Smart Powder Processing for Advanced Materials †

Recently, various novel powder processing techniques were rapidly developed for advanced material production due to the growing of high-tech industry, especially in consideration of energy consumption and the environmental issues such as the recycling of waste materials. Smart powder processing stands for novel powder processing techniques that create advanced materials with minimal energy consumption and environmental impacts. Particle bonding technology is a typical smart powder processing technique to make advanced composites. The technology has two main unique features. Firstly, it creates direct bonding between particles without any heat support or binders of any kind in the dry phase. The bonding is achieved through the enhanced particle surface activation induced by mechanical energy, in addition to the intrinsic high surface reactivity of nanoparticles. Using this feature, desired composite particles can Abstract


Introduction
Recently, various novel powder processing techniques were rapidly developed for advanced material production due to the growing of high-tech industry, especially in consideration of energy consumption and the environmental issues such as the recycling of waste materials.Smart powder processing stands for novel powder processing techniques that create advanced materials with minimal energy consumption and environmental impacts.Particle bonding technology is a typical smart powder processing technique to make advanced composites [1][2][3][4][5] . Te technology has two main unique features.Firstly, it creates direct bonding between particles without any heat support or binders of any kind in the dry phase.The bonding is achieved through the enhanced particle surface activation induced by mechanical energy, in addition to the intrinsic high surface reactivity of nanoparticles.Using this feature, desired composite particles can

Abstract
Smart powder processing stands for novel powder processing techniques that create advanced materials with minimal energy consumption and environmental impacts.Par ticle bonding technology is a typical smart powder processing technique to make advanced composites.The technology has two main unique features.Firstly, it creates direct bonding between particles without any heat support or binders of any kind in the dry phase.The bonding is achieved through the enhanced particle surface activation induced by mechanical energy, in addition to the intrinsic high surface reactivity of nanoparticles.Using this feature, desired composite particles can be successfully fabricated.The second feature of this technology is its ability to control the nano/micro structure of the assembled composite particles.As a result, it can custom various kinds of nano/micro structures and can produce new materials with a simpler manufacturing process in comparison to wet chemical techniques.In this paper, its application examples for making advanced materials will be explained.These two features lead to the achievement of minimizing energy consumption and environmental impacts when producing advanced materials.By making use of the particle bonding principle, a new one-pot processing method to synthesize nanoparticles without applying extra heat was developed.Furthermore, by carefully controlling the bonding between different kinds of materials in the composite particles, effective separation of elemental components can be achieved.It leads to the development of a novel technique for recycling advanced composite materials and turns them to high-functional applications.In this paper, these approaches will also be introduced.It is our goal to signify the particle bonding technology as a potential advanced processing technique for producing powder materials.be successfully fabricated.The second feature of this technology is its ability to control the nano/micro structure of the assembled composite particles.As a result, it can custom various kinds of nano/micro structures and can produce new materials with a simpler manufacturing process in comparison to wet chemical techniques.Fig. 1 showed the examples of unique microstructures created by the par ticle bonding technique, which led to various kinds of applications such as multi-layered composite particles for the drug delivery systems (DDS) 6) , self-assemble structures for electronic devices, composite porous materials for fuel cell electrodes, and nano-porous materials.In this paper, first, its application examples for advanced material production would be explained, including the development of fuel cell electrodes [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] and high efficient thermal insulation materials [25][26][27] .
Fur thermore, by making use of these features, a new one-pot processing method to synthesize nanoparticles without applying extra heat was developed.A rapid synthesis of perovskite type lanthanum manganite and related compounds star ting from a mixture of industrial grade powders was demonstrated [28][29][30][31][32][33]. Also, particle bonding with electric discharge is an interesting phenomenon for controlling surface structure of nanoparticles 34).These results suggest that particle bonding technique is a promising approach to minimize energy consumption when synthesizing nanoparticles, while existing processes require a long manufacturing route including firing at high temperatures.In this paper, the development of novel powder processing for energy reduction would be introduced.Finally, by carefully controlling the bonding between dif ferent kinds of materials, separation of composite structure into elemental components is possible, which leads to the development of novel technique to recycling advanced composite materials and turns them to high-functional applications.In this paper, the development of novel method to recycle glass-fiber reinforced plastics (GFRP) would be introduced and particle bonding principle was found to be a potential advanced processing technique for producing powder materials 35) .

Production of Composite Particles
So far, various kinds of composite particles have been produced by par ticle bonding process.The typical one is a core particle coated with fine guest particles.Bonding fine particles on the surface of core particle was already proposed.Fig. 2 showed the particle bonding process 3) by Mechanofusion System 4) .The process is carried out in two steps.First, the surfaces of fine particles and core particles are mechanically activated, as a result, fine particles adhere onto the surfaces of core particles.Then, as the second step, fine particles and core particles interact to each other while fine particles also adhere onto the fine-particle layer on the surfaces of core particles.Therefore, by changing the kind of fine particle materials during the processing, we can easily make multilayered composite particles or functional gradient

particles.
There are a lot of factors affecting the particle composing process, and, its bonding mechanism depends on the combinations of core/fine particulate materials.However, as is well known, the contact surface between the powder materials receives extremely high local temperature and pressure, where mechanical stresses were actually given 36) .For example, authors previously reported that the local temperature at the interface between particles during particle bonding processing could be ten times higher than the apparent temperature of processing chamber 37) .Such a locally high temperature is expected to cause unique phenomena between fine particles and core particles, or among fine particles.For example, when coating nano titania particles on the glass beads, the peak of binding energy of Ti 2p shifted away from its original position after only 5 min of mechanical processing 3) .It suggested that there was a chemical interaction on their surface during the processing.

Structural Control of SOFC Electrodes
Solid oxide fuel cell (SOFC) is a promising candidate for power generation in the 21 st century because of its high energy efficiency and clean exhaust.Current R&D efforts focus on reducing its production cost and increasing the long-term stability of cells and stacks by lowering its operation temperature without losing power density.Prefabrication of the composite particles followed by electrode forming using particle bonding process is an ideal way to go, especially for controlling the microstructure of composite electrodes.Recently, we successfully fabricated various kinds of composite particles such as large core-particles coated with nanoparticles and inter-dispersed composite mixture consisting of several kinds of nanoparticles using the particle bonding technique [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] . Nckel-yttria stabilized zirconia (Ni-YSZ) is the most widely used SOFC anode material due to its excellent electrochemical properties at high temperatures.The electrochemical reaction (hydrogen oxidation) takes place at the triple-phase boundary (TPB) where Ni, YSZ and fuel gas meet.The reaction rate strongly depends on the catalytic activity of anode materials and the TPB length.Since the former significantly decreases with decreasing operation temperature, the latter must be increased as much as possible in the limited effective electrode volume to keep high electrochemical performance even at lower temperatures.For the TPB enlargement, the anode microstructure such as size and arrangement of Ni and YSZ must be controlled precisely.Fig. 3 showed the process to fabricate SOFC anode using coated composite particles (CC), i.e.NiO large core-particle coated with YSZ nanoparticles.The composite par ticles provided well-organized electrode microstructure as shown in the figure, and resulted in good electrochemical performance 12) .As a result, a prototype SOFC power plant system with a capacity of about 1kW was fabricated using the anode made by the CC particles and has been in services at an operating temperature of 700℃.Fig. 4 showed a LED display exhibiting the Japanese letters of HO-SOKAWA, which was installed at the top floor of the main building of Hosokawa Micron Corporation in Osaka and powered by the prototype SOFC system for long-term evaluation.Particle bonding process can also create another type of composite par ticles.For example, Fig. 5 showed NiO-YSZ inter-dispersed composite particles  (IC) consisting of NiO and YSZ nanopar ticles 24) .Fig. 5 (a) was the SEM micrograph of the composite particles.Fig. 5 (b) was the detailed structure of a composite particle observed by TEM.NiO and YSZ phases in the composite particle were identified by EDS analysis in the micrograph.The micrograph indicated successful fabrication of the IC particles.NiO and YSZ nanoparticles were well dispersed and their sizes were in good agreement with those estimated from the specific surface area of starting particles (NiO: 160 nm and YSZ: 75nm) 24) .Fig. 6 showed SEM micrographs of the anode made of IC particles before the reduction, just after the reduction, and after the long-term stability test at 700℃ for 920h.The anode made of IC particles before the reduction showed grain sizes smaller than 1μm without abnormally large ones.NiO shrunk when it was reduced to Ni, and, porous structure evolved in the anode.Thus, the uniform porous structure after reduction suggested that NiO was uniformly distributed around the three-dimensional YSZ frame work in the entire anode.Even after the long-term stability test, no significant structural change was observed in the anode.The grain size was kept at about 0.5μm, and no abnormally large grain was obser ved.The insignificant micro-structural change indicated that grain growth of Ni was insignificant in the anode made of IC particles under the testing conditions 24) .Fig. 7 showed anode polarization curves as a function of current density.The anode made of IC particles showed lower polarization than the anode made  Fig. 4 LED display by SOFC power plant of CC particles 24) .It indicated that careful control of microstructure by the use of composite particles as starting materials was very important for improving the performance of anode.Further improvement on the electrode performance was expected by optimiz-ing Ni-YSZ microstructure using particle bonding technique.

Nano-Porous Structural Control and its Application for Thermal Insulation
The second example is to make high efficient thermal insulation materials [25][26][27] . Iterest in thermal insulation materials has been intensified globally, because escalating energy costs signified the importance of efficient thermal insulation.In this study, nanoparticle bonding process was used to make composite fibers coated with porous fumed silica layer in the dry phase.Fig. 8 showed the proposed dry processing method to fabricate fumed silica compact by using composite fibers 26) .Fiber glass composites porously coated with silica nanoparticles were fabricated at the first stage and then compacted into a board by dry pressing.The composites were produced by a particle bonding process without collapsing the fiber glass and nano-scale pores made by the fumed silica.The proposed method had the advantage of preventing contacts between fibers in the compacts due to the existence of coating layer.In addition, since fumed   silica was fixed on the fibers, particle segregation rarely occurred during forming.Therefore, highly uniform dispersion of fibers in the compact could easily be achieved.Fig. 9(a) showed a cross-sectional SEM image of a glass fiber coated with fumed silica found in the processed powder mixture.Fig. 9(b) showed the magnified TEM image of the coated fumed silica layer 25) .This layer was porously formed with a pore size of about 100 nm. Thecomposite fibers were then successfully dr y-pressed to make fiber reinforced fumed silica porous compact.Table 1 showed the thermal conductivity of compact specimens with 80% porosity at 100℃and 400℃ 26) .They were lower than molecular conductivity of still air (0.03 W/mK at 100 ℃, 0.05 W/mK at 400℃) and at the same level as those obtained from silica aerogel 38) and fumed silica compacts 39) .These results indicated that the obtained compacts had nano-scale porous str ucture.The remarkable attribute of composite fibers was achieving ver y low thermal conductivity with a relatively large amount of glass fibers.Mechanical strength of the compacts depends on their apparent density determined by the compressive strength.In this case, fracture strength ranged from 0.4 to 1.6 MPa, corresponding to apparent densities from 400 to 480 kg/m 3 , could be obtained.This made it possible to machine the compacts for various applications.
The present study demonstrated that the dry powder processing method as shown in Fig. 8 provided fibrous fumed silica compacts with mechanical reliability and efficient thermal insulation.The specific feature of this method was to apply effective mechanical processing for making fumed silica fiber composites, instead of conventional mixing techniques.The powder mixture consisting of composite fibers was in a good mixing state and resulted in fumed silica compact with well-dispersed fibers.The existence of fumed silica layer on the glass fiber could prevent direct fiber-fiber contacts in the compact which avoided solid thermal transpor t through the fibers even at high fiber loadings.Thus, as mentioned above, fumed silica compacts with efficient thermal insulation and good mechanical strength were achieved in this study 26) .Furthermore, the thermal conductivity of fumed silica compacts at higher temperature could also be kept at lower value by adding SiC powders as an opacifier.It was found that thermal insulation compacts made of powder mixture consisting of fumed silica: glass fiber: SiC at a mass ratio of 70:10:20 prepared by the particle bonding technique could achieve a thermal conductivity of 0.04 W/mK at 600℃ 40) .By changing the kind of nanoparticle additives, it was expected that thermal conductivity of the compacts could be kept lower at even higher temperatures by particle bonding process in the future.

Development of One-pot Process for Synthesizing Nanoparticles
The first attempt to synthesize materials at low   41,42) .It was found that particle bonding process could combine the elemental powder components to form superconductive MgB2 phase.Large magnesium particles (-330 mesh) and submicron amorphous boron particles (average particle size: 0.8μm) were mechanically processed by particle bonding technique.As a result, boron particles were embedded into the surface of magnesium particle at a depth of about 1μm, and, MgB2 phase was found at this embedded region after annealing at a relatively low temperature under atmospheric pressure of argon.
The second attempt of applying par ticle bonding principle for low temperature reaction was to dope TiO2 nanoparticles with nitrogen without any heat support, which usually requires annealing 43) at 500~600℃ under NH3 flow.The high annealing temperature can lead to undesirable grain-size growth of TiO2 nanoparticles.To overcome the problem, a new apparatus applying mechanical particle bonding technique with electric discharge was developed for this application 1) .NH3 (10%)/Ar plasma was generated at different gas pressures in a mechanical particle bonding processing chamber, where an anatase TiO2 powder with a BET surface area of 300m 2 /g (equivalent to about 7 nm in diameter) was uniformly irradiated.When generated plasma irradiated at 300 Pa, the TiO2 powder had a specific surface area of 283m 2 /g, and, noticeable absorption in visible light range was obser ved.In addition, the powder showed an improvement of the photo-catalytic oxidation activity of CH3CHO under visible light.These results indicated that the presented plasma processing was capable of modifying TiO2 nano-powder to improve its photoreactivity without much reduction in specific surface area, which typically occurred when powder modification was carried out with annealing treatment 34) .Using the features of particle bonding technique, new one-pot processing to synthesize nanoparticles without any heat support was developed [28][29][30][31][32][33] . Frst, a rapid synthesis of perovskite type lanthanum manganite starting from a mixture of industrial grade powders was demonstrated.A traditional route to synthesize LaMnO3+ δ was through the solid state reaction of constituent oxide powders at 1300℃ Fig. 10 showed conventional production process of LaMnO3 materials from La2O3 and Mn3O4 powders.This process needs many manufacturing steps and the thermal reaction involved leads to particle size enlargement and limits the degree of chemical homogeneity.
On the other hand, a rapid mechano-chemical synthesis proposed by the authors was shown in Fig. 11.In this method, synthesis was achieved by one-pot processing of a mixture of industrial grade La2O3 and Mn3O4.The one-pot processing was based on particle bonding technology, which applied mechanical forces such as compression and shear stresses repeatedly on the powder mixture without using media balls.Fig. 12 showed the phase evolution as a function of processing time examined by XRD with the powder mixture processed under the presence of water vapor 28) .In the experiment, the humid air (RH 70% at 25℃) was injected into the chamber before the mechanical processing.At the beginning, only the peaks corresponding to La2O3 and Mn3O4 were observed from the powder mixture.These peak intensities decreased drastically as processing time increased and almost disappeared after 15 min.in processing.Concurrent with the intensity decreases, the peaks related to La(OH)3 appeared.Also, the peaks corresponding to LaMnO3+ δ started appearing after 15 min in processing and their intensities increased as processing time increased.After 30 min, the peaks completely transformed to single phase LaMnO3+ δ with disappearance of the La(OH)3.The crystal structure of the synthesized powder was further identified to be LaMnO3+ δ by orthorhombic symmetry (JCPDS card 35-1353).Fig. 13 showed the change of specific surface area (SSA) of the powder mixture with an increase in processing time 28) .The starting SSA of the powder mixture was 7.0m 2 /g.It increased quickly until 10 min.in processing and then kept almost constant.After 30 min in processing, it became 9.0m 2 /g.However, when processed in the dry air, the SSA of powder mixture increased only slightly; and, it was 7.5 m 2 /g after 30 min in processing.In this case, the XRD pattern of the powder mixture was almost identical to that of the starting state.Adding to the synthesis of LaMnO3+ δ , strontium doped materials were also synthesized by the one-pot processing 29,30) .Furthermore, BaTiO3 was also rapidly synthesized by the one-pot processing shown in Fig. 11.In this case, industrial grade of TiO2 and BaCO3 powders were used 32,33) The one-pot reaction mechanism has not yet been fully understood.However, as mentioned in Figs. 12 and 13, the presence of a small amount of water played an important role on the reaction of La2O3 and Mn3O4.Further analysis would lead to the establishment of one-pot reaction mechanism to synthesize nanoparticles in the future.

The Development of Novel Recycling Process for GFRP
By making use of particle bonding principle between different kinds of materials, disassembling them is also possible and it can be applied to recycle waste composite materials.For example, glass fiber reinforced plastics (GFRP) is a typical composite material having the advantages of lightweight, high

Weighing of La 2 O 3 and Mn 3 O 4
One-pot dry processing LaMnO 3 nano powder Fig. 11 One-pot processing to synthesize LaMnO 3 nano powder     strength and high weather resistance.Therefore, it has been used in various applications including boats, bath tubs, and building materials.Its production volume reached 460,000 tons in Japan in 1996, but decreased gradually since then.However, the volume of waste GFRP has increased every year.So far, almost all of the waste GFRP has been incinerated or disposed in landfill.Only 1-2 % of the waste GFRP is recycled as cement raw material or additives for concrete.Japan Reinforced Plastics Society started producing cement recycled from GFRP in 2002.The incineration of GFRP has problems of low calorific values on burning, and its residue needs to be disposed.In order to recycle the waste GFRP, some advanced chemical solvents and supercritical fluid 44) have been studied.However, they have not been used in practice, because the chemical approach requires high temperature and high pressure operating conditions, which are not only costly but also generate byproducts.In addition, the recycled materials do not have similar quality to that of the starting materials.GFRP usually contains 40-50% of calcium carbonate filler and 20-30% of glass fibers.These materials must be recycled through simple and low energy process for profits.Therefore, we aimed to develop new recycling method to make advanced materials from the waste GFRP.Fig. 14 showed the concept of an innovative recycling process of GFRP proposed by the authors 35) .It consisted of two unit processes based on particle bonding principle.First, GFRP was separated into glass fibers and matrix resins, and then, the surface of separated glass fibers was coated by low cost nanoparticles.The coated composite glass fibers would be compacted to make porous materials as shown in Fig. 8. High functional materials having the properties of very low thermal conductivity, light weight, and easy machining are expected to obtain by applying the new process shown in Fig. 14.The waste GFRP chip crushed down to about 1 cm was processed by an attrition-type mill, which applied similar mechanical principle to that of particle bonding process.When strong shear stress was applied to the chip layers for surface grinding, glass fibers began to separate from matrix resins on the chip surfaces.As a result, all glass fibers were effectively separated from other matrix components.The SEM photographs of the processed waste GFRP were shown in Fig. 15 35) .It was obvious that the glass fibers separated from matrix resins had their own shape by using this method (Fig. 15(a)).The length of the glass fibers ranged from about 100μm to over 1 mm.On the other hand, the glass fibers were destroyed when applying a vibration ball mill (Fig. 15(b)).In the vibration ball milling, mainly impact and compressive forces were applied to the material, which reduced glass fibers to particle form.These results showed that the proposed method using particle bonding principle was very effective in Fig. 14 Innovative recycling process for GFRP proposed by authors selective separation of glass fibers from other matrix components.Fur ther experiments are now being carried out to make nanoparticle coated composite fibers from the recycled glass fibers for new material development.Fig. 16 showed the concept of smart recycling of composite materials by using the particle bonding and disassembling between different kinds of materials.From the conventional recycling standpoint of view, each element of waste composite material must be returned back to its original state for repeated usage.However, it needs high recycling costs and the obtained element has lower quality than the virgin material.As a result, the recycling process cannot be practically used.On the contrar y, the proposed recycling concept does not aim to obtain each original element, but develops further advanced materials using disassembled blocks of the waste composite materials.In this case, how to apply bonding and disassembling the waste materials is the very key issue.As shown in Fig. 16, recycling waste back to its intermediate structure and then assembling it with another material to make further advanced materials would be more energy efficient than reclaiming the original elements.This concept will be a basis for the next generation of recycling system for advanced materials.

Conclusions
In this paper, particle bonding process was explained as a typical example of smart powder processing.This process enabled us to develop new composite materials.Based on its principle, the electrodes for SOFC and the high performance thermal insulator were developed.This process is also applicable  to other applications.In this paper, two examples were explained.The first one was to develop one-pot processing for nanoparticle synthesis from elemental powders without any heat support, and, the second was to develop new recycling process for waste composite materials and turn them into other advanced materials.It is believed that dr y particle bonding technology can open the doors for various kinds of smart powder processing applications in the future.

Fig. 1
Fig.1 Unique microstructure created by particle bonding process

Fig. 1
Fig. 1 Unique microstructure created by particle bonding process.

Fig. 6
Fig.6 Cross-sectional SEM images of the anode (a) before reduction, (b) after reduction, (c) after the long-term stability test at 700 for 920h

Fig. 8 Fig. 8
Fig.8 Proposed dry processing method to fabricate fibrous fumed silica compacts: (a)mixing of raw materials;(b)particle bonding to coat glass fiber with fumed silica; (c)dry pressing of the composites from (b) to produce bulk bodyFig.8 Proposed dry processing method to fabricate fibrous fumed silica compacts: (a)mixing of raw materials;(b)particle bonding to coat glass fiber with fumed silica; (c)dry pressing of the composites from (b) to produce bulk body.

Fig. 12
Fig.12XRD patterns of the powder mixture processed under RH 70%.

Fig. 12 XRD
Fig.12XRD patterns of the powder mixture processed under RH 70%

Fig. 13
Fig.13 Change of the specific surface area of powder mixture as a function of mechanical processing time.

Fig. 13
Fig.13 Change of the specific surface area of powder mixture as a function of mechanical processing time 0 5 10 15 20 25 30 Milling time / min 0

Fig. 14
Fig.14 Innovative recycling process for GFRP proposed by authors.

Fig. 15
Fig. 15 SEM photographs of processed powders by using the proposed method (a) and vibration ball mill (b).

Fig. 15 Fig. 16 Fig. 16
Fig.15 SEM photographs of processed powders by using the proposed method (a) and vibration ball mill (b) .Yokoyama is the Director of Powder Technology Research Institute of Hosokawa Micron Corporation (HMC), a global supplier of systems and materials related with powder and particle science and engineering.He graduated from Kyoto University in chemical engineering and received M.S. there in 1975.Then he spent six years in Europe to study powder technology at Karlsruhe University and to work as an engineer for Hosokawa Europe Ltd. in Germany and England.After another six-year work in the engineering and R&D divisions of HMC in Osaka specializing in powder technology, he worked at Nagoya University and obtained Ph.D. on the subject of ultrafine wet grinding.From 1992 to 2003, he was a general manager of Hosokawa Micromeritics Laboratory for R&D mainly in the field of powder and nanoparticle technology.His major interests are particle design and processing to produce advanced functional materials by mechanical composing methods as well as fine grinding and particle characterization.Dr. C. C. Huang Dr. C. C. Huang is the Director of Research and Development, Nanoparticle Technology and Micron Products at Hosokawa Micron Powder Systems, which is an operating unit of Hosokawa Micron International Inc., a global supplier of systems and equipment related to material sciences and engineering.He holds an M.S. degree in engineering from Illinois Institute of Technology and a Ph.D. degree in chemical engineering from West Virginia University.He has many years experiences in industrial R&D, as well as academia, in the field of powder technology and science.Dr. Huang specializes in powder and nanoparticle processing, powder characterization, powder granulation, and fluidization.He has published over 30 articles and 8 patents, chaired several meetings, and continues to be an active member in a number of scientific and engineering societies.