SURFACE ENGINEERING SCIENCE AND

TECHNOLOGY II

Edited by: Ashok Kumar

Yip-Wah Chung John J . Moore

Gary L. Doll Kyoshi Yatsui

D.S. Misra

TABLE OF CONTENTS

Preface ix

Nanomaterials

Modifying the Surface of Nanoparticles by Coating 3 D. V. Szabó, S. Schlabach, B. Xu and D. Vollath

Formation of Photonic Nanocomposites by Surface Engineering Over Inorganic Nanoparticles 15

N. Kambe

Micro To Nano-Small Research For Fuels and Combustion 25 CE. Bunker, JR. Gord, T.R. Meyer, M.S. Brown, V.R. Katta, D.A. Zweifel, B.A. Harruffand Y-P. Sun

Formation of Gold Nanowires with MgO Surfaces 35 A. Ueda, R.R. Mu, V.C. Saunders, T.C. Livingston, M.H. Wu and D.O. Henderson

Dense Deposition of Nanocomposites By a Compact YAG Laser 43 M. Senna and K. Hamada

Aqueous Chemical Growth of Advanced Nanostructured Metal Oxides Thin Films 51

L. Vayssieres

Purification of Aluminum Nitride Nanosized Powder Synthesized by Pulsed Wire Discharge 61

C. Cho, Y Kinemuchi, T. Suzuki, H. Suematsu, W. Jiang and K. Yatsui

Engineered Nanomaterial Hard Coatings on Metal and Polymer Substrates 69

A. Singhal and G. Skandan

Synthesis of Nickel Ferrite Nanosized Powders by Pulsed Wire Discharge 79 K. Ishizaka, Y Kinemuchi, T. Suzuki, H. Suematsu, W. Jiang and K. Yatsui

Reinforced Epoxies Using Carbon Nanotubes 89 R.A. Bley, D. Beascoechea and R. Niedner

v

Tunable Self-Assembly of Carbon Nanotubes on Silica Surface 99 Z.J. Zhang, B.Q. Wei and P.M. Ajayan

Issues in Surface Engineering

Laser Surface Modification of Electronic Properties in Wide Band Gap Materials 113

I.A. Salama, N.R. Quick and A. Kar

Preparation of TiFe Thin Films of Hydrogen Absorbing Alloy by Intense Pulsed Ion-Beam Evaporation 125

T. Saikusa, T. Suzuki, H. Suematu, W. Jiang and K. Yatsui

Surface Modification of Austenitic Stainless Steels 133 H. Ocken and R. Asay

Synthesis of Functional Materials using Pulsed Power Technology 143 K. Yatsui, W. Jiang, H. Suematsu, T. Suzuki, Y Kinemuchi and S.C. Yang

Surface Modification of P/M TiC Reinforced Iron Matrix Composites 153 O.N. Dogan, D.E. Alman, J.A. Hawk and P. Danielson

Functionally Graded Materials Produced with High Power Lasers 163 J.T. De Hosson and Y Pei

Advances in Coating Technology

Nucleation and Growth of Cubic Boron Nitride Under Different Substrate Bias 17 9

Q. Li, Y. Lifshitz, L.D. MarL·, I. Bello andS.T. Lee

Role of Precursors on the Formation of CNX Deposited by PE-HF-CVD 189 D. Dumitriu and A. Karami

Optimization of Thermoelectric Properties in Boron Carbide Thin Films Prepared by Ion-Beam Evaporation 199

H. Suematsu, I. Ruiz, K. Kobayashi, M. Takeda, D. Shimbo, T. Suzuki, W.Jiang and K. Yatsui

Developing a MoSi2+ SiC Oxidation Resistance Coating for MO, a Prototype Refractory Metal 207

E.C. Hixson, GGW. Mustoe, J.J. Moore, H.J. Kleebe, D.L. Olson andC. Suryanarayana

VI

The Influence of Particle Energy Distributions on the Microstructure and Properties of Reactively Sputtered Titanium Oxide and Titanium Nitride Thin Films 217

C. Mur atore, J.J. Moore andJ.A. Rees

Effects of Bias-Enhanced Nucleation and Growth of Diamond Films on Copper and Transition-Metal Alloy Substrates 227

A. Kumar and A.K. Sikder

Epitaxial Nucleation of Diamond Polytypes on Silicon by Direct Ion Beam Deposition 239

Q. Li, X.F Duan, Y Lifshitz, F.Y. Meng, N.G Shang, I. Bello andS.T. Lee

Characterization of Materials

Development of Cermet Thin Films Coatings 369 B. Mishra, J. Zhou and F Kustas

Relative Nanomechanical and Raman Studies of Diamond-Like Carbon Thin Films 251

J. Goldsmith, E. Suiter, J. Moore, B. Mishra and M. Crowder

Fabrication and Characterization of Mechanical Properties of Ultra-Thin Silicon Microcantilevers 261

P.B. Zantye, A.K. Sikder, R.K Sharma and A. Kumar

Influence of Surface Roughness on Coercivity and Magnetic Interactions in CoCrX (X=Pt,Pd,Ta,B) Thin Film Media 273

M. Tarahci and S. Guruswamy

Mechanical Properties of Microstructural Evaluation of Chromium Containing Hard Coatings on Fe-Mn-Al Alloy 283

J-W. Lee andJ.-G. Duh

Curvature Method as a Tool to Evaluate Shape Memory Effects for TiNiCu Thin Films 293

Y. Fu, H. Du and S. Zhang

Observation of Surface-Related Phenomena During In-situ Nanoindentation in a Transmission Electron Microscope (TEM) 305

A.M. Minor, E.T. Lilleodden, E.A. Stach and J.W. Morris

Comparison in Characteristics of Electroless Deposited and Magnetron Sputtered Ni-P-W Coatings 315

F-B. Wu, Y.-Y. Tsai andJ.-G. Duh

VII

Oxidation/Corrosion/Nitridation/Diffusion

Factors Controlling In-Plane Cracking of Thermal Barrier Coatings 327 Z Zhang, J. Kameda, A.H. Swanson, S. Sakurai and M. Sato

The Corrosion Behavior of PVD-Grown WC-(Tij χΑ1χ) Ν Films Multilayers in 3.5%NaCl Solution .?.....*. 337

S.H Ahn, J.H. Yoo, Y.S. Choi, J.G. Kim andJ.G. Han

Ferritic Nitrocarburising in Fludised Bed 347 D.M. Fabijanic, P.D. Hodgson and G.L. Kelly

Surface Phenomena in Diffusion Limited Capillary Flow in a Reactive Porous Film 357

R. Asthana

Author Index 379

Subject Index 381

VIII

PREFACE

The symposium titled "Surface Engineering: Science and Technology IF was organized by Ashok Kumar, Y. W. Chung, John J. Moore, Gary L. Doll, D. S. Misra and Kiyoshi Yatsui, and held in Seattle, Washington, during the TMS annual meeting, February 17-21,2002. This volume consists of invited and contributed papers from national and internationals researchers represent­ing universities, federal laboratories and industries. Surface engineering covers a wide range of surface modification processes that are directed at the creation of tailor-made surface characteris­tic of materials. The objective of this symposium is to provide a multi-disciplinary discussion on surface related phenomena by which materials performance may be enhanced through engineered interface and surface modification technologies.

The objective of this symposium is to provide a forum for multi-disciplinary discussions on phe­nomena where materials-performance may be enhanced through engineered surfaces and inter­faces. The materials science of hard and wear-resistant coatings, including nanostructured coat­ings, is a scientifically challenging problem with immediate technological applications in ma­chine and machining tools, magnetic recording and others manufacturing industries. This sympo­sium has served as a forum to review key aspects, discuss ongoing current research and speculate on future trends relating to the science, technology and applications of coatings, surface treat­ments and surface analysis for the enhanced performance of engineered components and devices. We wish to thank all the authors, particularly the invited speakers and referees, for their contribu­tions to the success of this symposium. We also acknowledge the University of South Alabama, the Northwestern University/Materials Research Center, Colorado School of Mines, The Timken Company, 1.1. T. Bombay and Nagaoka University of Technology for the resources the organizers used to organize the symposium and complete this book. Special thanks are also extended to Mr. Steve Kendall from TMS for his assistance in editing this book.

Dr. Ashok Kumar Dept. of Mechanical Engr. & Center for Microelectronics Research University of South Florida Tampa, FL 33620

Dr. Yip-Wah Chung Dept. of Materials Sci.& Engr. Northwestern University Evanston, IL 60208 Tel: (847)491-3112, Fax: (847)491-7820

Dr. John J. Moore Advanced Coating & Surface Engr. Lab Colorado School of Mines Golden, CO 80401-1887

Dr. Gary L. Doll The Timken Company P.O. Box 6930 Canton, OH 44706

Dr. Kiyoshi Yatsui Extreme Energy Density Research Institute Nagaoka University of Technology Nagaoka, Niigata 940-2188, Japan

Dr. D. S. Misra Department of Physics 1.1. T. Bombay Powai, Mumbai 400076, India

IX

NANOMATERIALS

Modifying the Surface of Nanoparticles by Coating

D. V. Szabó, S. Schlabach, B. Xu, D. Vollath

Forschungszentrum Karlsruhe GmbH, Institut für Materialforschung III, P.O. Box 3640

D-76021 Karlsruhe, Germany

Abstract

The Karlsruhe Microwave Plasma Process is well suited to synthesize non-agglomerated ox­ides, nitrides, sulfides, or selenides with narrow particle size distribution in one reaction step. In many instances, applications of these materials require a modification of the surface. The rea­sons may either be the reduction of the particle interaction using the coating as distance holder or an alteration of the chemical properties of the surface. It is a special feature of the micro­wave plasma process that the particles leave the reaction zone with electric charges of equal sign. Therefore, the particles are repelling each other. This makes it possible to coat the parti­cles individually in a second reaction step, leading to a large variety of materials with combined properties. Depending on the application, this coating may consist either of a polymer or a sec­ond ceramic. This coating - in the simplest case - may act as distance holder in superparamag-netic composites, where the coating reduces interaction in-between the particles. Therefore, magnetic materials made from coated nanoparticles are free of hysteresis. The production of noble metal decorated nanoparticles is possible. These materials are interesting because of then-catalytic properties. Other typical examples of application of coated nanoparticles exploit the improvement of the dispersion behavior in water.

Surface Engineering: Science & Technology of Interfaces II Edited by A. Kumar, Y-W. Chung,

J.J. Moore, G.L. Doll, K. Yasui and D.S. Misra TMS (The Minerals, Metals & Materials Society), 2002

3

Introduction

Generally, nanomaterials are defined as materials with particle sizes below 100 nm [1]. A more stringent definition describes nanomaterials as ones exhibiting special properties depending on their small grain size. In many cases, the latter definition restricts nanomaterials to grain sizes below 10 -15 nm.

Often these properties are properties of single, isolated particles. Interacting nanoparticles, as it is in consolidated parts for technical application, behave like large particles. This may destroy the special, size dependent properties. Additionally, it is nearly impossible to sinter such com­pacts without significant grain growth. This leads to the loss of the special properties, too. The solution of these problems is to reduce the interaction between nanoparticles by keeping them "on distance" using coated nanoparticles. These are nanoparticles with sizes in the range from 5 to 20 nm coated with a second phase. Usually, the thickness of the coating is selected in the range of a few nanometers. Coating nanoparticles results in the following improvements:

• The active kernels are kept on distance minimizing the interaction of the particles

• During sintering, the grain growth of the kernels is thwarted. Whereas grain growth of the coating is not avoided.

• Adjustment of the surface chemistry of the particles according the needs for application. The appropriate coating has to be chosen separately for each application.

• The mixing of two phases is homogeneous on a nanometer scale.

• The combination of different physical or physical properties in the core and chemical ones on the surface of one particle is possible.

There are two strategies possible to select the coating of nanoparticles:

• The first strategy is to coat ceramic nanoparticles with a ceramic layer. The necessary pre­requisite is that kernel and coating do not have a mutual solubility or phase formation in the temperature regime of synthesis, sintering, and application.

• The second strategy is to coat ceramic nanoparticles with a polymer. This type of coating is possible provided the polymer does not dissociate catalytically at the surface of the parti­cle. The main application of such particles is in applications at ambient temperature.

Metals are not wetting ceramic nanoparticles. Therefore, an approach to coat ceramic nanopar­ticles with metals results in a decoration with metal clusters.

To produce such particles a very versatile synthesis, the Karlsruhe Microwave Plasma Process, has been developed [2,3]. This process is capable to produce non-agglomerated oxides, nitrides, sulfides, or selenides with narrow particle size distribution in one reaction step. Furthermore, the synthesis of ceramic or polymer coated nanoparticles and metal decorated nanoparticles is possible cascading this process, leading to a large variety of materials with combined proper­ties. This paper will explain in detail some examples of these various possibilities together with perspectives for applications.

4

The Karlsruhe Microwave Plasma Process

Fundamentals of the Microwave Plasma Process

The microwave plasma process uses gas phase reactions. It draws advantages from partly ioni-zation or dissociation of the reactive molecules in plasma. As consequence, thermodynamically possible but under normal conditions kinetically thwarted reactions can be performed at re­duced temperature. In an oscillating electrical field with the frequency/ the energy transfer E to charged particles with a mass m is inversely proportional to the mass and the squared fre­quency:

As the mass of electrons is much lower than the mass of ions or radicals, significantly more energy is transferred to electrons than to the latter ones. Thus, the "temperature" of the free electrons is much higher than the "temperature" of the ions [4]. The higher the frequency, the lower is the energy transfer. Thus, higher frequencies promote lower temperatures. Addition­ally, taking the collision frequency z with uncharged molecules and atoms in the plasma into account leads to the modified relation:

h m f2+z2 (2)

As the collision frequency increases with increasing gas pressure, the energy transfer is a func­tion of the gas pressure. The energy transfer increases with the gas pressure for z < / exhibits a maximum a t / = z, and decreases for z > f. Therefore, a gas pressure with maximum energy transfer exists.

Two standard industrial frequencies are available: the 2.45 GHz and 0.915 GHz. Due to the significantly different energy content of the different species a non-equilibrium plasma is ob­tained with relatively low overall temperatures in the system. In the case of the 2.45 GHz equipment, the temperature measured directly behind the plasma zone is adjusted in the range from 100 to 500 °C. The temperature range from 500 to 800 °C is accessible for the 0.915 GHz equipment. These temperatures are low as compared to conventional gas phase reactors where temperatures around 1200 °C are necessary. The pressure in the system is adjusted in the range from 10 to 50 mbar. The experimental conditions are selected to obtain residence times of the particles in the plasma zone of a few milliseconds.

As a unique feature of this process, the particles formed in the plasma leave the reaction zone carrying electric charges of equal sign. Therefore, they repel each other impeding particle growth by agglomeration. Consequently, the particle size distribution of the product is very narrow. Furthermore, the electric charges of the particles make it possible to coat the particles individually with a second phase in a cascaded process.

Synthesis of Nanoparticles and Coated Nanoparticles

The plasma is generated in a reaction tube made of quartz glass, passing through the microwave cavity. In the reaction zone, a gaseous precursor is forming the desired component with an ap­propriate reaction gas. E.g. in the case of oxide synthesis, a mixture of argon with oxygen is used as a reaction gas. The central problem with this kind of synthesis is the selection of appro-

5

priate precursors. Water-free chlorides or carbonyls are preferred precursors. In cases where these compounds are not existent or not sufficiently volatile, metal-organic compounds are used. Assuming a chloride as precursor, the following reactions may occur in the plasma:

MeCL +—02 => MeOm + - CL fV. " 2 2 (3)

To synthesize nitrides, nitrogen is used as reaction gas. As the chlorides are more stable than the nitrides, it is necessary to add hydrogen to the system to shift the equilibrium to the side of the nitride and HCl formation.

MeCl„ +—N2 +-H2 => MeNm + nHCl ( 4 )

Besides the problem of the formation of NHACl in most cases, it is beneficial to use ammonia as hydrogen carrier.

Using similar reactions it is also possible to synthesize sulfides, selenides, carbides, and in spe­cial cases metallic nanoparticles.

The mechanism of the formation of the nanoparticles in microwave plasma is assumed to occur in the following steps [5,6]:

• Ionisation and dissociation of the reactive components in the plasma.

• Reaction of the dissociated species forming molecules of the intended compound.

• Homogenous nucleation of the particles by random collision of two or more molecules.

• Growths of the nuclei by further collision with molecules.

• Further growth by coagulation of the particles. The increase of the electrical charge of the particles with size limits this coagulation process.

Taking advantage of the electrical charges of the as-produced nanoparticles and the relatively low temperatures it is possible to coat the individual particles in a second step. In this context, one may think e.g. of the synthesis of oxide particles consisting of a core MaOn coated with a second oxide MbOm. Two identical plasma stages consecutively on the reaction tube, each one with its own supply of reactive gases are used to produce ceramic-coated nanoparticles. In the second plasma stage, the chemical reactions are the same as compared to the first one. In the first reaction stage, the particles are formed by homogenous nucleation. In contrast, in the sec­ond reaction stage the particles formed in the first step act as heterogeneous nuclei for the molecules of the second oxide MbOm. The gas kinetic collision cross section of the particles formed in the first step is about two orders of magnitude larger than the cross section of a molecule. Therefore, the probability of the condensation of Mb oxide molecules on the Ma oxide particles is also at least two orders of magnitude larger. Therefore, one may expect a high yield in coated particles. Based on these considerations, it is obvious that a small amount of pure particles of Mb oxide is unavoidable. However, it is impossible for any of the Ma oxide particles to remain uncoated.

6

Figure 1 shows the schematic experimental set-up for the synthesis of ceramic-coated nanopar-ticles. The two microwave branches are adjusted independently, using the power variator and the tri-stub-tuners. Isolators in each branch reduce the mutual influence of the microwave tun­ing. The evaporated precursors are introduced with a preheated carrier gas before each plasma zone. Particle collection is performed by thermophoresis.

Magnetron 2.4S GHz

, Isolator*

Powder Collection

Reaction Tube Quarz Glass

TE„ Applicator PlaemaZone2

Insert Precursor 2

TEn Applicator Plasma Zonal

Insert Precursor 1

Figure 1: Schematic drawing of the experimental set-up for the synthe­sis of ceramic coated nanoparti-cles.

For the polymer coating, the basic considerations are similar, but the experimental arrangement is slightly modified [7]. Monomers as methyl methacrylate (MMA) or methacrylic acid (MAA), perfluoroalkyl acrylates (PBAF7) or methacrylates (PBMAF7) and PTFE-analogous compounds are used, depending on the desired properties of the polymer coating [8]. The va­porized precursors are introduced directly after the last plasma zone into the system. They con­dense on the surface of the nanoparticles and polymerize due to the elevated temperature in the system and UV radiation from the plasma. Figure 2 depicts the experimental setup schemati­cally.

Microwave waveguide

Input of precursors and reaction gases

Input of the monomer

Particles formed in the plasma UV-light from the plasma Output of polymer

coated nanoparticles

TEn cavity Condensation of the monomer on the nanoparticles and polymerization

Figure 2: Experimental set-up for the synthesis of polymer coated ceramic nanoparticles.

Morphology of Coated and Metal Decorated Nanoparticles

As examples of the two types of coated nanoparticles figures 3 and 4 show transmission elec­tron micrographs of ceramic and polymer coated nanoparticles [9,10]. The ΗΓΌ2/ΑΙ2Ο3 nano­particles depicted in figure 3 were produced from chloride precursors. The Η1Ό2 particles are crystallized and show lattice fringes. The coating with AI2O3 is amorphous and about 0.5 nm thick. The y-Fe203/polymer nanoparticles in figure 4 were made from Fe(CO)s and methacrylic

7

acid precursors. The y-Fe203 cores are 5 - 6 nm in diameter; the polymer coating has a thick­ness of 2 - 3 nm corresponding to ca. 70 vol% of polymer.

Figure 3: High-resolution electron mi­crograph of individual HfÖ2 nanoparti-cles coated with amorphous AI2O3. The lattice fringes stem from the crystalline

^ HfÖ2. The AI2O3 coating is amorphous.

Figure 4: High-resolution electron micrograph of polymer coated γ-Fe2Û3 nanoparticles. The 7-Fe2Ü3 cores exhibit lattice fringes. The polymer coating has a thickness of approximately 2 nm.

?A5ns!^*£.

Ceramic/ceramic nanoparticles are consolidated by cold pressing and high temperature sinter­ing. Whereas the ceramic/polymer nanoparticles are consolidated by "hot-pressing" in the tem­perature range from 70 to 140 °C at 0.7 MPa. Figures 5 and 6 show the typical structures of consolidated parts made of nanocomposites [11,12].

Figure 5: Transmission electron micro­graph of a sintered Ζ1Ό2/ΑΙ2Ο3 nano-composite. The ZrC>2 particles (dark) remain monocrystalline and clearly sepa­rated by the surrounding AI2O3.

Starting powder for the material shown in figure 5 were Ζ1Ό2 particles, 6 - 8 nm in diameter, coated with 2 nm thick AI2O3. After sintering the ZrO^ cores exhibit a particle size around 10 nm. This means that nearly no grain growth occurred in the ZrÜ2 kernels. Therefore, poten­tially interesting physical properties of the core are preserved in a densified body. Grain growth

8

of the AfeOs-coating during the sintering process is not prevented and meaningless.

In the case of the "hot-pressed" y-FezCVpolymer nanocomposite (figure 6), made of powder containing approximately 50 vol% polymer, particle growth of the Fe2Û3 nanoparticles is not observed. Striations visible in the micrograph are artifacts from the sample preparation tech­nique. It is important to point out that the particle size distribution is very narrow.

Figure 6: Transmission electron micro­graph of a 20 nm microtome section of 7-Fe2(V polymer nanocomposite. The sample preparation process causes stria­tions visible in the micrograph. The size distribution is very narrow.

Figure 7 shows as an example for noble metal decorated ceramic nanoparticles T1O2 nanoparti­cles, decorated wit Pt-clusters. The Pt-clusters are visible as dark dots in the micrograph. The size of the Pt-clusters is approximately 3 nm. These types of noble metal decorated transition metal oxide nanoparticles have a high potential for application in catalysis.

Figure 7: T1O2 nanoparticles, decorated wit Pt-clusters. The Pt-clusters have a size of approximately 3 nm.

Properties of Nanoparticles and Coated Nanoparticles

Dispersion Behavior

The dispersion behavior of bare and coated nanoparticles in pure distilled water was investi­gated. These suspensions were standardized at a particle concentration of 100 mg/1. Colloid stabilizer or dispersant were not added. The pH and ζ-potential of the dispersions, given in ta­ble I was measured after complete suspending the particles. A detailed analysis of the results in table I show several factors influencing the suspension behavior of nanoparticles. One is the type of precursor used for synthesis. Uncoated nanoparticles made from inorganic precursors (e.g. chlorides) exhibit excellent suspension behavior. In contrast, nanoparticles made from

9

organic precursors (metal-t-butoxides, carbonyls, and diethyl-metal) flocculate. Ceramic coat­ing made from an inorganic precursor improves the dispersion behavior of particles with poor behavior (e.g. Fe2<I>3 ex Fe(CO)s/Zr02 ex ZrCL*). Consequently, coating with a ceramic layer made from organic compounds impairs the suspension behavior (e.g. Fe203 ex Fe(CO)s/ZnO ex Zn(C2Hs)2). Plotting ζ-potential versus the pH value shows an interesting clustering of the measured values with respect to the suspension behavior. Figure 8 shows that the combination of pH < 5 and ζ-potential > 25 mV of the dispersion generally indicates a good dispersion be­havior (exception: Zr02 made from Zr-t-butoxide, dotted circle in figure 8). In contrast pH > 5.5 and ζ-potential < 25 mV indicates a poor dispersion behavior. Interestingly, Fe203/Zr02 suspensions are stable for several months. The ζ-potential of these dispersions is independent of the concentration.

Table I: Suspension behavior of various nanoparticles made under different experimental conditions.

Material A1203

Fe203

ZnO Ti02

TÍC-2 Zr02

Zr02

Al203/Zr02

Zr02/Al203

Fe203/ZnO Fe203/Zr02

Fe203/Zr02

Fe203/Zr02

Ti02/Zr02

Fe203/PMMA Fe203/PBAF7 Fe203/PBMAF7 Fe203/PMAA Fe203/PTFE

Precursor(s) A1C13

Fe(CO)5

Zn(C2H5)2

TiCU Ti-t-butoxid ZrCU Zr-t-butoxid AlCl3/ZrCl4 ZrCVAlCla Fe(CO)5/Zn(C2H5)2 Fe(CO)5/ZrCl4 Fe(CO)5/ZrCl4 Fe(CO)5/ZrCl4 TiCVZrCU Fe(CO)s/monomer Fe(CO)s/monomer Fe(CO)5/monomer Fe(CO)5/monomer Fe(CO)5/PTFE

pH 4,9 6,7 6,3 4,9 5,9 4,4 4,5 4,9 4,9 6,3 3,7 4,1 3,6 4,3 4,9 4,5 4,7 4,6 -

ζ-potential [mV) 37,9 14,3 22,4 26,8 -20,6 37,0 28,9 36,0 35,7 21,8 34,0 34,0 37,8 32,9 28,6 36,1 29,6 25,0 -

suspension behavior good poor poor good poor good poor good good poor good good good good good good good good hydrophobic

Coating nanoparticles with an organic layer also leads to surface modification. Depending on the polymer materials used for coating, different behavior is observed. Coating with polymer materials derived from MMA or MAA, PBAF7 or PBMAF7 lead to stable dispersions without any colloid stabilizer. The criteria pH < 5 and ζ-potential > 25 mV for stable dispersions are fulfilled also in this case. The excellent dispersion behavior of magnetic polymer coated nano­particles is a property promising for biological and medical applications. Nanoparticles, coated with polymer materials similar to PTFE are hydrophobic. Additionally, these composites show increased thermal and chemical stability. Thus, they cannot form suspensions, but a powder layer on the surface of the water.

Decreasing the pH by adding an acid (HO) to the dispersion does not change the stability of the dispersion. In contrast, additions of alkaline solutions (e.g. NH4OH) destroys the suspension,

10

the particles flocculate. Suspending the nanoparticles directly in acid or alkali solutions with various pH values of the acid or alkali leads to similar results.

| 20

Έ 10

S 0 o * -10

-20

Λ

: •

:

* * * *

f 4 K .

Dispersion Behavior * good ♦ poor

;

/ Λ >

♦

♦"♦"

*

/ ...·····

\

} S ·

:

Figure 8: Dispersion behavior of nanoscaled powders and ceramic coated nanoparticles listed in ta­ble I.

4.5 5.5 6 6.5 7

pH-Value

Magnetic Properties

For technical application, one of the most interesting particle size depending physical properties of nanomaterials is superparamagnetism [13]. In superparamagnetic materials, the vector of magnetization fluctuates thermally; the material is free of hysteresis in the magnetization curve. A particle is superparamagnetic provided the energy of magnetic anisotropy Kv (K: constant of magnetic anisotropy, v: volume of the particle) is smaller than the thermal energy kT:

Kv<kT (5)

To preserve this typical "nano-property" in a technical part coating of the nanoparticles is nec­essary. Due to the coating, acting as distance holder, dipole-dipole interaction between the magnetic particles is reduced. Figure 9 shows the static magnetization curves of uncoated and Zr02-coated y-Fe2C>3 [11]. This figure demonstrates clearly that the uncoated material (marked with an arrow) exhibits some hysteresis, whereas the two types of coated materials are free of any hysteresis.

Figure 9: Comparison of magneti­zation curves of non-coated and different zirconia-coated Y-Fe2C>3 nanoparticles at 300 K.

11

1.005 1.002

OJOS

OJ04

OJ02

OJO

'- -i -i- j- |....a..4.....íí..

r y "·" í *—T"T" " i — '

'·—-—i -i-- f 1 5·-*

^ p * j w ] i < 4

1 f | — — HttMl SMCblMI

• ExpwlnwaM dab

I i- f

i ■. « ■ i ) . . . i ■ ■ ■ ■ ¡ ■ ■ ■ ■ 1

■5 0 5 Velocity [mms -1]

15 -16 - 4 - 2 0 2 4 Velocity [mm»-1]

Figure 10: Mössbauer spectra of a partly superparamagnetic 7-Fe2C>3 nanopowder (left) and a superparamagnetic y-Fe203/polymer nanocomposite (right).

The ultimate proof of superparamagnetism is the Mössbauer-spectroscopy. Normal ferrimag-netic material exhibits a sextet of lines, whereas superparamagnetic material is characterized by the quadrupole split doublet. Figure 10 compares the Mössbauer spectra of two different mate­rials: partly superparamagnetic bare ferrite nanoparticles (left) and a superparamagnetic poly­mer coated nanoscaled ferrite (right) [12]. The coated material exhibits only the doublet char­acteristic for superparamagnetic materials.

High Frequency Properties

The complex permittivity, er=e- je" of different ceramic/polymer nanocomposites was de­termined with a vector network analyzer in the frequency range from 10 MHz to 8 GHz, using the coaxial-line-S-parameter method [14,15]. Correction for phase shift was necessary, since the length of the specimen was not equal to the one of the sample holder [16]. For the specimen holder in use, the algorithm for data evaluation looses precision at frequencies above 4 GHz.

Figure 11 clearly demonstrate a decreasing complex permittivity with increasing polymer con­tent of different AkCVpolymer nanocomposites [17] containing 50 +/- 5 vol% polymer. For comparison, the values of pure AI2O3 and C20F42 are shown, too. Compared to the permittivity of the pure polymer, additions of ceramic nanoparticles increase permittivity and frequency dependencies but remain smaller than these values of the pure ceramic material. Except for C20F42 coated AI2O3 particles (curve marked with a circle), the tendency in the behavior is similar for all polymer materials used. This material exhibits the most pronounced decrease of the permittivity with frequency

Figure 12 shows a similar behavior of the samples containing 70 +/- 5 vol% of polymer [17]. With this high polymer content, the observed frequency dispersion is very small despite a sig­nificantly increased permittivity. In general, the various polymer materials are not showing significant differences in their behavior. As expected from the pure materials, tan δ of the com­posites containing 70 vol% polymer are lower than tan δ of the material with 50 vol% polymer.

12

ω

1Ë+007 1Ε+008 1Ε+009

Frequency [Hz]

1E+010 -0.5 1E+007 1E+O08 1E+009

Frequency [Hz]

1E+010

Figure 11 : Real part and tan δ of the permittivity for various y-AkCVpolyiner nanocompo-sites with approximately 50 vol% of polymer.

ω

1E+007 1E+008 1E+0O9

Frequency [Hz]

c CO

-0.5 1E+010 1E+007 1E+008 1E+009

Frequency [Hz]

1E+010

Figure 12: Real part and tan δ of the frequency dispersion behavior for different fluoro-polymer materials with approximately 70 vol% of polymer.

Summary

The Karlsruhe Microwave Plasma Process is capable to synthesize coated nanoparticles with narrow distribution of the particle size. The narrow size distribution is caused by the physics of the plasma process; the particles leave the plasma zone with electric charges of equal sign. This property of the process makes it possible to coat each particle individually in a cascaded proc­ess. For coating, two strategies are possible: either coating with a second ceramic phase or a polymer one. Coating influences the properties of the material in several ways: It reduces inter­action between the kernels, thwarts grain growth of the kernel during consolidation or sintering. Therefore, this saves particle size dependant physical properties, such as superparamagnetism in a macroscopic part made from nanoparticles for technical application.

Coating modifies surface chemistry of the particles: It is possible to produce particles, forming stable dispersions without addition of any colloid stabilizer or dispersant. This property is nec­essary for biological or medical applications. Additionally, it is possible to synthesize hydro-phobic nanoparticles using fluorinated polymer materials.

This makes it possible to combine different physical and chemical properties in one particle. This opens a wide field for various applications.

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References

1. H. Gleiter, "Nanocrystalline Materials," Prog. in Mater. Sei.. 33(1989) 223-315.

2. D. Vollath, K.E. Sickafus, „Synthesis of Nanosized Ceramic Oxide Powders by Microwave Plasma Reactions," Nanostructured Materials. 1 (1992) 427-437.

3. D. VoUath, K.E. Sickafus, „Synthesis of Nanosized Ceramic Nitride Powders by Micro­wave Supported Plasma Reactions," Nanostructured Materials. 2 (1992) 451-456.

4. D. McDonald, Microwave Breakdown in Gases (New York, NY: John Wiley & Sons, 1966).

5. D. Vollath, D.V. Szabó, "Nanocoated Particles: A Special Type of Ceramic Powder," Na­nostructured Materials. 4 (1994) 927-938.

6. D. Vollath, "Synthesis of Nanocrystalline Ceramic Powders in a Microwave Plasma," (in German), KfK-Nachrichten, 25 (1993) 139-144.

7. D. Vollath, D. V. Szabó, B. Seith, German Patent DE0019638601C1 (1998).

8. I. Lamparth, D.V. Szabó, D. Vollath, "Ceramic Nanoparticles Coated with Polymer Based on Acrylic Derivatives," Macromolecular Symposia, submitted Oct. 2001.

9. D. Vollath, D.V. Szabó, "Nanocomposites - New Functional Materials on the Edge of In­dustrial Application," (in German), FZK-Nachrichten. 31 (1999) 197-205.

10. D, Vollath, D.V. Szabo, J. Fuchs, "Synthesis and Properties of Ceramic-Polymer Compos­ites," Nanostructured Materials. 12 (1999) 443-438.

11. D. Vollath, D.V. Szabó, J. Haußelt, "Synthesis and Properties of Ceramic Nanoparticles and Nanocomposites." J. Europ. Ceram. Soc. 17 (1997) 1317-1327.

12. D. Vollath, D.V. Szabó, J. Fuchs, "Polymer Coated Nanoparticulate Ferrite Powders: Prop­erties and Application," Mat. Res. Soc. Svmp. Proc. 577 (1999) 443-448.

13. L. Néel, Compt. Rend. 228 (1948) 664.

14. A.M. Nicolson, G.F. Ross, "Measurement of the Intrinsic Properties of Materials by Time Domain Techniques," IEEE Trans. Instrum. Meas.. IM-19 (1970) 377-382.

15. Hewlett Packard, "Materials Measurement: Measuring the Dielectric Constant of Solids with the HP 8510 Network Analyzer," Product Note 8510-3

16. R. Pelster, University of Cologne, Germany, private communication with author, January 2000.

17. D. V. Szabo, I. Lamparth, D. Vollath, "Complex High-Frequency Properties of Ceramic-Polymer Nanocomposites: Comparison of Fluoropolymers and Acrylic Based Compounds," Macromolecular Symposia, submitted Oct. 2001.

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FORMATION OF PHOTONIC NANOCOMPOSITES BY SURFACE ENGINEERING OVER INORGANIC NANOPARTICLES

Nobuyuki Kambe NanoGram Corporation, Fremont, CA 94538, U.S.A.

ABSTRACT

Surface engineering over inorganic nanoparticles has been developed and proven to form optical quality nanoparticle-polymer composites. NanoGram process developed here is unique in the sense that preformed inorganic nanoparticles and organic polymers form chemical bonds via linkage molecules. A major guideline in the properties of preformed nanoparticles, and processing of particle dispersion and nanoparticle-polymer composites is provided. Formation of chemical bonds between inorganic nanoparticles and organic polymers is found to facilitate homogeneous photonic nanocomposites without agglomeration. In addition, a number of peculiar self-assembled macro/microstructures have been observed. Some interesting structures are presented and interpreted in terms of the strength of particle-particle interaction, which can be controlled by surface modification of nanoparticles through linkage molecules.

INTRODUCTION

Surface engineering of inorganic nanoparticles can add or substantially enhance their physical functions. Nanocomposites are a new class of materials that comprise surface-engineered materials and are expected to lead to a wide range of photonic applications from optical coatings to miniaturized microphotonic device [1]. Numerous approaches for inorganic-organic nanocomposites have been developed in an attempt to incorporate advantageous micro- and nano-structure designs, unique performances, and synergistic effects of the merged components [2]. Many cases, however, involve development of the nanoparticles in-situ or forming them in a solution and then mixing them with the organic phase without isolating the evolved particles [3]. Except for gold or other metal particles, most of the in-situ developed particles are only precursors to ceramic crystalline phases. They are far from providing the anticipated functions expected from the crystalline material. The incorporation of preformed nanoparticles with fully developed crystalline phases into polymeric materials is one solution to overcome those issues and a rapidly growing field in nanotechnology [4]. The primarily focus has been on improving mechanical properties such as toughness and hardness of elastomeric and transparent materials [5].

NanoGram has developed an advanced processing technology of nanoparticle-polymer composites by chemically bonding preformed nanoparticles with organic polymers through surface-modifying linkage molecules [6]. This allows precise control of the refractive index that is required for photonic device fabrication. Lately we have been able to finely tune the refractive index through a linear combination of the index for nanoparticles and for the polymer hosts. Successful index tuning requires, among other properties, (a) high levels of uniformity in nanoparticle size and shape and (b) high levels of particle dispersion in solvent both before and after the surface modification.

The challenge here is to obtain high loadings of optically functional nanoparticles in

Surface Engineering: Science & Technology of Interfaces II Edited by A. Kumar, Y-W. Chung,

J.J. Moore, G.L. Doll, K. Yasui and D.S. Misra TMS (The Minerals, Metals & Materials Society), 2002

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polymeric host materials while maintaining a high level of transparency in a wavelength region of interest. It is therefore important to avoid agglomeration of nanoparticles within a nanocomposite. However, it is known that nanoparticles in general tend to agglomerate or dramatically increase the polymer viscosity. Many of the currently available nanoparticles (fumed silica, for example) appear to have hard agglomeration even before they are incorporated into the organic phase. There is a need to develop a methodology to overcome this problem.

This paper outlines the important properties of preformed nanoparticles and nanocomposites synthesized by the NanoGram technology. Dispersions in solvents and also in polymer media are significant parameters for successful fabrication of photonic nanocomposites. Surface modifications play a key role in the whole processing. Levels of particle-particle interaction affect micro or macrostructures of the synthesized nanocomposites. Many peculiar self-assembled structures have been observed in coatings of nanocomposites and are discussed in relation to surface modifications, i.e., the strength of particle-particle interaction.

UNIFORM NANOPARTICLES

NanoGram has developed and proven laser-driven chemical reaction processes as an enabling tool to generate a versatile range of nanoparticles with unparalleled uniformity in size and shape [7,8]. A high level of uniformity is found to facilitate the subsequent processing of nanomaterials that allows homogeneous dispersion in liquid and polymeric media.

A CO2 laser beam provides a heat source for rapid heating (at a rate on the order of 105

K/sec) within a tiny and controlled reaction zone, resulting in high-temperature pyrolysis and chemical reaction between the precursors such as TiCU and oxygen gas for the preparation of nano-TiO? particles. Immediately after the laser reaction zone, nanoscale particles are formed due to a rapid cooling (at a rate on the order of 105 K/sec) without any contact to a cooling agent. The highest level of uniformity in particle sizes and shapes can be obtained by precise control of the laser reaction zone and the precursor flow.

Characterizations of synthesized nanoparticles are generally made by transmission electron microscopy (TEM), scanning electron microscopy (SEM), x-ray diffraction (XRD), and BET surface area measurements.

Figure 1 represents typical size and morphology of laser-made nanoparticles. Because of

Figure 1. Typical TEM images for laser-made uniform nanoparticles: (a) δ-phase AI2O3 with the surface area of 137 m2/g that is stable at the temperatures between 760 and 930 °C; (b) γ-phase A1203 that is generally obtained below 760 °C; and (c) a lattice image for a crystalline nano-LiMn204 that has a diameter of 25 nm.

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the thermally-nonequilibrium synthesis process, the shape of our nanoparticles is generally spherical even if the corresponding bulk crystal shows an anisotropic shape or facet. Figure 1(a) and (b) correspond to two distinct phases of alumina nanoparticles: (a) higher-temperature phase ô-A^Ch and (b) lower-temperature phase γ-Αΐ2θ3. This indicates that the NanoGram process can control crystalline phases while maintaining the stoichiometry and uniformity in size and shape. In general, the standard deviation around an average particle size (φ3ν) is around 30 %. There is always a sharp cut-off at upper size limit; typically no oversized particles beyond 2φ3ν. Figure 1(c) represents a lattice image for most of multicomponent metal compounds such as this case of nano-LiMmO^ This compound has a spinel structure, and the characteristic facet can be slightly observed.

These well-crystallized preformed nanoparticles are found to be very promising precursor materials for photonic device applications because of ease in handling and processing for coatings and composite formation. Very high levels of homogeneity and optical transparency in dispersion and nanocomposites have been achieved using the laser-made nanoparticles.

TWO TYPES OF NANOPARTICLES - POLYMER COMPOSITES

Pre/ormednanoparticles-polymer composites can be classified into two types as illustrated in Figure 2. Type I is merely a mixture of the two elements without forming strong chemical bonds between nanoparticles and polymers (Figure 2(a)). Type II is strongly-interacted composites through covalent bonding between nanoparticles and polymers

Figure 2. Two types of inorganic - organic nanocomposites made from preformed nanoparticles. (a) Type I nanocomposites that have few chemical bonds between nanoparticles and polymers; (b) Type II nanocomposites that have covalent bonds between nanoparticle surfaces and polymer chains. This paper describes Type II.

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(Figure 2(b)). By comparison, Type II has a wider range of materials portfolio because there is little limitation in the selection of both inorganic nanoparticles and organic polymeric materials. This is the case so far as appropriate linkage molecules are available to form covalent bonds between the two elements. The NanoGram process is uniquely fit to Type II.

Type I seems to result in clustering because only weak barrier exists to prevent interparticle interactions. In contrast, Type II can reduce the interparticle interactions so that nanoparticles are likely distributed without forming clusters, as implied from Figure 2(b).

In Type II that can be represented by our nanocomposites, the actual number of surface-modifying molecular arms bonded to the surface of individual nanoparticles is very large, potentially over a 10 -103 arms per the surface. Such system may be analogous to a neural network system, although the number of linkage is much larger in the current case.

Type II nanocomposites can provide interesting thermal and electronic applications in addition to photonic ones, when a linkage molecule is thermally or electrically conductive respectively. This system may be an interesting N-dimensional (N » 3) system. For example, electrically conductive molecules play a role of electric charge transfer channels that may be interpreted by the percolation model in soft matters [9].

In the later section, several examples of anomalous macro/microstructures are described indicative of a rich field of phase transition that involves self-assembled structures. In principle, the largely van der Waals-based interactions between nanoparticles can be controlled over a long range by means of linkage molecules, and likely determine both microscopic and macroscopic structures of coated films and other devices.

SURFACE MODIFICATION OF NANOPARTICLES

[A] Dispersion of preformed nanoparticles

Level of nanoparticle dispersion in solvents is a significant parameter in addition to surface modifications. Any presence of agglomerates in solution likely results in inhomogeneous coatings. Laser-made transition metal oxide nanoparticles have been chosen to

c o '55 V. Φ a

> a -i

2.4 7 18.3 24.3 Poíarity

47 78.5

■TÎ02 (rutile) ■Ti02 (rutile) *Ti02 (anatase)

Figure 3. Dependence of nanoparticle dispersion upon polarity of solvents

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