Nanoparticle Technology for the Production of Functional Materials

The nanoparticle technology, relating to the preparation, characterization, processing, and applications of nano-sized particles, plays an increasingly important role in the emerging nanotechnology. Although the nanoparticles have many unique functional properties superior to the coarser particles, they also suf fer from dispersion and stability problems because of their strong cohesiveness and high specific surface areas. To make the best use of nanoparticles and solve their application problems, the development of nanomaterial processing techniques is essential. New chemical synthesis methods for producing nano-sized oxides particles in the gas phase and producing biocompatible polymeric nano-composite particles in the solution phase were elucidated in the paper. In addition, mechanical breakdown method (e.g. nano-grinding) was brief ly discussed. Furthermore, newly developed dry particle processing systems for making high per formance nanocomposites as well as their applications in Fuel Cells, Drug Delivery Systems, and Cosmetics were introduced.


Introduction
The nanotechnology has drawn much attention since the beginning of this century as the critical technology to advance industrial outputs and to extend human life in the 21st century.Developing commercial applications of nanotechnology, the nanosized particles play a significant role because of their unique functional properties.
The definition of nanoparticles depends on their applications.In general, nanoparticle refers to the particle having a size smaller than 100 nm.However, it could be, in the narrow sense, less than around 10 nm as its physical properties, such as melting point, differ from those of the bulk solids.On the other hand, particles ranging from nm to one µm could also be called "nanoparticles" in the broader sense.In this paper, the particles less than the shortest wavelength of visible light (around 400 nm) are called "nanoparticles", which are finer than the so-called "submicron particles".
The technology to prepare, characterize, process and apply these nanoparticles is called "Nanoparticle Technology", which is expected to be one of the key technologies to materialize the commercial applications of nanotechnology.In fact, nano-sized particles, such as ink, pigment, carbon black, fine silica etc., are not new to some industries and have been used as additives to improve product structure and qualities.The nanoparticle technology is to bridge the new nanotechnology applications and the traditional powder technology.
In this paper, the nanoparticle technology for the production of nanoparticles and nanocomposites and their applications are to be discussed.

Production of nanoparticles and nano-sized composite particles
Nanoparticles can be produced by the break-down (top-down) or the build-up (bottom-up) method.In this paper, the production methods were discussed according to their working environments, i.e. the solid, liquid or gas phase, as shown in Fig. 1.

1) Solid phase method
The most popular way to break-down solid materials is the grinding method.The sizes of ground product have been getting finer and finer since long.Thirty years ago or so, the dry grinding limit of mineral materials 1) was around few microns.However, the grinding limit was reduced down to the submicron range with the development of compressionshear type ultra-fine grinding mills 2) in early 1980's.Furthermore, the nanosized products were reported by using wet grinding with ball media mill 3) in late 1990's.
However, it was also understood that the grinding process could reach its finest possible product size (so-called grinding equilibrium), after that the size of the ground product increased with an increase in the grinding time because of particle agglomeration.It was also experimentally proven that the smaller the grinding media and the maximum force exerting on them were, the finer was the grinding equilibrium diameter as shown in Fig. 2 4) , as long as the grinding intensity given by the grinding media was large enough to break the particles.Becker et al. also confirmed this phenomenon in terms of the grinding intensity 5) .Nowadays, fine ceramic beads below 30 µm are available for nano-grinding applications.
In addition to the grinding intensity, the physicochemical conditions of the grinding processes could affect the outcomes of nano-griding.Fig. 3 showed that the size reduction of products stopped at a certain grinding time, but its particle size could further be reduced when the pH value of the slurry was adjusted to better disperse the particles in it.The pH value of the slurry could be controlled by measuring its zeta potential 6) .

2) Gas phase method
A number of methods are available to prepare the nanoparticles in the gas phase using chemical reactions or physical state changes.The gas phase methods can generally produce higher quality nanoparticles but at a lower capacity in comparison to the solid phase and liquid phase methods because of their extremely low bulk densities.Nearly 20 years ago, we developed a system to produce metallic nanoparticles using the evaporation method with the plasma-arc 7) .This lab-scale system produced nano-sized metallic particles at a rate of tens of grams per hour for usual metals such as iron and cupper.
The new gas phase method we recently developed is based on the plasma assisted chemical reaction technique.The liquid mixture of raw materials is fed together with the reaction gas into the combustion chamber, where the raw materials are gasified and the nanoparticles are generated by the chemical reaction in the gas phase with the assistance of plasma emission.The critical step of this method is to collect

Liquid phase synthesis
Fig. 1 Production methods for nano particles and micron particles the nanoparticles by rapid quenching before the occurrence of any grain size growth or particle agglomeration.Our pilot-scale system has the advantage of producing various kinds of high-purity nanoparticles at a rate of several kgs per hour, depending on the type of materials and their particle size requirements.
Based on the BET specific surface area measurement, nanoparticles in tens of nanometers have been produced by this system.In some cases, nanoparticles, such as ceria, less than ten nanometers could be obtained as well.
The other advantage of the newly developed system is to produce nanoparticles consisting of multi-components with three structural patterns, as shown in Fig. 4. The pattern a) is the particle of uniform structure, such as solid solutions or compounds, made of mutually soluble components.The pattern b) is the core-shell structure consisting of the core particles covered by another shell component.The pattern c) is the finely dispersed nanocomposites, where oxide nanoparticles are scattered in the matrix material.In principle, these patterns are formed depending on raw materials and their concentrations during the particle formation.But, the ratio of individual components can be widely varied to optimize the quality of the final products.

3) Liquid phase method
There are also various methods, such as evaporation decomposition, crystallization, precipitation, sol-gel process, polymerization etc., can be used to prepare nanoparticles in the wet phase.Their produc-tion rates are normally higher than the gas phase method and suitable for mass production.Many types of nanoparticles, not only ceramics but also organic polymers, can be produced by the liquid phase method.However, the nanoparticles tend to agglomerate during the drying process if dry powder is desirable.
One of the liquid phase methods for nanoparticle production we are recently involved in is the spherical crystallization of polymeric nanoparticles by using emulsion solvent diffusion technique, originally developed by Kawashima et al 8) .With this method, the polymer, such as PLGA (Poly Lactide-co-Glycolide) is first dissolved in a solvent, such as the mixture of acetone and ethanol.The solution is then introduced into water with PVA to make polymeric nanoparticles in the suspension by the crystallization mechanism, as shown in Fig. 5.The PLGA nanoparticles produced by this method are in the spherical shape and have a narrow particle size distribution with a median diameter of about 200 nm (Fig. 6).When a drug is dissolved in the solvent with the PLGA, drug encapsulated biocompatible PLGA nanoparticles can be produced.The applications of these nanoparticles for DDS (Drug Delivery Systems) will be discussed in the later section.

Fabrication of nanocomposites
It is well known that composites can create superior functional and structural properties of materials.Fig. 7 showed the typical methods to fabricate particle composites 9) .The composites could be made by intercalation in the nanometer scale.
The nanocomposite particles can be prepared by the methods such as gas phase reaction or crystallization as discussed in the previous sections.In those cases, the particle size of the nanocomposites ranges from tens to a few hundreds of nanometers.On the other hand, micron-sized composite particles consisting of nanoparticles can also be produced efficiently by mechanical processing.For example, f luidizedbed granulation method and the like can produce nanocomposite particles from tens of microns to mm.
Although the nanoparticles have unique properties and big potential for many new applications, they are usually difficult to handle because of their strong cohesiveness, low f lowability, and low stability.The search for a way to overcome the handling problem becomes a priority in the commercialization of nanoparticles.Making nanocomposites is often a good solution to the problem, because it can modify parti-  cle surface properties to improve their f lowability or control the particle structure for better functional performance.The nanocomposite production methods can be in the wet or dry phase as described below.

1) Dr y method
Nanoparticles have low stability and tend to agglomerate or make aggregates because of their strong cohesiveness and high surface areas.It is usually not easy to disperse them, especially in a solid mixture.Recently, new machines and systems, based on MCB (MechanoChemical Bonding) Technology were developed to address these application problems.The MCB Technology is to create direct solid bonding between fine particles by the mechanical energy without using any binders, which may further be assisted by additional physical energy such as plasma and the like.The plasma has the function of cleaning particle surfaces and enhancing the strength of the solid bonding.The plasma assisted MCB system is called Nanocular, which can make nanocomposite materials continuously.The plasma effect during the mechanical treatment was confirmed by studying the photo-catalytic characteristics of the nano titanium dioxide 10) .Fig. 8 showed the batch type lab-scale Nanocular for the purpose of research and development of new materials.
Another system developed based on MCB mechanical treatment alone is called Nobilta 11) , which can impose high mechanical energy intensity on the material mixtures.Although the Nobilta is a batch system, it is designed to handle various types of materials.
The degree of particle bonding in the nanocomposites can be evaluated by the particle size analysis, BET specific surface area measurement 12) , image analysis of particle cross-section with electron microscope 13) , photo-correlation of elements 14) , sieving separation of core and guest particles, and so on.The BET measurement is chosen to use in this study, because it is often applied in the nanoparticle characterization.As shown in Fig. 9, the raw material mixture consisted of SiO 2 with an average particle size of 26 µm and TiO 2 nanoparticles with a nominal diameter of 15 nm at a mass ratio of 10 to 1.The specific surface area of the powder mixture decreased with the increase of mechanical energy input, which indicated the progression of bonding the nano TiO 2 onto the surface of SiO 2 particles.Particle bonding performances of three different batch types of equipment, as shown in Fig. 10, were compared in this study.The Cyclomix is a high-speed powder mixer based on the impact and shearing mechanisms; the MechanoFusion AMS system is a particle-bonding machine having a rotating chamber with stationary press heads; and, the Nobilta is specially designed to give high specific energy input on the powder mixture.The test results showed that the Nobilta could reduce the specific surface area of powder mixture from about 11.0 m 2 /g to about 1.0 m 2 /g, which was nearly that of starting

2) Wet method
With this method, nanocomposites are produced from the suspension or slurry containing nanoparticles via a drying process.The dry nanocomposites can be obtained by spray drying of mixed material suspension or by coating the core particles with the suspended materials as the shell particles.In this case, the dispersion of the nanoparticles and homogeneity of the components in each nanocomposite particle are important to insure the product quality.One of the useful machines for this application is the Agglomaster, which is a f luidized-bed type granulator with spray drying assembly 15) and pulse-jet mechanism for better dispersion of particles 16) .Using this machine, granules having an average size of tens of µm with various interesting granule structures can be obtained because of the simultaneous granulation and particle dispersion during the drying process 17) .Fig. 11 showed an example of composite particles made of platinum-doped nano-sized carbon black and resin using the Agglomaster.This material was successfully used to improve the electro-conductivity and catalytic characteristics of the electrode for high performance fuel cells.This method can modify particle surfaces for various functional applications as well.

Applications of nanoparticles
The nanoparticles have many unique features comparing to the bulk solids because of their particle size effect and high surface reactivity.They can be used in various applications, such as batteries, sensors, catalysts, paints, inks, films and plastics, cosmetics, nanobiotechnology and so on.A few highlighted examples were discussed below.

1) Fuel cell
Fuel cell is a clean energy generator and expected to use widely in the near future.Among several different types of fuel cells being developed, the SOFC (solid oxide fuel cells) has the highest energy efficiency, but its conventional operating temperatures have been as high as 800 to 1000°C, which requiries expensive seals and insulation materials for the cell construction.The SOFC anode electrode is usually made of YSZ (yttria-stabilized zirconia) Ҁ NiO cermet.For developing high performance SOFC, it is necessary to create large reaction area with high reactivity in the electrode and to achieve low internal resistance and good morphological stability of the fuel cell at the elevated temperatures, while securing the passages of gases and liquid reactant.Therefore, porous electrodes made of well-dispersed fine particles is highly desirable.However, sintering "green" anode electrode often causes the grain sizes of fine NiO to grow.This leads to the formation of large Ni particles in the anode and results in inhomogeneous electrode structure.It was experimentally confirmed that bonding the YSZ nanoparticles on the surfaces of NiO particles using the above-mentioned MCB system could suppress the grain size growth of Ni and improve the power density of SOFC and its stability to a great extent 17) .Fig. 12 showed the microstructure of the Ni-YSZ cermet anode fabricated from NiO-YSZ composite particles processed by the MCB technology.
In concert with the thin solid electrolyte, the SOFC using MCB treated electrode materials was found to have exceptional performance at 700°C, which could significantly reduce the manufacturing costs of the SOFC.Fig. 13 showed the performance improvement of our prototype SOFC.As seen in the figure, the anode polarization of MCB treated SOFC was remarkably reduced at 800°C comparing to that of the conventional SOFC at the same temperature.In addition, it was still better than the conventional one even at 700°C.

2) DDS and cosmetics
Site specific drug delivery has been investigated for a while.One of the recent developments is to use biocompatible PLGA polymeric nanoparticles, as mentioned in the previous section, for the applications of DPI (dry powder inhalation).It is known that the absorbance of nanoparticles in the human body is much higher than that of micron-sized particles, but the nanoparticles tend to agglomerate, stick to the drug capsule, and adhere to the throat and trachea of the patient during pulmonary drug administration.They cannot effectively reach the depth of human lung.Therefore, it is necessary to make coarse agglomerates having a diameter of 30ȁ50 µm for easy handling while allowing the inhaler to break them down to several µm, suitable for pneumatic-conveying to the lung.The micron-sized agglomerates are fully dispersed to their original nanoparticles in the lung.ite particles consisting of drug encapsulated PLGA nanospheres and the photo of composite agglomerates.For this application, both dry and wet methods for making nanocomposites as mentioned previously were applied.Test results showed that the respirable fraction of the drug could be improved from less than 10% to over 40% by using PLGA nanocomposites in vitro study with a cascade impactor 18) .In addition, the drug efficacy was found significantly improved with the use of insulin encapsulated PLGA polymeric nanoparticles.Fig. 15 showed the blood glucose level of rats after introducing insulin via different drug administration methods.The drug efficacy could be evaluated by calculating the area between the initial blood glucose level and that after insulin administration in the figure.It was confirmed that the therapeutic efficacy of using drug encapsulated nanoparticles could be 1.6 times of that applying insulin solution injection 19) .This might attribute to the controlled release effect of the PLGA co-polymer, which gradually decomposed in the water by hydrolysis.
The nanocomposite particles can also be used as transdermal drugs and cosmeceuticals.Fig. 16 showed the amount of ascorbic acid in the dermis of human skin 20) as a function of time after applying pro-vitamin C. It clearly indicated that applying the pro-vitamin C encapsulated PLGA nanoparticles could produce much more reduced form vitamin C in the dermis than using pro-vitamin C liquid suspension.The reduced form vitamin C is known to prevent the oxidation of biological tissues caused by the UV radiation and to suppress the generation of melanin pigment, which causes black spots on the skin.It also promotes the formation of collagen to reduce the wrinkles for anti-aging applications.The reason for the differences in the vitamin-C absorption attributed to the excellent skin permeability of PLGA nanosphere, which was used as a carrier of pro-vitamin C. The pro-vitamin C has poor skin permeability because of its hydrophilic characteristics.It was also found in-vitro that the pro-vitamin C encapsulated PLGA nanoparticles could remarkably reduce the DNA damage on the skin caused by the UV radiation.

Conclusion
Although nanoparticles have a lot of unique properties and present great potential for many applications, the relationship between the material properties and product performance has to be established to justify their usages.While nanoparticle production had impressive progress in the past few years, the nanoparticle technology is still in its infancy.It is necessary to demonstrate the benefits of nanoparticles to the commercial products before their commercialization.Therefore, the technologies relating to the nanoparticle characterization, design, modification, and processing will play an important role in the nanoparticle commercialization; and, the nanoparticle technology is expected to further develop rapidly in all the industries handling fine particles in the near future.

Fig. 2 Fig. 3 Fig. 4 Fig. 5
Fig. 2 Correlation between the grinding equilibrium diameter (x eql ) and the maximum force exerting on a single grinding ball (F B ) in the planetary ball mill

Fig. 11 Fig. 10
Fig. 11 Example of composite particles made of platinum-doped nano-sized carbon black and resin using the fluidized-bed granulator (Agglomaster)

Fig. 16
Fig.16 Comparison of the amount of ascorbic acid in the dermis delivered by the PLGA nanoparticle and liquid suspension