Synthesis of Nanoparticles in High Temperature Ceramic
Microreactors:Design,Fabrication and Testing
Kartavya Jain,Carl Wu,Sundar V.Atre,*Goran Jovanovic,Vinod Narayanan,and
Shoichi Kimura
Oregon Nanoscience and Microtechnologies Institute,Oregon State University,Corvallis,Oregon
Vincent Sprenkle and Nathan Canfield
Pacific Northwest National Laboratory,Richland,Washington
Sukumar Roy
Bharat Heavy Electricals Ltd.,Bangalore,India
Microreactors as a novel concept in chemical technology enable the introduction of new reaction procedures in chemistry, pharmaceutical industry,and molecular biology.These miniaturized reaction systems offer many exceptional technical advantages for a large number of applications.One major appli
cation is in the bulk synthesis of nanoparticles.Despite the availability of a plethora of nanoparticle synthesis processes,there exist many difficulties in controlling the shape,size,and purity of nanoparticles in large quantities in a safe and cost-effective manner.These difficulties have been the principal factors adversely limiting the applications of ceramic nanoparticles.Recent experiments have shown that to study the process of growth and formation of nanoparticles,a reactor having much smaller dimensions,namely a microreactor is more appropriate. These studies have also shown that a microchannel reactor provides control over the mean residence time and hence over the nanoparticle size and shape.This paper deals with the design,fabrication,and testing issues related to a high temperature, ceramic microreactor by investigating the use of reactive gas streams in arrays of microchannel reactors.These innovations offer
the potential to overcome the barriers associated with synthesis of ceramic nanoparticles in large quantities.
Introduction
Microscale process engineering is the science of con-ducting chemical or physical processes(unit operations) inside small volume.These processes are usually carried out in continuousflow mode,as o
pposed to batch pro-duction,allowing a throughput high enough to make microscale process engineering a tool for chemical pro-duction.It involves the use of microsystems that are now available in many devices for commercial applica-tions including micromixers and microreactors as alter-native to batch production in pharmaceutical andfine chemical industry,lab-on-chip devices,microsensors, advanced rapid throughput chemical and catalyst screen-ing tools,distributed or portable power and chemical Int.J.Appl.Ceram.Technol.,6[3]410–419(2009)
reaction mass
DOI:10.1111/j.1744-7402.2008.02285.x Ceramic Product Development and Commercialization
*Sundar.Atre@oregonstate.edu
r2008The American Ceramic Society
production,distributed heating and cooling,and even space applications.1
The subfield of microscale process engineering that deals with chemical reactions,carried out in microstruc-tured reactors or microreactors,is also known as micro-reaction technology.Microreaction te
chnology,with its unprecedented heat and mass transfer advantages as well as uniform residence time andflow pattern,is one of the few technologies with the potential to develop efficient, environmentally benign,and compact processes.It is a highly interdisciplinary research area,where chemistry, physics,and engineering merge together.It involves the use of microreactors that allow reactions to be performed at the micrometer scale.Microreactors offer many advantages for the performance of heat and mass-transfer-limited reactions.Large gradients in concentra-tion and temperature can be achieved by shrinking the characteristic dimensions of a microreactor down to the microscale.This is especially advantageous in the case of highly exothermal reactions as well as in the case of mass-transport-limited processes.The use of micro-reactors for in situ and on-demand production is gaining increasing importance as thefield of microreaction tech-nology emerges from the stage of being regarded as a theoretical concept to a technology with significant industrial applications.2While the dimensions of the individual channels are small,a microreactor can contain many thousands of such channels,and its overall size can be on the scale of meters.However,the objective of microreaction technology is not primarily to miniaturize production plants,but to increase yields and selectivities of reactions,thus reducing the cost of production.
An important application of microreactors is for the synthesis of nanoparticles.3For example,nanoscale
ceramic powders can significantly help in the miniatur-ization of current products and can be used for various high performance applications due to their excellent mechanical,thermal,dielectric and corrosion proper-ties.4The performance of these nanoscale ceramic pow-ders is strongly affected by particle properties,such as the particle size,the distributions of the size,and the shape.Conventionally large-scale batch reactors have been used for the synthesis of ceramic nanoparticles.5 However,the particle size distribution of nanoparticles synthesized in batch reactors often does not satisfy the requirements for many applications.Prior work has shown that the shape,size,and yield of nanoparticles are strongly influenced by the mean residence time and temperature required to produce the nanoparticles.6A microreactor provides control over the mean residence time,mixing and reaction temperature and hence over the nanoparticle size and shape.The most important advantages of a microreactor are rapid reaction times and scalable throughput of highly pure ceramic nano-particles,which is due to several microreactor features such as,high temperature gradient,short and control-lable residence time,small reactor size,and high heat transfer rates.
The focus of this paper is to design and fabricate a microreactor with multiple microchannel arrays and investigate the use of reactive gas streams of silicon oxide (SiO)and ammonia(NH3)to synthesize silicon nitride (Si3N4)nanoparticles.This work also demonstrates the feasibility and effectiveness of mu
ltiple microchan-nel reactors in synthesis of nanoparticles as compared with single microchannel reactors and the use of porous alumina as a prospective ceramic material for fabricating the microreactor.It is anticipated that the principles demonstrated in this study can help overcome the bar-riers associated with the production and safe handling of nanoparticles in large quantities.
Microscale Reactor vs.Macroscale Reactor It will be possible to construct a plant consisting of microreactors that is small enough to be moved from place to place in less time with low capital cost.7These portable plants can be used for on-site production of hazardous chemicals,which currently incur considerable risk to human beings and the environment.The hazard potential of strongly exothermic or explosive reactions can also be drastically reduced and higher safety can also be achieved with very less toxic substances at higher operating pressures.8Another advantage of microreac-tors is the high surface area to volume ratio due to the microchannel dimensions.The small size of these microchannels makes it easier to control the reaction parameters such as pressure,temperature,residence time,andflow rate,as compared with the large batch reactor,thus influencing reaction rate and selectivity.9 The difference between the macroscale or batch reactor and the microreactor is not just the diameter of channels but also the length of reaction zone.The mean residence time of the reactants in the reaction zone of the microreactor can be at least two orders of m
agnitude smaller as compared with the macroscale reactor.Addi-tionally,scale-up only involves adding more microchan-
nels in the reactor.Owing to all of these advantages,a microreactor is a useful tool that allows for a safe and fast transfer from a microscale to pilot or production scale. Material Selection for the Microreactor
Microreactors represent a scaled-down method to perform chemical reactions in a just-in-time fashion. Theyfind applications in a variety of sectors such as au-tomotive,aerospace,electrical,mechanical,and chemical industries.Therefore,microreactors can be generally made out of ceramics(silicon carbide,silicon nitride,alumina, titania),glass,quartz,silicon,and metals(stainless steel), as well as polymers(polydimethylsiloxane,parylene, polyimide,PMMA).1Each material has its own advanta-ges and disadvantages;however ceramics have the potential to add beneficial properties to microreactors for applica-tions involving high temperature reactions thanks to their key properties such as high chemical resistance,10high
temperature structural stability,and wear resistance.11 Based on the design and fabrication considerati
ons and the material requirements for the microreactor,the microreactor was manufactured using ceramic alumina.
Alumina is one of the most cost effective and widely used materials in the family of engineering ceramics.It possesses strong ionic inter-atomic bonding giving rise to its desirable material properties such as high hardness (11–14GPa),good thermal conductivity(27W/m1C), high temperature stability(17501C),low thermal expan-sion coefficient(8.4Â10À6/1C),and excellent dielectric thus making it the material of choice for a wide range of high temperature applications like microreactors. Basis of the Work
In this work,the microreactor will be used to synthesize silicon nitride nanoparticles.Silicon nitride is chosen as the basis of this work because of its significance in high-temperature structural applications.It exhibits unique combination of thermal,mechanical,dielectric, and corrosion properties such as high temperature capability(10001C),high hardness(14–16GPa),good thermal conductivity(29–30W/m.K),low thermal ex-pansion coefficient(3.3Â10À6/1C),good oxidation,and wear resistance,under severe environment.12
Prior work in a macroscale tubularflow reactor has identified that nanoscale particle formation is limited
by diffusion between reactant and gas streams.6 The effects of mean residence time as well as the reac-tion temperature on the growth of nanosized particles were investigated.
Figure1shows that the average mean particle size of Si3N4nanoparticles increases with an increase in the res-idence time of reactants in the reaction zone.The growth process may be divided into two stages:an induction stage,and a cluster growth stage.During the induction stage,the homogenous vapor-phase nucleation takes place as sequence of additions of single molecules for developing an embryo.This process leads to the forma-tion of nucleus with a radius equal to the critical nucleus radius,above which it becomes stable.Based on the ther-modynamic conditions,the critical radius for a silicon nitride nucleus to be stable and grow is0.3–0.6nm.The minimum residence time to produce particles of critical size turned out to be about6ms(refer Fig.1).This is an indication that the mean residence time achieved in the tubularflow reactor6was much larger than the residence time needed for the formation of stable clusters.So,to study the process of nanoparticle formation and growth, a reactor having smaller dimensions is more appropriate, namely,as small as those of microchannel reactors.
When a microreactor is used,the mean residence time can be easily controlled upwards of0.1ms,and the average particle size can be reduced to those close to critical size.The yield of nano-sized silicon n
itride powder in the tubular reactor was found to be at most 43%because of the formation of whiskers and crystals. However,the use of microreactor is expected to increase and control the residence times and yield
scalable Fig.1.Growth of average Si3N4particle size with residence time of reactant gas mixture.
412International Journal of Applied Ceramic Technology—Jain,et al.Vol.6,No.3,2009
throughputs of high purity nanoparticles.In addition to requiring small quantities of reagent,the microre
actor having sub-millimeter reaction channels will allow for the precise control of reaction variables,such as reagent mixing,flow rates,reaction time,and heat and mass transfer which is ideal for integration with a post pro-cessing system.
Design
A computer model of the microreactor and its parts was generated using commercially available 3-D CAD software package,SolidWorks.This computer drawing is at least a two-dimensional drawing and more typically a three-dimensional drawing of the different parts and fully assembled micro-reactor.Figure 2shows the 3D model of a microreactor.As seen in the Fig.2,the microreactor consists of three parts:the top plate,the bottom plate,and the extruded body.The circular array of holes in all these parts acts as microchannels through which the reactants are carried into the hot reaction zone of the microreactor,where they react to obtain the final product.These computer-generated parts of the micro-reactor were then patterned and bonded together using different microfabrication tech-niques to obtain the final microreactor.
Table I shows the dimensions of all these three parts of the ceramic microreactor.This microreactor design is expected to produce silicon nitride nanoparticles with a small mean diameter and narrow size
distribution as compared with the batch reactor,because it provides a uniform reaction field in which the reaction conditions can be precisely controlled.Owing to its portability and compactness,this microreactor allows for in-situ synthesis of silicon nitride nanoparticles,thus avoiding several
safety and health concern issues related with handling of these nanoparticles.Another advantage is the low patterning cost involved,because the number of parts to pattern is reduced to just three as compared with nor-mally used 10–100parts or shims in previously reported microreactor designs and fabrication methods.13Fabrication
Fabrication,through its role in microelectronics and optoelectronics,is an indispensable contributor to microsystems technology.It is ubiquitous in the fabri-cation of sensors,microreactors,combinatorial arrays,14microelectromechanical systems (MEMS),microanalyt-ical systems,15and micro-optical systems.Fabrication of microreactors involves different patterning and bonding techniques.The fabrication of functional models of a microreactor is very important.Processing effects that play a minor role in the macroscale cannot be neglected at the microscale regime as they may have a much larger impact due to the increased surface-to volume ratio.Ceramic microreactors like almost all ceramic compo-nents are formed in the green,unfired state by consol-idating the ceramic powder with the help of binder additives into the desired shape.Following are the steps involved in fabrication of the ceramic
microreactor used in this study.
Patterning
Conventionally,ceramic microreactors have not been used due to the costs associated with their design and development and because methods for the produc-tion of larger series have not yet been fully established.10
During product development,high costs can be incurred for the fabrication of models and prototypes for design optimization.However,to speed up this process and to reduce the costs involved,different patterning
techniques
Fig.2.3D model and sectional view of the microreactor created using SolidWorks.
Table I.Dimensions of the Top Plate,the Bottom Plate,and the Extruded Body of the Microreactor
Top plate (mm)Bottom plate (mm)Extruded body (mm)
Diameter 172727Length
0.50.520Diameter of truth holes
1
3
3
such as laser machining,micropunching,micropowder injection molding,extrusion,etching,photolithography,electro discharge machining (EDM),are nowadays available for fabricating ceramic microreactors.16,17
In the microreactor used in this study,the top plate and the bottom plate were fabricated by laser machining green tape alumina at PNNL,Richland,WA.The green tape alumina used for these plates was Alcoa A-16Super Grind (Alcoa,Leetsdale,PA)with Rohm &Haas Dura-max B-1000aqueous binder (Rohm &Haas,Philadel-phia,PA).The laser used for fabricating the top and bottom plates was a CO 2laser with 35W beam power.Figure 3shows the experimental set-up of the laser ma-chining process.One repetition cut along with 28%beam power and 20%speed were used as the cutting parameters.Both these plates were then sintered with the following schedule:0.51C/min ramp to 4001C with a 1h hold followed by 3.01C/min ramp to 16001C and a 1h hold.Figures 4and 5show the top and bottom plates fabricated using this procedure.
The extruded body of the microreactor was fabri-cated using alumina at BHEL,New Delhi,India.Figure 6shows the schematic representation of the extrusion process and the porous extruded body (with 50–60%porosity)obtained using this process.The reason behind using highly porous alumina is to allow the ammonia gas to diffuse into the microchannels through the pores of extruded body and react with the silicon oxide stream.
All these parts were characterized using scanning electron microscopy (SEM).The SEM image of ex-truded body at Â5000magnification (refer Fig.7)confirms the presence of pores in the extruded body.
Bonding and Post Bonding
Following patterning,the next step in fabricating the ceramic microreactor was the bonding of different parts together to obtain the final microreactor assembly.Many difficulties have been previously observed in bonding or joining of the ceramics due to their high melting temperature.Further,traditional methods,such as riveting,bolting,and threading,are not suitable to be applied in the joining of ceramic materials for that the stress concentration is an inevitable problem.At present,the main techniques used for bonding ceramics are diffusion bonding,metal brazing,diffusion and friction welding,adhesive bonding,and solvent welding.18
The
Fig.3.Experimental set-up for laser
machining.Fig.4.Top plate fabricated using laser
machining.
Fig.5.Bottom plate fabricated using laser machining.
414International Journal of Applied Ceramic Technology—Jain,et al.Vol.6,No.3,2009

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