decreasing; liquid evapor-
ation front recedes into the interior of the gel body,
but primarily takes place over the solid surface. Endo-
phragms of unsaturated pores are covered by a
continuous thin liquid layer, which can provide a
channel for transfer of liquid from pores. Therefore,
in the first falling rate-drying period, liquid transfer
occurs primarily by flow accompanied by vapor dif-
fusion. In the first falling rate-drying period, a vapor–
liquid interface appears in the interior of gel body, so
cracking of the gels appears mainly during this
period. Moreover, it is dependent on the thickness
of the gel.

In the first falling rate-drying period, a thin liquid

layer covers the unsaturated region of gel pores. There-
fore, the outside surface of the gel does not change
immediately so long as liquid flux is comparable to
evaporation rate, and this state may be continuously
preserved. However, as evaporation goes on, distance
from the solid outside surface to drying front increases,
the pressure gradient decreases and so does the flow
rate. Hence, distribution of liquid on the outside sur-
face becomes discontinuous, and drying enters the
second falling rate period. In this period, evaporation
completely occurs inside the gel body. The solvent
evaporation rate is no longer sensitive to external con-
ditions, the liquid near the external surface of the gel
pores appears discontinuous, and transfer of liquid
from the gel pores to the outside is taken by flow
with diffusion as predominant. The force exerted on
the gel is significantly mitigated during the second
falling rate-drying period. Therefore, the gel body
may dilate slightly. Because compressive stress on the
nondrying side of skeleton is larger than that on
the drying side, this may deform and collapse the gel
skeleton. In this period, the gel volume no longer
changes but its quality gradually decreases. Along,
with this excess part of solvent evaporation, various
degrees of collapse may appear, micropores vanish,
and the original skeletal structure shrinks into large
agglomerations.

30.2.2 M

ECHANISM OF

RACKING OF

ELS

AND

GGLOMERATION OF

ANOPARTICLES

30.2.2.1 Capillary Pressure Theory

Capillary pressure is the major cause leading to crack-
ing of the gel skeletons or agglomeration of nanopar-
ticles during drying [21]:

2s cos u

(30:1)

2006 by Taylor & Francis Group, LLC.

where P is the capillary pressur e, s is the liqui d–vapo r
interfaci al en ergy (or surfa ce tensi on), u is the contact
angle, and r

is the rad ius of cu rvature. As we note

from

Equat ion 30.1

, in order to prevent nanop articles

from agglomerating or gels from collapsing, capillary
pressure must be decreased to a minimum. The fol-
lowing means can decrease capillary pressure:

Reducing surface tension (using supercritical
drying, freeze-drying, and solvent-replacement
drying)

Changing wetting angle and making it to ap-
proach 908 (surface modification drying for
nanomaterials)

Enlarging pore size of gels properly (for capillary
pressure is inversely proportional to the radius of
pores) and making it uniformly (uneven pore size
of gels leads to different capillary pressure and
damage of the nanostructure)

Also we can use the microwave drying technique since
it can shorten the drying period and heat materials
evenly.

30.2.2.2 Models for Cracking of Gels

There are many divergent opinions about the causes
of gel cracking and many models have been suggested
based on experiments and observed results. The
macroscopic model and the microscopic model are
among the more representative models reported.

30.2.2.2.1 Macroscopic Model
The macroscopic model [6] assumes that drying stress
is responsible for cracking of gels. Stress produced
during drying is macroscopic and is exerted on the
whole body of gels and not only on local drying
regions. Its experimental basis is that gels either are
integrated or crack into pieces, but never turn into
powders. This model can illustrate quota relationship
among cracking and evaporation rate, thickness of
gels, and penetration coefficient. It can also explain
some drying phenomena adequately. But it also has
limitations, for example, the macroscopic model fails
to explain why cracking mainly appears at the critical
point, because there is not a sudden stress change that
can cause gel cracking according to this model.

30.2.2.2.2 Microscopic Model
The microscopic model [6] assumes that the main
cause of gel cracking is asymmetry of pores. Accord-
ing to this model, behind the critical point evapor-
ation of liquid occurs in the larger pores initially, thus
the stress on the larger pores can be relaxed. How-
ever, stress is still present in the smaller pores, which

can cause shrinkage of pores before cracking. Unlike
the macroscopic model, the microscopic model as-
sumes that the stress produced by local asymmetry
of gel microstructure during drying is microscopic,
and thus only affects this local region. So the distri-
bution of pores plays a key role in this process; crack-
ing is likely to occur with wider distribution of pores.
This model can explain why the cracking takes place
at the critical point. However, it fails to explain why
cracking is reduced when the drying rate and the
dimension of gel decrease. Clearly, this is also a defi-
ciency of the microscopic model.

30.2.2.3 Mechanism of Agglomerations

of Nanoparticles

At present, there are no generally accepted views on
the formation of hard agglomerates of nanoparticles.
There are several representative theories proposed,
e.g., including crystal bridge theory, capillary pres-
sure theory, hydrogen bond theory, chemical bond
theory, etc. [21]. We shall only mention the essential
concept of each theory.

Crystal bridge theory: During drying, capillary

pressure enables particles to approach each other;
crystal bridges form and tighten due to deposit of
dissolved surface hydroxyls. With time passage these
crystal bridges combine and large agglomerates can
be formed.

Capillary pressure theory: When gels are heated, a

solid surface is partially exposed due to evaporation
of the absorbed water. Thus, surface tension resulted
from the existence of capillary pressure in water may
cause shrinkage of capillary walls. This is thought to
be the main cause of agglomerates formation.

Hydrogen bond theory: Nanoparticles become at-

tached as a result of hydrogen bond effects, and thus
form agglomerates.

Chemical bond theory: Jonts and Norman believed

that nonbridging hydroxyls existing on a nanoparticle
surface are the basic sources of hard agglomerates.
The reaction of nonbridging hydroxyls on the surface
of neighboring particles taking place in this case is

OH þ HOMe ! MeOMe þ H

(30:2)

Here Me–O–Me group accounts for the formation of
agglomerates.

In fact, it is very difficult to illustrate mechanism

of agglomerations by a single theory. Most authors
attribute agglomeration to capillary pressure, but
capillary pressure theory fails to explain why gels

2006 by Taylor & Francis Group, LLC.

exhibit large differences in agglomeration states when
using different organic solvents with similar surface
tension to replace water. Agglomerates can be easily
dispersed in water if the particles are combined only
by hydrogen bond effect, but actually it is very diffi-
cult, and therefore this type of agglomerate cannot be
attributed solely to hydrogen bond theory.

30.3 DRYING METHODS FOR

NANOMATERIALS

There are many advanced drying techniques for
nanomaterials that have not been classified systemic-
ally so far. In this chapter, we try to classify these
methods into direct drying, solvent-replacement dry-
ing, and modification drying for nanomaterials.

30.3.1 D

IRECT

RYING

Direct drying method involves drying or calcinations
of precipitates prepared by liquid-phase method after
simple washing or filtering, but without solvent-
replacement and surface modification. There is direct
contact between the wet solid and the convective
drying medium. The following are some of the more
common methods in use.

30.3.1.1 Oven Drying

It is difficult to preserve slender porous gels when we
are removing liquid solvents from gel pores. During
oven drying, tremendous capillary pressure is pro-
duced in micropores when the solvents are removed,
which may lead to serious agglomerations. Rapid dry-
ing can cause cracking of gels because gels and liquid
solvents have different thermal expansion coefficients,
so drying rate must be controlled as low as possible.
But according to literature, low rate of safe drying will
take as long as 1 year [14,21]. Oven drying is mainly
used in the laboratory. Nanoparticles obtained by this
method result in severe agglomerations, so quality of
products is not very good.

30.3.1.2 Spray Drying

Spray drying is an established method that is initi-
ated by atomizing and spraying suspensions into
droplets followed by a drying process, resulting in
solid nanoparticles. This method is widely used due
to its many advantages, including the fact that it
is a simple system, low cost, and easy to be indus-
trialized. Several authors [22–27] have reported
the fabrication of nanoparticles via spray drying.

Chow et al. [22] used spray drying to prepare

nanosize hydroxyapatite (HA) particles. They used a
nozzle to spray the acidic calcium phosphate solution
and an electrostatic precipitator to collect nanosize
powder. Their high-resolution transmission electron
microscopy (TEM) showed that the particles, some of
which were only 5 nm in size, exhibited well-ordered
HA lattice fringes. The thermodynamic solubility of
the nanosize HA is also presented better than that
prepared by the conventional methods.

Li et al. [25] synthesized nanoscale LiCoO

pow-

ders via spray drying in which equivalent amounts
of lithium acetate and cobalt were dissolved in
deionized water and some polyethylene glycol (PEG)
was added. The suspension was spray-dried to
produce mixed precursor in a spray dryer with a two
fluid nozzle. Atomizing pressure was controlled at
0.1 MPa, inlet air temperature was 3008C, and the
outlet air temperature was 1008C. LiCoO

powder

was synthesized by calcining the mixed precursor at
8008C for 4 h. The resulting particle had homoge-
neous particles size in the order of hundreds of nano-
meters.

There are also other methods such as spray pyr-

olysis and electrospray pyrolysis besides the above
method [26]. To prepare nanoparticles by spray pyr-
olysis, a starting solution is prepared by dissolving,
usually, the metal salt of the product in the solvent.
The droplets atomized from a starting solution are
introduced to furnace. Drying, evaporation of solv-
ent, diffusion of solute, precipitation, reaction of pre-
cursor, and surrounding gas, pyrolysis may occur
inside the furnace before the formation of product.
It is similar to spray drying except the type of precur-
sor. For this, colloidal particles are typically used as
precursors. Some products prepared by spray pyroly-
sis are listed in Table 30.1.

TABLE 30.1
Nanoparticles Prepared by Spray Pyrolysis

Particles

Chemical

Mean Size (nm)

CuO

Nitrate

CoO

Chloride

MgO

Chloride

Nitrate

MgFe

Chloride

MnFe

Chloride

(Ni,Zn)Fe

Chloride

BaO

6Fe

Chloride

Source: From Wang, B., Zhang, W., Zhang, W., Mujumdar, A.S.,
and Huang, L., Drying Technol., 23(1–2), 7, 2005.

2006 by Taylor & Francis Group, LLC.

In spray pyrolysis, mean size of final nanoparticles

can be determined from droplet size of solution
sprayed [23,24]. But a typical atomizer such as twin
fluid or ultrasonic nebulizer that is used to generate
droplet in spray pyrolysis is only capable of gene-
rating droplet with mean size in the range of several
microns. For example, a droplet with a diameter of
5 mm can produce a particle with a diameter
of 100 nm, the initial concentration of solute must
be 0.0008%. But in practice, low concentration may
lead to low rate of particle formation and affect the
purity of product. So, ultrafine nanoparticles can only
be produced from ultrafine droplets. The electrospray
technique has been examined as a method to generate
ultrafine droplets; during this process, a meniscus of a
spray solution at the end of a capillary tube becomes
conical when charged to a high voltage (several kilo-
volt) with respect to a counterelectrode. The droplets
are stably formed by the continuous breakup of a jet
extending from this liquid cone, generally referred to
as a ‘‘Taylor cone.’’ According to previous studies,
mean size of droplets sprayed by this method can be
controlled in a range of 1 nm to several microns.

Faezeh [28] used the technique of electrospray

drying to prepare carbon molecular sieve (CMS)
nanoparticles. The electrospray drying experiments
were operated in the cone-jet mode by making use of
a strong electric field (metallic nozzle connected to
a high-voltage source), in which a pendular droplet
deforms into a conical shape and then passes through
a weaker electric field (shielding electrode) at the
same polarity as the first one. The charged particles
were neutralized with the aid of corona discharge.
During the investigation of the effects of applied
voltage, liquid flow rate, and polymer concentration,
he found that the liquid flow rate has the most
important effect in determining the particle size.
The narrower particle size distribution with an aver-
age size of 200 nm was obtained under the experi-
ment with flow rate of 0.05 mL/h, at the voltages
of 13.6 and 7 kV (on the capillary and ring, respect-
ively), with a 0.05 wt% polyetherimide solution. The
better morphology was also found at this operation
condition.

30.3.1.3 Freeze-Drying

This method involves atomizing of the solution into a
freezing agent (for example, liquid N

) where tiny

droplets are turned into solid particles and after ne-
cessary filtering, these solid particles are moved to a
vacuum freeze-drying chamber in which they are
heated and ice sublimes. The dried material is sintered
to produce oxidized nanoparticles. Obviously freeze-
drying consists of two steps namely spray freezing

and vacuum drying, respectively. Table 30.2 lists
some powder products prepared by freeze-drying.

As regards drying of gels, the situation is more com-

plex [29,30]. Water undergoes some volume expansion
when freezing, which can separate adjacent particles
when water turns into ice. Nanoparticles are prevented
from agglomerating due to the formation of the solid
phase. On the other hand, high-energy vapor–liquid
interfaces are replaced by low-energy vapor–solid inter-
face, so agglomerations that are induced by surface
tension during drying can be mitigated in theory.

30.3.1.4 Microwave Drying

Zhang et al. [31] prepared CeO

nanoparticles by sol–

gel process using citric acid and cerium nitrate. The
solution was heated during reaction and dried by
microwave energy. It was found that the reaction
period is shortened from original 1–3 d to 30–60
min and drying period also shortened to only several
minutes from 2–3 h. Titanium dioxide, aluminum
oxide, zirconium oxide, and silicon dioxide have been
successfully synthesized by this method [32]. Com-
pared with conventional direct drying, microwave
drying has the following merits:

Rapid heating rate that only takes 1/10–1/100
the time needed by conventional ways.

There is no temperature gradient in the mater-
ials during microwave heating, so the materials
can be heated up more evenly due to volumetric
heat absorption.

30.3.1.5 Supercritical Drying

Supercritical drying [33,34] was initially developed by
Kistler to obtain materials having large pore volume
and specific surface area. A fluid is qualified as super-
critical when its pressure and temperature exceed
values, e.g., carbon dioxide has the low critical para-
meters (T

¼ 318C, P

¼ 7.29 MPa), but the critical

pressure for water is as high as 21.77 MPa. Supercritical

TABLE 30.2
Particles Prepared by Freeze-Drying

Particles

Reactants

Size (nm)

Ammoniacal brine

3.8–6

W-25%Re

Ammoniacal solution

Brine

70–220

MgO

Sulfate

100

Source: From Wang, B., Zhang, W., Zhang, W., Mujumdar, A.S.,
and Huang, L., Drying Technol., 23(1–2), 7, 2005.

2006 by Taylor & Francis Group, LLC.

drying prevents capillary pressure between the vapor–
liquid interface and the solid part of gel, which
occurs during supercritical drying. During this process,
the solvent within the gel is removed leaving only the
linked gel network. This process can eliminate the solv-
ent from the sol–gel without generating a two-phase
system and the related capillary forces. The process is
simply described in Figure 30.1. The sol–gel mixture at
point A is initially heated and pressurized to the super-
critical state (Point B). Then it is depressurized and
cooled to room condition (Point C). However, the solv-
ent vaporization curve (V) is not been crossed, i.e., no
two-phase system appears. Only a low-pressure solvent
vapor is present in the porous gel, which will be changed
by air diffusion since the gel is highly porous with open
pores. So gel skeleton is free of capillary tension during
supercritical drying. Cracking of microporous structure
and agglomeration of particles can be avoided theoret-
ically.

The supercr itical drying may be separat ed into

four stage s [18]:

Solvent -replacem ent stage : Liqu id CO

replac es

the solvent to cover the gel.

Diffus ional replacemen t : Liq uid CO

replaces

the solvent to fill in the pores of gel.

Supercri tical transit ion: Liquid CO

and gel mix-

ture are pressur ized and heated to the sup ercrit-
ical cond itions.

Isotherm al exp ansion : The mixture of liquid CO

and gel under sup ercritical co ndition is depres-
suriz ed to the atmosp heric con dition.

30.3.1 .6 S ubcritic al Drying

In contrast to supercritical drying, operating param-
eters for subcritical drying including temperature and

pressure are below the critical point. Wei et al. [35] used
E-40 (multi-polysiloxane) as silicon source and isobutyl
alcohol as solvent to prepare SiO

gel. The prepared gel

was placed in autoclave, adding an appropriate amount
of isobutyl and surfactant, preserved sometime under
pressure of 2.3–2.6 MPa, temperature of 240–2608C,
and then deflated slowly and cooled naturally; hydro-
phobic SiO

aerogel was finally obtained.

30.3.1.7 Direct Calcining

Direct calcining involves placing the precipitates or
wet gels in a muffle furnace in which dehydrating and
calcining simultaneously take place at high tempera-
ture. As sho wn in

Table 30.3

, Zhang et al. [36] also

prepared TiO

nanoparticles, using a muffle furnace

at 4508C for 3 h. Particle size of both TiO

and MgO

increases with calcination temperature.

Direct calcining is economical and convenient, but

seldom gives satisfactory nanoscale products because
of serious incidence of agglomeration.

30.3.2 S

OLVENT

-R

EPLACEMENT

RYING

Solvent-replacement drying involves replacement of
water in gels or precipitates with special organic solv-
ents and removal of the organic solvents.

When we prepare nanoparticles by the liquid-phase

method, wash with deionized water and filtration are
necessary steps before drying in order to get rid of the
residual ions. This usually causes severe agglomer-
ations. Replacing water with selected solvents having
low surface tension can yield products with minor ag-
glomeration. Regarding gels, solvent replacement is
required for subsequent treatment. Solvent replacement
can be carried out by one of the following ways:

Supercritical
domain

Temperature

Pressure

Solid

Liquid

(V)

Gas

FIGURE 30.1 The supercritical drying procedure.

2006 by Taylor & Francis Group, LLC.

Washing with organic solvents: Soaking, washing,
and filtering with organic solvents several times
are necessary steps before drying precipitates or
gels. The functional groups of organic molecules
replace nonbridging hydroxyls partially as well
as produce sterically hindered effects to prevent
agglomeration of particles. The commonly used
organic solvents include methyl alcohol, ethyl
alcohol, butyl alcohol, tert-butyl alcohol, acet-
one, cetane, silicone oil, etc. The need for wash-
ing with organic solvents is one of the reasons
for the high preparation cost.

Azeotropic distillation: Precipitates or wet gels are
dissolved in solvents, which possess higher boiling
points than water, but are immiscible in water.
Under heating and vigorous stirring, water and
organic solvents vaporize as an azeotropic mix-
ture. Solvents that can be used for azeotropic
distillation include butyl alcohol, isoamyl alco-
hol, isopropyl alcohol, propyl alcohol, glycol,
ethyl alcohol, benzene, and toluene. Butyl alcohol
is the most popular one. Researchers have con-
cluded that surface hydroxyls on particles are re-
placed by butyl alcohol molecules, which produce
steric hindrance effects to prevent agglomeration.
But butyl alcohol can cause serious pollution
problem and its recycling is quite problematic,
therefore commercialization is very difficult.

Liquid carbon dioxide replacement: Due to poor
miscibility of liquid carbon dioxide with water,

water must be replaced with suitable organic
solvents before supercritical CO

drying.

Supercritical carbon dioxide extraction: Because
gel pores are very fine, it takes a long time for
liquid carbon dioxide to percolate into the interior
of the micropores. The whole drying process can
take much longer time. This problem is solved if
supercritical carbon dioxide is used to extract
organic solvents, which are used to replace water.

30.3.2.1 Solvent-Replacement Oven Drying

Dong et al. [37] aged TiO

hydrosol for 2 h in mother

liquor, washed, and filtered with deionized water
many times to obtain a hydrogel, and then replaced
water with ethanol to get an alcogel. Alcogel was
dried in oven at 908C for 3 h, again kept in oven for
24 h at 1108C, finally calcined in a muffle furnace for
3 h at 5508C. Particle size of product was reduced and
other properties also improved compared with direct
oven drying (as shown in Table 30.4).

30.3.2.2 Solvent-Replacement Freeze-Drying

The medium of direct freeze-drying is water, but melt-
ing temperature of water differs from its boiling tem-
perature by 1008C.The sublimation heat of water is
high as a result of hydrogen bonding. Moreover, the
cold trap temperature in freeze-dryer must be con-
trolled under –508C in order to prevent the saltwater

TABLE 30.3
Relation between Particle Size and Calcining Temperature

Temperature (8C)

400

430

450

500

600

700

800

900

MgO particles size (nm)

—

TiO

particles size (nm)

—

22.1

23.9

34.4

41.1

46.9

49.6

Source: From Zhang, M., Yang, J., and Yang, X.J., Missiles Space Vehicles, 4, 51, 2001.

TABLE 30.4
Properties of TiO

Nanoparticle Obtained by Different Drying Methods

Samples

Drying Methods

Particle Size (nm)

Specific Surface Area (m

/g)

Porosity (m

/g)

CP1

Direct oven drying

þ calcining

50–100

4.88

0.027

CP2

Ethanol-replacement supercritical drying

þ calcining

10–20

113.8

0.41

CP3

Ethanol-replacement oven drying

þ calcining

20–40

60.7

0.34

Source: From Dong, G.L., Gao, Y.B., and Chen, Sh.Y., Acta Phys. Chim. Sin., 14(2), 142, 1998.

2006 by Taylor & Francis Group, LLC.

system from melting owing to its low plait point and
extremely slow sublimation rate. Sublimation heat of
tert-butyl alcohol is much lower than that of ice and
has small difference between its melting temperature
and boiling temperature. So cold trap temperature
only needs to be controlled at –158C to guarantee
no melting to take place. tert-Butyl alcohol experi-
ences smaller volume change than water when frozen,
which is in favor of maintaining integrity of porous
structures. Saturated vapor pressure of tert-butyl al-
cohol is higher than that of water (see Table 30.5), so
drying time can be greatly reduced. Products dried by
this method have excellent mesoporous structure.

Tamon et al. [30] synthesized resorcinol–formal-

dehyde (RF) hydrogel by sol–gel polycondensation of
resorcinal with formaldehyde, and the gel had been
immersed in ten times volume of tert-butyl alcohol for
over 3 d to replace water in its pores. Then the gel was
frozen at –308C for 1 d, at –108C for 1 d, and at 08C
for 1 d to obtain RF cryogel. Carbon cryogel was
prepared by pyrolyzing RF cryogel at high tempe-
rature. Although pores undergo shrinkage during
high-temperature carbonization, they maintain within
mesoscope. Property of this type of carbon cryogel
lied between carbon aerogel and carbon xerogel. Its
specific surface area and mesovolume are smaller than
carbon aerogel, but larger than carbon xerogel dried
by direct evaporation.

30.3.2.3 Solvent-Replacement

Supercritical Drying

30.3.2.3.1 Organic Solvent-Replacement
Supercritical Drying
Because hydrogels are not suited for supercritical
drying, hydrogels prepared from inorganic salt must
be converted to alcogels by replacing water with al-
coholic solvent before supercritical drying. Methyl
alcohol was used firstly, alcogels are put in dryer,
and some methyl alcohol is added; and it is essential
to boost temperature and pressure at a proper rate

until methyl alcohol reaches its supercritical state and
preserve for sometime in order to assure liquid methyl
alcohol can turn into supercritical fluid completely.
Release methyl alcohol gradually under constant tem-
perature and reduced pressure. It was Kistler who
prepared silicon aerogel by this type of supercritical
drying for the first time in 1933. Al

, TiO

, ZrO

organic, and carbon aerogels were also prepared later
by organic solvent-replacement supercritical drying
[38]. Besides, nanoparticles of nickel hydroxide, nickel
oxide, zinc oxide, zirconia, and a-Fe

were prepared

by ethanol-replacement supercritical drying.

30.3.2.3.2 Liquid Carbon Dioxide-Replacement
Supercritical Drying [21]
In general, alcohol (methyl alcohol) can cause esterifi-
cation on gel surface. The surface of this type of gel is
hydrophobic, therefore it is unlikely to absorb water in
air. But alcohol has high critical temperature and pres-
sure; in addition alcohol is flammable and toxic, espe-
cially methyl alcohol. Moreover, the gel skeleton may
be compact during supercritical drying, in particular
under the condition of the existence of base catalysis
and water. Mechanism of this kind of compact is simi-
lar to aging. In 1985, Tewari et al. took CO

as a drying

medium in supercritical drying which greatly reduced
the supercritical temperature and enhanced reliability
of drying equipment. Under operating conditions, CO

is chemically inert to gel skeleton. Due to the poor
solubility of CO

in water, water must be replaced by

organic solvents before supercritical drying. Aerogels
prepared by this method have strong water affinity and
tend to absorb water in air, so aerogels may gradually
turn to cream color after sometime in air and even
crack. Absorbed water can be removed by heating up
to 100–2508C without damage to aerogels’ skeleton.
Liquid carbon dioxide-replacement supercritical dry-
ing is currently the most common method to prepare
aerogels and nanoparticles.

TABLE 30.5
Properties of tert-Butyl Alcohol and Water

Substances

Melting Point (8C)

Boiling Point (8C)

Density Change (g/cm

)

SVP

/Pa

tert-Butyl alcohol

25.5

3.4

(268C)

821

Water

100

7.5

(8C)

SVP is the saturated vapor pressure.

Source: From Luan, W.L., Gao, L., and Guo, J.K., NanoStruct. Mater. 10(7), 1119, 1998; Tamon, H., Ishizaka, H., Yamamoto, T.,
and Suzuki, T., Carbon 37, 2049, 1999.

2006 by Taylor & Francis Group, LLC.

30.3.2 .3.3 Sup ercritica l Carbon Dio xide-
Replace ment Extract ion Drying
If we substitute supercritical CO

for liquid CO

which

is used in carbon dioxide-replacement supercritical dry-
ing, the drying operation is called supercritical carbon
dioxide-replacement extraction drying. Novak et al.
[33] prepared BaTiO

particle aerogel by this method.

Compared with liquid carbon dioxide-replacement
supercritical drying, drying period of supercritical car-
bon dioxide-replacement extraction drying is greatly
shortened, so preparation cost is reduced accordingly.

30.3.2 .4 Aze otropi c Distil lation Drying

Azeotro pic distillati on drying is used to remove wat er
in precipitat es or wet gels a s co mpletely as possible
and then the resid ual solvent sti ll ne eds furt her dr ying
and calcin ing afte r remova l of water. Organ ic solvent s
can be reu sed afte r con densation and lami nation.
Nanopart icles of zirconium oxide, aluminum oxide,
nickel oxide, sil ica dioxide , indium ox ide, aluminu m
and magnes ium hydro xide (AM H) had already been
prepared by this method. Hu et al. [39] remove d wat er
in TiO

gel using butyl alcohol aze otropic dist illation

followe d by supercr itical drying an d obtaine d TiO

nanop article.

30.3.2 .5 So lvent-Repl aceme nt Mic rowave Dry ing

Yamamo to et al. [40] replac ed water in the already
prepared organic hyd rogel wi th tert -butyl alcohol and
followe d it by 10 min of microwav e drying, after
carbonizi ng at high tempe rature and obt aining a car-
bon x erogel. The result indica tes that effici ency of
solvent -replaceme nt micr owave drying is much high er
than that of solvent -replaceme nt oven drying and
solvent -replaceme nt freeze-dryi ng.

30.3.3 M

ODIFIED

RYING FOR

ANOMATERIALS

Althou gh aerogel s or nano particles obtaine d by
supercr itical drying have ex cellent qualit y, rigor ous
operati ng condition s, long preparat ion time, an d ex-
pensive equipment cost are needed . To realize sub crit-
ical or ambien t dr ying for g els and nanop articles , a
series of modification measur es must be impl ement ed,
includin g strengtheni ng hardn ess of gel skeleton as
well as chan ging size and unifor mity of pores, surface
modify ing, etc. Some drying con trol c hemical addi-
tives (DCCA) can also be added to redu ce cracki ng
and shorte n drying pe riod [41, 42].

The modificat ion of nanomat erials is a pretr eat-

ment process that wet gels or precipi tates are su b-
jected to some modificat ion measu res in order to dry
them at lower pressur e or in ambie nt co nditions.

30.3.3 .1 Impr oving Uniform ity of Gel Por es

It is imprac tical to e xpect skel etal structure of
gel to be highly unifor m , as the skel etal structure of
gel i s ob t ained by hydrolyzation and condensation
reaction of pentamethide . According to E qua tion
30.1, the capillary pressure is inversely propor t ional
to the r adius of pores, and hence there are different
stresses on gel pores which lead to cracks or breaks
during drying. Moderate increase of pore size can
decrease capillar y pressure as well as increase pene-
tration c oefficient of t he gels, which in turn reduces
drying stress. On the other hand, adding formamide
can r estrain hydrolysis r ate and speed up condensate
rate of silicon a lkoxide, henc e la r g er g el ske l et on ca n
be obtained. This method also has a disadvantage,
since l arge pores need hi gher sintering t emperature
[43].

30.3.3 .2 Alter ing Vola tilization Order

of So lvent Mixture

If the solvent in gel pores is a mixture of wat er and
metha nol (or ethano l), appreci able amou nt of wate r
may be left when drying is finished because methyl
alcohol is easier to volatilize than water. But if some
surfactants of low surface tension and volatility (for
example, N,N’-dimethylformamide (DMF)) are added
to the wet gel before drying, the possibility of gel
cracking is reduced.

30.3.3.3 Surface Modification

Deshpande et al. [42] replaced water in hydrogel
with ethanol (or acetone, cetane). The hydrogel
was placed in trimethylchlorosilane (TMCS) (solv-
ent is benzene, toluene, or cetane) for modification
treatment, and then washed with ethanol. Surface
wettability of gels was improved significantly and
the contact angle nearly reached 908 (

see Table 30.6

The capillary pressure was also reduced. Then it was
dried at ambient pressure and room temperature
for 24 h, and then at 50 and 1008C for 24 h each
to produce a porous SiO

xerogel having properties

of aerogels which are obtained via supercritical
drying.

Shen et al. [43] prepared silica gel from polysilox-

ane E-40 precursor. After aging and solvent replace-
ment, the silica gel was placed in TMCS solution
(10%) for surface modification for 3 d, followed by
washing with silicone oil repeatedly, and dried under
ambient pressure to obtain SiO

xerogel. Its pore size

was 10.4 nm, the specific surface area 969 m

/g, and

shrinkage ratio 9%.

2006 by Taylor & Francis Group, LLC.

30.3.3 .4 Re inforc ement Streng th of Gel Skele ton

In order to reinforce the m ec hanical strength o f the g el
skele ton, the following mea sures a re usually taken [ 41]:

Adjust ing hydrol ysis cond ition : Hydrot hermal
and ch emical treatmen t are a pplied to adjust
hydrolys is conditio n in the inter est of accele rat-
ing conde nsation react ing to obtain gels that
have high polyme rization degree, and reinf or-
cing gel skeleto n strength accordi ngly.

Aging: A ging is a process of dissolving and rede-
positing of gel granules, by t his m eans connective
statu s of th e gel sk eleton can be improved. Einars-
rud et al. had s ucceeded in reinforcing gel skeleton
using water–ethanol, ethylate–et hanol to age gels.

Adding DC CA : For mamid e, dimet hyl form a-
mide, dimethyl acetam ide, glycero l, and oxalic
acid are gen erally used as DCCA reagent s.
DCCA can co nstrain hydrolys is of alkoxid e and
speed up con densatio n rate and all these are in
favor of high stre ngth of gel skeleton . On the
other hand , it can make size distribut ion of gel
pores more na rrow and reduce drying stress.
How ever, DCCA can also bring certa in side
effects, for example, some org anic impuriti es
are difficult to get rid off an d produce bladd ers
during sint ering. Some of them even cause nig-
rescence of gels at high tempe ratur e due to its
carboniza tion, etc.

30.4 COMPARISON AND SELECTION

OF DRYING METHODS FOR
NANOMATERIALS

30.4.1 C

OMPARISON OF

RYING

ETHODS

FOR

ANOMATERIALS

Differ ent drying methods wer e selec ted by diff erent
researc hers to produ ce various nano particles or nan o-
structure material s. Differ ent drying method s have

strong effects on the pro perties of nan omaterial s,
includin g particle size, partic le morph ology,
porous struc ture, specific surface area, etc. Here we
compare the results obtaine d by some research ers
who employ ed different drying methods to prepa re
nanomat erials.

Don g et al. [37] aged their TiO

hyd rogel in the

mother liquor for 2 h, filtered , a nd was hed the hyd ro-
gel with deionize d wat er severa l tim es. The hyd rogel
was sep arated into two pa rts: A and B. Part A was
dried in oven at 90 8 C for 3 h, then at 110 8 C for 24 h
and obtaine d as A1. B was con verted to B1 by re-
placin g wat er wi th ethan ol. B1 was sep arated into C
and D. Sample C was subjected to ethanol-replacement
supercr itical drying (260 8 C, 8 MPa , constant tem-
peratur e and pressur e for 0.5 h) to obtain C1. Sa mple
D was dried in oven at 90 8C for 3 h, and then at
110 8 C for 24 h to obt ain D1. A, C1, and D1 were
calcined in a mu ffle furnace for 3 h to prod uce prod -
ucts CP1, CP2, and CP3.

Figure 30.2a

shows the

morpholog y of CP1, whi ch displ ays seri ous agglom -
eration, wi th irregu lar pa rticle shapes. The reason for
this phe nomeno n is that dur ing drying strong capil -
lary forces develop that pull the pa rticles into closer
contact . Figure 30.2b shows relative ly minor agglom -
eration; it can be attr ibuted to replac ement of wate r
with ethano l, whi ch ha s lower surfa ce tensi on. Figu re
30.2c shows the mo rphology of the ethanol -repl ace-
ment supercr itically dried CP3; it is observed that
the cross-li nked structure is well preser ved during
supercr itical drying. Fur thermore, sup ercritic al dry-
ing can increa se the specific surface area and porosit y,
as well as effecti vely prevent s the formati on of hard
agglom erates. The particle and pore size dist ribution s
are more even.

Table 30.4

shows the pro perties of

three pro ducts.

Luan et al. [14] dried BaTiO

precipitate in three

different ways: direct oven drying, azeotropic distillation
drying, and tert-butyl alcohol-replacement freeze-drying.
Their results showed that the solvent-replacement freeze-
dried nanoparticle’s size is approximately 30 nm, and

TABLE 30.6
Contact Angles of Modified and Unmodified Gels

Solvents

Surface-Modified (Atmospheric Drying)

Nonsurface-Modified (Supercritical Drying)

Ethanol

76.78, 78.48

30.38, 35.18

Acetone

79.38, 77.28

29.18, 37.28

Hexane

89.68, 82.78

41.38, 48.48

1:4 Dioxane

81.18

66.48

Source: From Deshpande, R., Smith, D.M., and Brinker, C.J., Preparation of High Porosity Xerogel by Chemical Surface Modification,
U.S. Patent 5,565,142, 1996.

2006 by Taylor & Francis Group, LLC.

its crystal form is perfect. Particle size of powder
obtained by direct oven drying was about 80 nm and
agglomeration of particles is quite obvious. Finally,
particle size of powder obtained by solvent-replace-
ment azeotropic distillation drying is between that
of the above two; however, partial agglomeration
and hollow spherical structure formation are found.
Besides, sintering results indicated that there is a
big difference in sintering activity of the three dried
particles. Particles obtained by solvent-replacement
freeze-drying exhibit higher density (larger than 85%
TD) when sintered at 12508C, which is several hun-
dred centigrade lower than that of the conventional
method. Compared with oven drying, sintering
activity of particles obtained by azeotropic distillation
drying is improved.

Tamon et al. [30,44–46] and Yamamoto et al.

[47,48] prepared RF aerogels, RF cryogels, RF xer-
ogels, and RF MW gels from RF hydrogels, respect-
ively, by supercritical drying freeze-drying, oven
drying, and microwave drying. Their porosity prop-
erties were determined by nitrogen adsorption. It was
found that RF aerogels have the best mesoporous
structure among the three gels, next in line are RF

cryogels, RF xerogels, and RF MW gels. Their scan-
ning electron microscopic (SEM) images showed
that the RF aerogels were composed of intercon-
nected spherical particles; RF cryogels had cross-
linked structure composed of interconnected primary
nanoparticles; and RF xerogels and RF MW gels had
cross-linked structures similar to the structures of RF
cryogels. The primary nanoparticles of RF xerogels
and RF MW gels are confirmed to link densely and
the mesopores seem to be similar to those of RF
cryogels, which facts support the results of nitrogen
adsorption. From the above results, we can conclude
that supercritical drying can preserve the skeleton of
gel and minimize its shrinkage effect during drying;
freeze-drying is also useful to prepare porous structure
from hydrogels, and that it is difficult to obtain meso-
porous RF xerogels by oven drying. Using microwave
energy, one can dry hydrogels at higher rates than
freeze-drying and oven drying while retaining the
mesoporous structure of hydrogels.

Zhang et al. [49] prepared magnesium hydroxide

precipitate and dried them in four different ways,
as shown in Table 30.7. Then the dried precursors
were calcined in a muffle furnace at 5508C for 4 h to

(a)

100

μm

100

μm

100

μm

(b)

(c)

FIGURE 30.2 Nanoparticle morphologies prepared by different drying methods: (a) direct oven drying and calcining,
(b) ethanol-replacement supercritical drying and calcining, and (c) ethanol-replacement oven drying and calcining. (From
Dong, G.L., Gao, Y.B., and Chen, Sh.Y., Acta Phys. Chim. Sin., 14(2), 142, 1998.)

TABLE 30.7
Drying Conditions of the Experiments [49]

Sample

Parameters

Solvent Replacement

Drying Methods

Temperature (8C)

No solvent replacement

Oven

Alcohol

Oven

N,N’-Dimethylformamide

Oven

Butyl alcohol

Azeotropic distillation

97–120

Butyl alcohol

þ alcohol

Azeotropic distillation

97–120

Alcohol

Microwave drying

180

Alcohol

Supercritical drying

2006 by Taylor & Francis Group, LLC.

produc e magnes ium ox ide nanoparti cles. It is observed
that MgO nano particles made from the precu rsor
using direct oven drying get the largest parti cle size
and they are also most irre gular among the tested
sample s. It is due to severe agglom eration of the
precurs or during oven dr ying. On the other hand, it
also indica ted that sample 7 has the smal lest parti cle
size (18 nm) and pe rfect cryst allinity; mil d tempe ra-
ture (608 C) an d free of capil lary pressur e during
supercr itical drying are responsi ble for this.

The morphologies for different samples using TEM

are shown in

Figure 30.3

. The agglomeration and size

distribution status can be clearly observed under the
TEM images. Figure 30.3a is the image of sample 1,
which displays serious agglomerations and irregular
particle shapes. The reason for this phenomenon
is that strong capillary pressure pulls the particles
into closer contact during oven drying without solvent
exchange. Comparing the images of sample 1 with
samples 2 and 3 (Figure 30.3b and Figure 30.3c), it is
seen that more uniform particle size distributions are
obtained due to fewer agglomerations. The TEM
images for samples 4 and 5 are shown in Figure
30.3d and Figure 30.3e. The finding is consistent with
the fact that they have large particle sizes, which are
calculated from Scherrer equation. It was also found
that sample 4 presents cross-linked structure. Figure
30.3f is the photograph of sample 7 from which it is
clearly observed that the particle size is the smallest
compared with the others. It was also found that
minor agglomeration occurs and spherical morph-
ology of the particle is obtained.

Finally, microwave drying is used to dry the pre-

cursor. From the TEM image of sample 6, shown in
Figure 30.3g, it is found that significant growth of the
magnesium oxide particle size occurs due to the
growth of the precursor during the high-temperature
drying. However, its agglomeration status is bit simi-
lar to that in sample 7.

30.4.2 C

HOICES OF

RYING

ETHODS

FOR

ANOMATERIALS

It is a difficult task to select the most suitable drying
method for nanomaterials. There is no unified selec-
tion basis or set of criteria at present. Generally
speaking, drying methods for nanomaterials should
be determined according to laboratory test results and
product quality parameters specified. The following
observations are useful:

Although direct drying has many advantages,
such as simple operation, low preparation cost,
and equipment investment, it is difficult to obtain

high-grade nanomaterials by this method due to
excessive agglomeration.

Solvent replacement is a necessary process in dry-
ing of nanomaterials, especially for nanoparticles
prepared by the precipitation method. Organic
solvent replacement (or washing) can improve
quality of product significantly. Azeotropic distil-
lation is one of the solvent-replacement methods
with potential for development.

Quality parameters of dried products (nanopar-
ticles and aerogels) obtained by solvent-replace-
ment supercritical drying (including organic
solvent-replacement supercritical drying, liquid
carbon dioxide-replacement supercritical drying,
and supercritical carbon dioxide-replacement
extraction drying) are the best. Next in line
are

solvent-replacement

freeze-drying

and

solvent-replacement microwave drying.

Among all solvent-replacement supercritical
drying methods, organic solvent-replacement
supercritical drying has limitations of poor
safety due to its high temperature and pressure
operation; in addition its equipment and oper-
ation costs are high. As for liquid carbon diox-
ide-replacement supercritical drying, operation
period is very long. Supercritical carbon diox-
ide-replacement extraction drying has advan-
tages of short operation time, low-operating
pressure, and milder operating temperature.

Microwave drying has advantages of even heat-
ing and a short-drying period and therefore it
has great potential for development.

When the morphologies are considered, Rabani et al.
[16] suggested four basic regimes of drying-mediated
nanoparticle assembly. Disk-like or ribbon-like nano-
particle would domain reminiscent of spinodal de-
composition form at the early time when the solvent
evaporates homogenously from the surface. However,
when solvent disappears inhomogenously because of
the infrequent nucleation events, network structures
are found at early time as vapor nuclei meet. These
patterns are unstable if the domain boundaries are
not frozen following the evaporation.

30.5 CONCLUSIONS

With ever increasing research and development of
preparation technology for nanomaterials, drying
technology for nanomaterials also needs to be en-
hanced accordingly. At present, research in this field
is at a low level. Experimental study is mainly on the
level of small batch operation and the whole drying
cycle is quite long with operation cost remaining very
high. Therefore, there is massive work to be done

2006 by Taylor & Francis Group, LLC.

in order to industrialize drying technology for
nanomaterials:

Strengthening theoretical research on mechan-
ism of drying for nanomaterials

Exploiting subcritical drying or atmospheric
pressure drying based on modification tech-
nology for nanomaterials due to their low oper-
ation cost and operation pressure, and mild
operation temperature

(a)

(d)

(g)

(e)

(f)

(b)

(c)

FIGURE 30.3 TEM photographs of the MgO samples: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, (e) sample 5,
(f) sample 6, and (g) sample 7.

2006 by Taylor & Francis Group, LLC.

Making full use of the sup eriority of micr owave
drying an d pro bing into further studies on
mechani sm of its drying

Consid ering synthes izing, washing, filter ing,
modif ying, an d drying for na nomate rials as a
whol e in order to optim ize the process

Increas ing trans fer rate of dry ing process for
nano materials

Realizi ng continuous ope ration of drying for
nano materials

Finally , the model of self-assembl y of nanop articles
by Raban i et al. [16] is the first step in model of
nanomat erials drying. We would like to emph asize
that num erical models or simu lations using molec ular
dynami cs ha ve sti ll not matur ed. But this can help us
to unde rstand more about the pheno mena during
drying of nano size material s. At present , the focus
appears to be on empirical methods to dev elop nan o-
produc ts by the wet pro cess of desir ed propert ies.
Agglomera tion remai ns a major ch allenge. As ne w
applic ations develop for new nanomat erials, ne w dry-
ing techni ques may evolve. Also current ly, a ll appli-
cation s ap pear to requir e smaller quantities , so even
expensi ve drying techni que s may be utilized. In fu-
ture, safety and environm ental issues will become
very critical since nanoparti cles are difficul t to colle ct
if emit ted in air. The fact that they can be easily
inhale d or even ingested by the skin is especi ally
worrisom e and must be taken into account in the
design and operatio n of any drying process .

REFERENCES

1. Willems, van den Wildenberg, 2004, Work Documents

on Nanomaterials,

http://www.nanoroadmap.it/

2. Helmut, D., 2001, Some general aspects of confinement

in nanomaterials, Appl. Surf. Sci. 182, 192–195.

3. Nanotechnology Research Directions: IWGN Work-

shop Report, 2003,

http://www.wtec.org/loyola/nano/

IWGN.Research.Directions

4. Gutmanis, I., 1999, Probability of the Nanotechnology

Manufacturing Processes in the Industrial Nations in
2015–2025 Time Period, Report of Hobe Corporation.

5. Bourell, D.L., 1996, Synthesis and processing of nano-

crystalline powder, in: Proceedings of Annual Meeting
of the Minerals, Metals and Materials Society , February
4–8, California.

6. Wang, B., Zhang, W., Zhang, W., Mujumdar, A.S.,

Huang, L., 2005, Progress in drying technology for
nanomaterials, Drying Technol . 23(1–2), 7–32.

7. Jiao, Y., Ubrich, N., Marchand-Arvier, M., Vigneron, C.,

Hoffman, M., Lecompte, T., Maincent, P., 2002, In vitro
an d in vivo evaluation of oral heparin-loaded polymeric
nanoparticles in rabbits, Circulation 1 05 , 2 3 0– 23 5.

8 . Ahl in, P., K r is tl, L. , K r is tl, A ., Vr ece r , F., 2002, I nves ti -

gation of polymeric nanoparticles as carriers of enalapri-
lat for or al adminis tr ation, Int. J. Pharm. 239, 113–120.

9. Cavalli, R., Bargoni, A., Podio, V., Muntoni, E., Zara,

G.P., Gasco, M.R., 2003, Duodenal administration of
solid lipid nanoparticles loaded with different percent-
ages of tobramycin, J. Pharm. Sci . 92, 1085–1094.

10. Pandey, R., Sharma, A., Zahoor, A., Sharma, S., Khuller,

G.K., Prasad, B., 2003, Poly(dl -lactide-co-glycolide)
nanoparticles based inhalable sustained drug delivery
system for experimental tuberculosis, J. Antimicrob. Che-
mother. 52, 981–986.

11. Chastellain, A., Petri, M., Hofmann, M., Development

of Synthesis Methods for Nanosized Superpara-
magnetic Particles for Medical Application,

http://

ltp.epfl.ch/page17706.html

12. Pandey, R., Ahmad, Z., Sharma, S., Khuller, G.K.,

2005, Nano-encapsulation of azole antifungals: poten-
tial applications to improve oral drug delivery, Int.
J. Pharm. 301, 268–276.

13. Huang, Z., Chen, H., Yip, A., Ng, G., Guo, F., Chen,

Z., Roco, M.C., 2003, Longitudinal patent analysis for
nanoscale science and engineering: country, institution,
and technology field, J. Nanoparticle Res. 5, 1–47.

14. Luan, W.L., Gao, L., Guo, J.K., 1998, Study on drying

stage of nanoscale powders preparation, NanoStruct.
Mater. 10(7), 1119–1125.

15. Chow, G.M., Noskova, N.I., 1997, Nanostructured

Materials: Science and Technology, Kluwer Academic
Publishers, Dordrecht.

16. Rabani, E., Reichman, D.R., Geissler, P.L., Brus, L.E.,

2003, Drying-mediated self-assembly of nanoparticles,
Nature 426, 271–274.

17. Whitesides, G.M., Grzybowski, B., 2002, Self-assembly

at all scales, Science 295, 2418–2421.

18. Pakowski, Z., 2004, Drying of nanoporous and nanos-

tructured materials, in: Proceedings of the 14th Inter-
national Drying Symposium (IDS2004), Sao Paulo,
Brazil, Vol. A, pp. 69–88.

19. Wright, J.D., Sommerdijk, N.A.J.M., 2001, Sol–Gel

Materials: Chemistry and Applications, Gordon &
Breach Science Publishers, Berkshire.

20. Scherer, G.W., 1990, Theory of drying, J. Am. Ceram.

Soc. 73(1), 3–14.

21. Wang, B.H., Yu, C.Y., Wang, X.Zh., 2002, Supercritical

drying for nanometer porous materials (in Chinese), in:
Proceedings of the 8th National Drying Symposium, Harbin,
China, January 8–10, pp. 22–31.

22. Chow, L.C., Sun, L., Hockey, B., 2004, Properties of

nanostructured hydroxyapatite prepared by a spray dry-
ing technique, J. Res. Natl. Inst. Std. Technol. 11–12.

23. Masters, K., 1991, Spray Drying Handbook, 5th ed.,

Longman Science & Technical, England.

24. Iskandar, F., Gradon, L., Okuyama, K., 2003, Control

of morphology of nanostructured particles prepared by
the spray drying of a nanoparticle sol, J. Colloid Inter-
face Sci. 265, 296–303.

25. Li, Y.X., Jiang, Ch.Y., Wan, H.R., Zhu, Y., 1999,

Preparation of ultrafine LiCoO

powders by spray dry-

ing (in Chinese), J. Inorg. Mater. 14(4), 657–660.

2006 by Taylor & Francis Group, LLC.

26. Okuyama, K., Lenggoro, I.W., 2003, Preparation of

nanoparticles via spray route, Chem. Eng. Sci. 58,
537–547.

27. J ef f r e y , O . H . S. , Z ha n g , Y . , W a r r en , H . F . , W i l s o n,

H . R . , R a i m ar , L . , 2 00 4 , F o r mu l at i o n a n d c h ar ac t e r -
ization of s pray- dried powders co ntaining nan opar ti-
cles for a erosol deliver y to the l ung, Int . J. Pharm. 269,
45 7 –4 6 7.

28. Faezeh, B.-T., 2005, Preparation of Polyetherimide

Nanoparticles by Electrospray Drying, and Their Use
in the Preparation of Nano-Size Carbon Molecular
Sieve (CMS) Adsorbents and Membranes, The 2005
AIChE Annual, Cincinnati,

http://aiche.confex.com/

aiche/2005/techprogram/P32246.HTM

29. Scherer, G.W., 1993, Freezing gel, J. Non-Cryst. Solids

155, 1–25.

30. Tamon, H., Ishizaka, H., Yamamoto, T., Suzuki, T.,

1999, Preparation of mesoporous carbon by freeze-
drying, Carbon 37, 2049–2055.

31. Zhang, H.H., Li, X.Zh., Pan, Zh.Ch., Xiao, Ch.M.,

2002, Preparation of nanocrystalline CeO

by sol–gel

method (in Chinese), Fine Chem. Intermediates 32(5),
30–31.

32. Schiffiman, R.F., 1995, Microwave and dielectric drying,

in: Handbook of Industrial Drying, Mujumdar, A.S.,
Eds., Marcel Dekker, New York and Basel, pp. 345–372.

33. Novak, Z., Knez, Z., Ban, I., Drofenik, M., 2001, Syn-

thesis of barium titanate using supercritical CO

drying,

J. Supercritical Fluids 19, 209–215.

34. Phalippou, J., Woignier, T., 1999, Glasses from aerogel,

J. Mater. Sci. 25, 3111–3117.

35. Wei, J.D., Deng, Zh.Sh., Xue, X.S., Sun, Q., Yang, J.,

Wang, J., 2001, Hydrophobic SiO

aerogel dried at sub-

critical condition (in Chinese), J. Inorg. Mater. 16(3),
545–549.

36. Zhang, M., Yang, J., Yang, X.J., 2001, XRD research

on nanometer TiO

prepared by stearic acid gel method

(in Chinese), Missiles Space Vehicles (4), 51–53.

37. Dong, G.L., Gao, Y.B., Chen, Sh.Y., 1998, Effects of

different drying methods on the properties of TiO

par-

ticles (in Chinese), Acta Phys. Chim. Sin. 14(2), 142–146.

38. Poco, J.F., Satcher, J.H., Jr., Hrubesh, L.W., 2001,

Synthesis of high porosity monolithic alumina aerogel,
J. Non-Cryst. Solids 285, 57–63.

39. Hu, Z.S., Dong, J.X., Chen, G.X., 1999, Preparation of

nanometer copper borate with supercritical carbon di-
oxide drying, Powder Technol. 102, 171–176.

40. Yamamoto, T., Nishimura, T., Suzuki, T., 2001, Effect

of drying method on mesoporosity of resorcinol–
formaldehyde dry gel and carbon gel, Drying Technol.
19(7), 1319–1333.

41. Uchida, N., Ishiyama, N., Kato, Z., Uematsu, K., 1994,

Chemical effects of DDCA to the sol–gel reaction pro-
cess, J. Mater. Sci. 29, 5188–5192.

42. Deshpande, R., Smith, D.M., Brinker, C.J., 1996, Prep-

aration of High Porosity Xerogel by Chemical Surface
Modification, U.S. Patent 5,565,142.

43. Shen, J., Zhou, B., Wu, G.M., Deng, Zh.Sh., Ni, X.Y.,

Wang, J., 2002, Preparation and investigation of nano-
porous super thermal insulation: silica aerogel (in Chi-
nese), Chin. J. Process Eng. 2(4), 341–345.

44. Tamon, H., Ishizaka, H., 1999, Preparation of organic

mesoporous gel by supercritical or freeze-drying, Dry-
ing Technol. 17(7–8), 1653–1665.

45. Tamon, H., Ishizaka, H., Yamamoto, T., Yamamoto, T.,

Suzuku, T., 2001, Freeze drying for preparation of aero-
gel-like carbon, Drying Technol. 19(2), 312–324.

46. Tamon, H., Ishizaka, H., Yamamoto, T., Suzuki, T.,

2001, Freeze drying for preparation of aerogel-like
carbon, Drying Technol. 19(1), 313.

47. Yamamoto, T., Nishimura, T., Suzuki, T., Tamon, H.,

2001, Control of mesoporosity of carbon gels prepared
by sol–gel polycondensation and freeze drying, J. Non-
Cryst. Solids 288, 46.

48. Yamamoto, T., Sugimoto, T., Suzuki, T., Mukai, S.R.,

Tamon, H., 2002, Preparation and characterization of
carbon cryogel microspheres, Carbon 40(8), 1345.0

49. Zhang, W., Wang, B., Fan, F., Zhang, W., Mujumdar,

A.S., Huang, L., 2004, Effect of different drying me-thods
on the morphology and particle size of magnesium oxide
nanoparticles, in: International Workshop and Symposium
on Industrial Drying, Mumbai (India), pp. 427–432.

2006 by Taylor & Francis Group, LLC.

Document Outline

Table of Contents
Chapter 030: Drying of Nanosize Products

Wyszukiwarka

Podobne podstrony:
039 Drying of Biotechnological Products
034 Drying of Textile Products
029 Drying of Pharmaceutical Products
Modeling with shrinkage during the vacuum drying of carrot (daucus carota) (Arévalo Pinedo, Xidieh M
Influence of drying methods on drying of bell pepper (Tunde Akintunde, Afolabi, Akintunde)
[Mises org]Hülsmann,Jörg Guido The Ethics of Money Production
Far infrared and microwave drying of peach (Jun Wang, Kuichuan Sheng)
ecdltest, produkty ryżowe, List of Rice Products in Stock
Microwave Application in Vacuum Drying of Fruits (Drouzaf, H SchuberP)
Microwave vacuum drying of model fruit gels (Drouzas, Tsami, Saravacos)
Duty Health Effects of Cleaning Products
041 Drying of Polymers
042 Drying of Enzymes
ANALYSIS OF FOOD PRODUCTS 116
Modeling and minimizing process time of combined convective and vacuum drying of mushrooms and parsl
022 Drying of Fish and Seafood
026 Drying of Herbal Medicines and Tea
Characterization of microwave vacuum drying and hot air drying of mint leaves (Mentha cordifolia Opi
Microwave irradiation of hazelnuts for the control of aflatoxin producing Aspergillus parasiticus

więcej podobnych podstron

030 Drying of Nanosize Products

Document Outline