bluuebunny/crossref_metadata_2025 huggingface dataset:
Room Temperature Photoluminescence Spectra of Annealed PECVD Silicon Nitride Thin Films
Authors: Savall C. , Bustarret E. , Stoquert J. P. , Bruyére J. C.
Published: March 2011

We present the changes upon isochronous annealing in the room temperature photoluminescence (PL) spectra of nearly stoichiometric silicon nitride. Samples are prepared by the 50kHz PECVD of a N2/SiH4/Helium gas mixture at 350°C. In the as-deposited films the hydrogen content was around 11% for a refractive index of 1.98. For a photoexcitation at 351 nm, the polarized PL spectrum of the as-deposited film is characterized by a main broad emission band in the visible region with a maximum at 2.55eV. A second narrow peak (FWHM = 55meV), at 3.02eV is observed. We study the evolution of the PL spectra with different isochronous anneals from 350°C to 1000°C. Even though the general shape of the PL spectra does not change, we observe an increase of quantum efficiency with maximum value upon 750°C annealing. Beyond this temperature the low energy part of the PL spectrum shows a slight increase. These variations are compared to those of the infrared absorption peaks measured on the same samples. Both the temperature dependence of the intensity of a well defined absorption peak in the Si-H stretching mode region and that of the PL features can be explained assuming that solid state chemical reactions involving hydrogen and Si-Si bonds occur in the bulk of the alloy.
Electrical Properties of Silicon Nitride Thin Films Fabricated by ECR PECVD at Room Temperature
Authors: Jeon Yoo-Chan , Lee Hoyoung , Joo Seung-Ki
Published: March 2011

Silicon nitride thin films were deposited on single crystalline silicon substrates at room temperature by ECR PECVD with SiH4 and N2 as source gases and the electrical properties were analyzed. The dominant conduction mechanism in a high field was Poole-Frenkel emission. A ledge in I-V curve was observed in the first voltage ramp and it was found to originate from the field reduction at the injecting electrode due to the charge trapped in deep traps in the film. It also turned out that the ledge is a characteristic of monopolar conduction. A new interpretation of the current at low field — tunneling into trap states — was proposed and the current variations according to the field and temperature could be well explained.
Excimer Laser Interaction with Metals
Authors: Duley W.W. , Kinsman G.
Published: March 2011

Excimer laser radiation may be used to process metal surfaces in a variety of novel ways. The simplest of these involves the use of UV laser pulses for ablation. Ablation occurs as the result of both vaporization and hydrodynamical effects. Experimental data related to these processes will be discussed. In addition, it will be shown how specific irradiation regimes can yield metal surfaces with unique radiative properties.
A Comparative Study of the Photoablation of Polyimide-Doped Poly(Tetrafluoroethylene) at 308 NM and 248 NM
Authors: D'couto G. C. , Babu S. V. , Egitto F. D. , Davis C. R.
Published: March 2011

Experimental data on the 248 nm and 308 nm wavelength excimer laser ablation of poly(tetrafluoroethylene) (PTFE) doped with polyimide (PI) are reported for a range of fluences and dopant concentrations. Threshold fluences were determined and correlated with the dopant concentrations. The threshold fluences and the limiting etch rates measured at high fluences decreased with increasing dopant concentration and there is a minimum absorption coefficient below which there is no ablation at both the wavelengths. The etch rates have been modeled using a two parameter Arrhenius thermal model to describe the etching process.
Development of a Simple and Highly Sensitive Microbalance for Measurements of Total Particle Yields in Matrix Assisted Laser Desorption
Authors: Huth-fehre T. , Quist A.P. , Linder S-O. , Sundqvist B.U.R.
Published: March 2011

In order to open a new window for getting insight into the process of matrix assisted laser desorption ionization (MALDI) of biopolymers, a highly sensitive quartz crystal microbalance (QCM) was developed to measure desorption yields of neutral particles. For the matrix substances commonly used the mass resolution of 0.95 ng obtained with the balance corresponds to a detection limit of ∼1012 particles.
Tunable Laser Desorption from Diamond Films and Pyrolytic Graphite
Authors: Ueda A. , Mckinley J. T. , Albridge R. G. , Barnes A. V. , Tolk N. H. , Davidson J. L. , Languell M. L.
Published: March 2011

We report the observation of wavelength-dependence in the early phases of the laser ablation of chemical vapor deposited (CVD) diamond films. The measurements were made with 3–4 μm infrared radiation from the Vanderbilt Free-Electron Laser (FEL), the brightest tunable mid-infrared FEL in the world. Though we do not propose any specific mechanism for the ablation, the C-H bond-stretching mode at 3.47 μm is clearly important in the early stagesof the damage process, as evidenced by a sharp drop in the ablation threshold near 3.47 μm.The ablation damage threshold was determined by directing a single FEL pulse at the sampleand measuring the diameter of the resulting hole using a scanning electron microscope (SEM).This measurement was repeated for several pulse intensities. The damage threshold was defined as the extrapolation to zero hole diameter. We also looked at the desorption productsusing a time-of-flight spectrometer and a quadrupole mass analyzer. C2 is the dominant desorbed species. Visible light emission was monitored with a monochromator and photodetector.
Atomic Layer Growth of High Tc (Ca,Sr)CuO2 Thin Films by Laser Mbe
Authors: Kana M. , Kawai T. , Kawai S.
Published: March 2011

Resistive and magnetic anomalies were observed at 150-170K in Ca0.3Sr0.7CuO2 thin films prepared by atomic-layer stacking using laser molecular beam epitaxy. Systematic current density dependence of the resistivity was observed below these temperatures with clear resistivity drops. At the same temperature range, the films showed diamagnetic response and hysteresis of magnetization between zero-field cooling and field cooling process. These behaviors suggest that the anomalies are due to high temperature superconductivity around 150-170K in the Ca0.3Sr0.7CuO2 thin films.
Rheed-Oscillations During Pulsed Laser Deposition of YBCO
Authors: Karl H. , Stritzker B.
Published: March 2011

Reflection high-energy electron diffraction (RHEED) oscillations during pulsed laser ablation of YBCO on SrTiO3(100) substrates were measured. We found that the oscillations are modulated by the laser-pulse repetition frequency, due to diffuse scattering by the initially disordered YBCO material. Since the disordered YBCO crystallizes the specular intensity recovers exponentially with time. An estimate of the diffusion length was obtained by measuring RHEED oscillations during deposition on slightly misoriented SrTiO3(100). From the rise time of the RHEED signal and the mean diffusion length a diffusion constant of 1.4*10−2cm2/sec was determined [1].
Growth of Laser Ablated YBa2Cu3O7−δ Films as Examined by Rheed and Scanning Tunneling Microscopy
Authors: Russek Stephen E. , Roshko Alexana , Sanders Steven C. , Rudman David A. , Ekin J. W. , Moreland John
Published: March 2011

Using scanning tunneling microscopy (STM) and reflection high energy electron diffraction (RHEED) we have examined the growth morphology, surface structure, and surface degradation of laser ablated YBa2Cu3O7−δ thin films. Films from 5 nm to ltm thick were studied. The films were deposited on MgO and LaAlO3 substrates using two different excimer laser ablation systems. Both island nucleated and spiral growth morphologies were observed depending on the substrate material and deposition rate used. The initial growth mechanism observed for a 5–10 nm thick film is replicated through different growth layers up to thicknesses of 200 run. Beyond 200 rnm, the films show some a-axis grains and other outgrowths. The thinnest films (5–10 nm) show considerable surface roughness on the order of 3–4 nm. For both growth mechanisms the ledge width remains approximately constant (∼ 30 nm) and the surface roughness increases as the film thickness increases. The films with spiral growth have streaked RHEED patterns despite having considerable surface roughness, while the films with island growth have more of a three dimensional diffraction pattern. RHEED patterns were obtained after the film surfaces were degraded by exposure to air, KOH developer, a Br-methanol etch, and a shallow ion mill. Exposure to air and KOH developer caused only moderate degradation of the RHEED pattern whereas a shallow (I nm deep) 300 V ion mill completely destroyed the RHEED pattern.
In-Situ Laser Processing and Microstructural Characteristics of YBa2Cu3O7−δ Thin Films on Si with TiN Buffer Layer
Authors: Tiwari P. , Zheleva T. , Narayan J.
Published: March 2011

We have prepared high-quality superconducting YBa2Cu3O7−δ (YBCO) thin films on Si(100) with TiN as a buffer layer using in-situ multitarget deposition system. Both TiN and YBCO thin films were deposited sequentially by KrF excitner laser ( λ = 248 nm ) at substrate temperature of 650°C. Thin films were characterized using X-ray diffraction (XRD), four-point-probe ac resistivity, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Rutherford backscattering (RBS). The TiN buffer layer was epitaxial and the epitaxial relationship was found to be cube on cube with <100> TiN // <100> Si. YBCO thin films on Si with TiN buffer layer showed the transition temperature of 90–92K with Tco (zero resistance temperature) of ∼84K. We have found that the quality of the buffer layer is very important in determining the superconducting transition temperature of the thin film. The effects of processing parameters and the correlation of microstructural features with superconducting properties are discussed indetail.
Epitaxial Growth of TiN Films on (100) Silicon Substrates by Laser Physical Vapor Deposition
Authors: Tiwari P. , Zheleva T. , Narayan J.
Published: March 2011

We have synthesized epitaxial TiN films having low resistivity on (100) silicon substrates using pulsed laser deposition method. The TiN films were characterized using X-ray diffraction, Rutherford back-scattering, four-point-probe ac resistivity, high resolution transmission electron microscopy techniques and epitaxial relationship was found to be <100> TiN // <100> Si. TiN films showed 10–20% channeling yield. In the plane, four unit cells of TiN match with three unit cells of silicon with less than 4.0% misfit. This domain matching epitaxy provides a new mechanism of epitaxial growth in systems with large lattice misfits. Room-temperature resistivity of these films was found to be about 15 μΩ-cm. Implications of low-resistivity epitaxial TiN in silicon device fabrication are discussed.
Phase Composition and Microstructure as a Function of Deposition Conditions for Potassium Tantalate Niobate Thin Films Grown by Pulsed Laser Deposition
Authors: Cotell C.M. , Leuchtner R.E.
Published: March 2011

Potassium tantalate niobate [K(Ta1−xNbx)O3 or KTNJ films were deposited by pulsed laser deposition (PLD) on (100)MgO substrates from targets of KTN with x=0.45. The effects of substrate temperature (300–700°C) and ambient oxygen pressure (50 and 300 mTorr) on the characteristics of films were investigated. At 500°C and 300 mTorr, films were amorphous with a few isolated, randomly-oriented crystalline grains of perovskite and pyrochlore. At 600°C and 300 mTorr, the films comprised a columnar microstructure consisting of a mixture of amorphous phase with (100) perovskite. At 650°C, films were almost entirely (100) perovskite. At 700°C, the films were predominantly (100) perovskite, but contained a much higher fraction of pyrochlore. At 50 mTorr, pyrochlore was found in significant fractions up to 650°C. At 700°C, films were predominantly (100) perovskite. Rutherford Backscattering analysis of the chemical composition showed that all the films were potassium-deficient and the tantalum to niobimn ratio exceeded that found in the targets for all deposition conditions. There appeared to be a relationship between oxygen pressure during deposition and the amount of potassium retained in the films.
Interfacial Reactions During Processing of Thin Nickel Oxide Films Grown by Pulsed-Laser Ablation
Authors: Kotula Paul G. , Carter C. Barry
Published: March 2011

The study of oxide solid-state reactions is a natural application for pulsed-laser ablation (PLA) because high quality films of virtually any oxide composition can be fabricated. In this study, the effect of interfacial morphology on reaction kinetics in the model oxide system of nickel oxide (NiO) and alumina (α-Al2O3) is investigated. Thin films of NiO have been grown on single-crystal substrates of basal (0001) α-Al2O3 by PLA in order to study the initial stages of the nickel-aluminate spinel (NiAl2O4) phase transformation. The initial state, a 100 nm NiO film on (0001) α-Al2O3, is characterized with both plan-view and cross-section transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The final state of the spinel reaction layer between the NiO and α-Al2O3 after various heat-treatments is also characterized by the same techniques.
Species Temporal and Spatial Distributions in Laser Ablation Plumes
Authors: Hastie John W. , Bonnell David W. , Paul Albert J. , Schenck Peter K.
Published: March 2011

The intermediate species present in laser ablation plumes, particularly those formed during pulsed laser deposition (PLD) of thin films, have been identified for a variety of advanced materials systems. Optical multichannel analysis spectroscopy has been used to monitor the atomic and ionic species present, via their spectral emissions. Molecular beam-sampling mass spectrometry has been used to monitor the molecular species present, in addition to atoms and ions. With both monitoring approaches, temporal and spatial species distribution information has been obtained. Velocity distributions, obtained from the time-dependent mass spectral studies, show the effects of isentropic expansion to be predominant when compared with gasdynamic models of the plume evolution process. Also, the plume structure was found to be particularly sensitive to target elemental distribution. Examples of systems studied include high temperature superconductors, refractory compounds, ferroelectrics and nanostructured magnetic films.
Unique Features of Laser Ablation
Authors: Hanabusa Mitsugu
Published: March 2011

The advantage and disadvantage of laser ablation as a thin Film deposition technique are discussed on the basis of my own experiences. High instantaneous growth rate is useful to produce amorphous films, and reactive deposition can be achieved by gases introduced in the deposition chamber. Combining these two features, hydrogenated amorphous silicon films were produced. Laser ablation is characterized by the presence of energetic neutrals and ions in beams, which helped to deposit diamond-like carbon films. Also selective-area deposition is made possible by placing a mask between the target and substrate. In pulsed laser ablation the mask must be placed close to the substrate to avoid the spreading of deposits. The thickness profilr can bc controlled by residual gas pressure, and using this technique microlenses were fabricated, in addition to ridge-type optical waveguides. The particle problem which plagues laser ablation is more serious for targets with low fractional ionic character. The usefulness of a molten target as the means to eliminate particles was confirmed for aluminum.
Ion-Assisted Pulsed Laser Deposition of Cubic Boron Nitride on Si (100) Substrates
Authors: Friedmann T. A. , Mccarty K. F. , Klaus E. J. , Johnsen H. A. , Medlin D. L. , Mills M. J. , Ottesen D. K. , Stulen R. H.
Published: March 2011

We are studying the boron nitride system by using a pulsed excimer laser to ablate from hexagonal BN (hBN) targets to form cubic BN (cBN) films. We are depositing BN films on heated (400°C) Si (100) surfaces and are using a broad beam ion source operated with Ar and N2source gasses to produce BN films with a high percentage of sp3-bonded cBN. The best films to date show ∼85% sp3-bonded BN as determined from infrared (IR) reflection spectroscopy. High resolution transmission electron microscopy (TEM) and selected area electron diffraction (SAD) confirm the presence of cBN in these samples. The films are polycrystalline and show grain sizes up to 500 Å.
The Optimization of Niobium Diselenide Thin Films Through Control of Pulsed Laser Deposition Parameters
Authors: Day Allan E. , Laube Samuel J.P. , Donley M.S. , Zabinski J.S.
Published: March 2011

Niobium diselenide has potential for use as a conductive lubricant, but to achieve the optimal properties of low friction coefficient, high conductivity and oxidation resistance, the SeJNb ratio and crystallinity must be carefully controlled. It has been shown that Pulsed Laser Deposition (PLD) permits the required degree of control, even over films with complex stoichiometries. (4–8). In this study, PLD was used to grow stoichiometric, crystalline thin films of niobium diselenide and to study the effects of laser deposition parameters on film properties. Film chemistry and crystallinity were evaluated using XPS, RBS, and glancingangle XRD. Friction and wear measurements were taken on a ball-on-flat tribometer. The deposition apparatus incorporates a fully computerized data acquisition and control system that facilitated the correlation of the laser deposition parameters to film properties. This study has shown that film chemistry could be changed from substoichiometric to superstoichiometric and crystallinity varied between amorphous to highly crystalline by appropriate choice of PLD parameters. The property correlations and acquisition system that permitted the identification of the optimal growth conditions will be described.
Structural Analysis of Carbon Thin Films Deposited by Pulsed Laser Deposition
Authors: Capano M.A. , Qian F. , Singh R.K. , Mcdevitt N.T.
Published: March 2011

The objective of the current study is to gain a greater understanding of the atomic structure of carbon films deposited by UV-pulsed-laser ablation. Films deposited onto Si substrates at 293 K, 473 K, 673 K, and 873 K are characterized using X-ray reflectometry and Raman spectroscopy. Film densities are shown to initially decrease from a maximum density of 2.39 g/cm3 as temperature increases. Above 673 K the density remains constant. When compared with Raman spectroscopy data, the reflectometry data are shown to be sensitive to microstructural changes within the films. The possibility of using density measurements as a boundary condition for more quantitative analysis is discussed. Also, Raman spectroscopy indicates that the graphitic domain size initially decreases, passes through a minimum, and then increases as temperature increases. The amount of bond-angle disorder is shown to decrease with increasing substrate temperature. However, an upward shift in frequency of the Raman D peak is not observed with the presumed decrease in sp3 bonding as the deposition temperature increases. These and other experimental results are presented and discussed.
Synthesis of Sic Fine Particles by Gas-Phase Reaction Under Short-Time Microgravity
Authors: Okutani Takeshi , Nakata Yoshinori , Suzuki Masaakt , Maniette Yves , Goto Nobuyoshi , Odawara Osamu , Mori Kiyoshi
Published: March 2011

SiC fine particles were synthesized by the gas-phase thermal decomposition of tetramethylsilane (Si(CH3)4) in hydrogen under microgravity of 10−4G for 10 sec. Rapid heating to the temperature over 800°C which is required for thermal decomposition of Si(CH3)4) under short-time microgravity was attained using a chemical oven where the heat of exothermic reaction of combustion synthesis of Ti-A1-4B composites was used as the heat source. Monodisperse and spherical SiC fine particles were synthesized under microgravity, whereas aggregates of SiC fine particles were synthesized under 1 G gravity. The SiC particles synthesized under microgravity (150-200 nm) were bigger in size and narrower in size distribution than those under 1 G gravity (100-150 nm).
Synthesis of Nanocrystalline Fe-based Particles by CO2 Laser Pyrolysis
Authors: Bi Xiang-Xin , Eklund Peter C.
Published: March 2011

Nanocrystalline Fe (α and γ phases), Fe-carbide (Fe3C, Fe7C3 ), Fe-sulfide (Fel-xS), Fe-nitride (Fe3N and Fe4N) particles with narrow size distributions have been produced using a CO2 laser pyrolysis of vapor mixtures of Fe(CO)5 with C2H4, H2S or NH3. Typical production rates of ∼ 1 - 3 g/hr were observed. The chemical phase and size of these nanoscale particles can be controlled by reaction parameters such as laser intensity and gas composition. Pyrolytic carbon coatings were found to be present on the Fe-carbide particles, but absent in the Fe-sulfide and -nitride particles.
Synthesis of Nanocrystalline Co-W Alloys
Authors: Osmola D. , Renaud E. , Erb U. , Wong L. , Palumbo G. , Aust K.T.
Published: March 2011

Electroplating is one of the most technologically advanced methods to synthesize nanophase materials both as coatings and in bulk form. This paper demonstrates the formulation of a processing “window” for nanocrystalline Co-W alloys using factorial design. The microstructural evolution from polycrystalline to nanocrystalline to amorphous Co-W microstructures is shown for electrodeposits produced from a bath containing cobalt sulphate, sodium tungstate, Rochelle salt and ammonium chloride. The effect of operating variables such as temperature and current density is discussed.
Transformations and Hydrogen Storage in Ball-Milled Iron-Titanium
Authors: Tessier P. , Zaluski L. , Yan Z.-H. , Trudeau M.L. , Bormann R. , Schulz R. , Ström-Olsen J.O.
Published: March 2011

Nanocrystalline and amorphous Fe-Ti at or close to the equiatomic composition have been prepared by ball milling, starting either from the elemental components or from the pre-melted intermetallic. The final structure, whose evolution has been monitored by Xray diffraction and transmission electron microscopy, is found to be critically dependent on milling conditions and contamination level.

Hydrogen absorption properties of both the amorphous and nanocrystalline alloys have been investigated. These properties are substantially modified by the reduction in grain size. We will also discuss the influence of the activation process and repeated hydrogen charging on substitutional disorder and grain size.
Luminescence of Rare Earth Metalorganic Complexes in Sol-Gel Derived Host Materials
Authors: Matthews Lowell R. , Knobbe E. T.
Published: March 2011

The narrow, intense emission bands of europium(III) make its compounds of interest for various optical source applications. Previous studies have shown that europium thenoyltrifluoroacetonate complexes, e.g. Eu(TTFA)3, have superior fluorescence properties in select media with respect to inorganic salts like EuCl3. It has been shown that such complexes may be incorporated into solid-state matrices via the sol-gel process. The present research examines the luminescence behavior of a metalorganic complex (Eu(TTFA)3) doped into a sol-gel derived silica-acrylate ORMOSIL host and extends our earlier work on doped silica gel materials. The present study focuses on the effects of dopant concentration and dopant-matrix interaction, and seeks to compare the behavior of Eu(ITFA)3 in silica and ORMOSIL gel host materials.
SOL-GEL DERIVED SENSOR MATERIALS FOR USE IN AQUEOUS TRANSITION METAL ION DETECTION
Authors: Yates L. M. , Arbuthnot D. , Harmon H.J. , Knobbe E. T.
Published: March 2011

Opto-chemical transducers, which exhibit changes in luminescence and/or absorption characteristics as a function of local conditions, are currently of great interest for use in performing quantitative chemical analyses. Present research is focused on the preparation and characterization of novel optrode materials which may be remotely interrogated via optical fiber carriers. The approach under investigation involves preparation of sol-gel derived films which are sensitized by the entrapment of analyte-complexing organic probe molecules. Zn(II)aq induced perturbations to the luminescence behavior of a porphyrin derivative entrapped within a porous sol-gel derived host matrix are detailed.
Nanocomposite Dielectric Material: Au/SiO2-gel
Authors: Vezzoli G.C. , Chen M.F.
Published: March 2011

A nanocomposite of clusters of gold atoms in a silica gel matrix has been prepared by the sol-gel technique through reduction of gold chloride. This composite was fabricated to explore novel techniques for materialsengineering new high dielectric strength substances for capacitor applications. The gold islands amidst the silica gel sea are materials-designed to utilize the percolation threshhold phenomenon to create a peak in the dielectric

constant as a function of optimal topology. Experimental results indicate an average dielectric constant of the order of 5000 at 1 kHz, and a maximum versus temperature at ∼ −100°C. Electron microscope and microprobe analysis indicate ellipsoidal Au particles or clusters of a dimension ranging from a hundred angstroms to several thousand angstroms.
Lessons From Biology for Materials Scientists
Authors: Birchall J. Derek
Published: March 2011

Living organisms make use of inorganic materials such as calcium carbonate and phosphate, silica, iron oxide etc. to construct support structures, protective armour, magnetic devices gravity devices and so on. The structures are remarkably adapted for the duties they have to perform, the adaptation being at all levels from the nanometer to the macroscopic. Microscopic detail is controlled in order that the macroscopic structure forms an efficient functional component of a machine and the structure develops in response to signals from the local environment. The study of these biogenic minerals and the mechanism of their formation can inspire new approaches to the synthesis and design of new materials and to the advantageous modification of old materials. Examples are given in this paper.
Oxide and Metal Intercalated Clay Nanocomposites
Authors: Malla P. B. , Komarneni S.
Published: March 2011

Truly nanocomposite materials that are stable to about 400 to 700°C can be prepared by intercalating oxides or metal clusters of about 0.4 to 2.0 nm in between ∼1.0 nm layers of smectite clays. Both the chemistry and size of intercalates (pillars) can be varied to introduce unique catalytic, molecular sieving, dehumidifying and adsorption properties in these materials. The intercalated clays also provide opportunities to prepare compositionally and stoichiometrically diverse nanocomposite precursors to high temperature structural and electronic ceramics. Although montmorillonite is the most widely used host, further designing in properties can be achieved by using other members of smectite family having subtle crystal chemical and compositional variations, such as beidellite, nontronite, saponite or hectorite. The sol-gel chemistry involving the preparation of positively charged mono- or multiphasic solution-sol or colloidal-sol particles is a viable approach to introduce chemically diverse oxide particles in the interlayers of smectite. Reduction of transition metal ions or complexes in the interlayers of smectite to zerovalent metal clusters/particles using polar liquids is another novel approach to develop catalytically active, high surface area materials.
Preparation of Nanophase SiC/Si3N4, SiC/B4C and Stoichiometric Sic From Mixtures of A Vinylic Polysilane and Metal Powders
Authors: Jrang Zhiping , Rhine Wendell E.
Published: March 2011

Pyrolysis of a vinylic polysilane (VPS, Union Carbide Y12044) under argon at 1600 °C resulted in the formation of SiC with 20% residual carbon. The excess carbon was transformed to B4C or SiC by mixing B or Si powder with VPS and pyrolyzing the mixture under a flow of argon. When excess Si was added and the mixture was pyrolysed under N2 at 1395 °C, a SiC/ Si3N4 composite was obtained. All the products showed a fine-grained microstructure and excellent homogeneity. The precursors and the pyrolysis products were characterized by means of FIIR, TGA, XRD, SEM, STEM, BET, and elemental analysis.
Effect of the Matrix on the Oxidation of Nanostructured Ge Granular Films
Authors: Waite M.M. , Shah S. Ismat , Doele B.A.
Published: March 2011

Nanostructured Ge granular thin films were prepared by magnetron sputtering of composite targets pressed from mixtures of Ge and ceramic powders. The ceramics used were Al2O3, MgO and SiO2. Films in the Gex(Al2O3)l-x system were composed of Ge particles 10-60 nm in size uniformly embedded in the ceramic matrix, particularly for low Ge concentration films. X-ray photoelectron spectroscopy (XPS) was used to characterize these films. Samples were identified according to the coordination of Ge with the oxygen as (i) Ge-Ge4, (ii) Ge-Ge3O, (iii)Ge-Ge2O2, (iv) Ge-GeO3 and (v) Ge-O4. Using the modified Sanderson model1 and its modifications2, we found Ge oxidation to be largely composition dependent. Similar results were obtained for Ge granular films in MgO and SiO2 matrices.
Microstructural Development in Nano-Crystalline Zinc Sulfide Powders
Authors: Han Y. , Voiles E.T. , Chumbley L.S. , Akinc M.
Published: March 2011

Monodispersed, spherical, submicron size zinc sulfide powders were previously prepared in our laboratory utilizing homogeneous precipitation by thermal decomposition of thioacetamide in acidic aqueous solutions. Electron microscopic investigations of these powders indicated that the individual particles were agglomerates of 8∼14 nm size crystallites. It has been known that zinc sulfide does not sinter under ambient pressure due to the covalent nature of its bonding, high melting point, and relatively high vapor pressures at elevated temperatures. The heat treatment of these nano-crystalline zinc sulfide powders, however, showed some degree of sintering at temperatures as low as 600°C under vacuum.
The Influence of Powders and Processing Methods on Microstructure and Properties of Dense Silicon Nitride
Authors: Woetting G. , Feuer H. , Gugel E.
Published: March 2011

Experiments with commercially available as well as with specially prepared Si3N4-powders were performed in order to estimate the extent of influence of the powder properties and the homogeneity of the additive distribution on the sintering behaviour, the microstructure and on mechanical properties of the dense sintered materials.

The quality of the processing steps was examined by dilatometry which also proved to be a useful tool for deducing appropriate sintering conditions for different Si3N4-powder/additive systems.

The effects of compositional variations and processing on microstructure and mechanical properties are discussed with respect to strength, reliability (Weibull analysis) and fracture toughness. Some important factors to be considered for the production of highperformance, reliable silicon nitride materials and components are pointed out.

The careful analysis of possible influences on the final ceramic resulted in the production of a high-quality Si3N4-material which is used for engine valves, bearing balls, etc.
Siaun Ceramics: Retrospect and Prospect
Authors: Jack Kenneth H.
Published: March 2011

The future for sialon ceramics is bright. Engine components are slowly increasing in numbers and variety, and applications for wear parts and molten metal handling are set to flourish.

Two major developments have been: (i) the production of useful sialon composites; and (ii) the ability to join pieces to make large and complex components.

In (i), properties of a′:8′ sialon composites can be varied by processing modifications in ways that are impossible for single-phase materials. Also, densifying additives such as yttria and rare-earth oxides can subsequently be incorporated into the α′-structure to minimize the amount of intergranular glass. The potentialities of other composites including O′:B′, O′:ZrO2, B′:TiN and B′:BN are assessed.

In (ii), the joining of sialons follows from an understanding of phase relationships, glass formation and glass-ceramics in the Y-Si-Al-O-N system. Its exploitation has greatly widened the field of sialon applications.
Structural Characterization of Silicon Carbonitride Ceramics from Polymeric Precursors using Nuclear Magnetic Resonance
Authors: Gerardin Corine M. , Taulelle F. , Livage J.
Published: March 2011

The pyrolytic conversion of a polyvinylsilazane precursor to Si-C-N ceramics is studied using solid-state NMR. 13C and 29Si magic angle spinning and 1H static NMR experiments allow us to investigate the structure of the disordered multiphased intermediates formed during the pyrolysis. The Si environments are quantitatively analyzed by 29Si MAS NMR spectra simulations. The relative abundance of SiHCN2, SiC2N2, SiN3C and SiN4 sites at different temperatures are used to calculate elemental compositions that characterize the Si-containing phase. The comparison of these compositions with those obtained from chemical analyses shows evidence of the presence of a free carbon phase.We have also determined the H and C contents in the free carbon phase.
Effect of Powder Processing on Slip Casting Parameters and Sintering of Si3N4-SiO2-Y2O3
Authors: Gilde Gary , Gazza George
Published: March 2011

A procedure was developed for slip casting Si3N4-Y2O3 - SiO2 compositions. The silicon nitride was oxidized to supply the silica. The procedure resulted in 65 w/o slips with viscosities of 35 mPa s which when cast, yielded compacts which were 56 % of theoretical density. Slips with 70 w/o solids loading were also prepared having viscosities of 120 mPa.s. The properties of the slips were influenced by the starting silicon nitride powder, dispersant, and milling media. The resulting compacts could be gas pressure sintered to full density, at temperatures as low as 1850 C, using nitrogen pressures as low as 4 MPa. Strengths as high as 750 MPa were achieved and the initial results show good creep resistance at 1350 C.
A Novel Processing Route for the Fabrication of Monolithic and Composite Silicon Nitride
Authors: Raman R. V. , Rele S. V.
Published: March 2011

Current hot isostatic consolidation methodology used for the fabrication of complex-shaped Si3N4-based components requires the use of an expensive glass encapsulation technique and extended thermal exposure (in hours) of the specimen. An alternative consolidation approach involving the use of solid pressure transmitting media under high pressure, has enabled the consolidation of Si3N4 alloys without the need for glass encapsulation.

Characterization of microstructures and mechanical properties of this (MOR, fracture toughness) material has been carried out and will be presented. It has been noted that in Si 3N4/8%Y2O3-4%Al2O3 composition, consolidated using this approach, a significantly larger volume fraction of α phase has been retained compared with typically observed conversion in α⇒ β in hot isostatically pressed material or sintered material.

Key issues for addressing densification and microstructure control using this process are presented. This rapid consolidation approach appears to be a promising alternative to hot isostatic pressing for the fabrication of complex-shaped Si3N4 components.
Characterization of HIP'ed, High Purity Si3N4 Grain Boundaries
Authors: Lu Ping , Danforth S.C. , Symons W.T.
Published: March 2011

Laser synthesized, ultra-fine, amorphous, Si3N4 powders were densified via HIP'ing without any oxide sintering aids. Exposed samples were made from powder that had been exposed to the atmosphere, thereby picking up an oxide surface layer, and unexposed samples were made from powders processed entirely under glove box conditions, i.e. without oxygen contamination. TEM (and sintering) studies indicate that the exposed samples HIP'ed at temperatures in excess of the melting point of Si02, densified via a solution-reprecipitation mechanism, with a resultant intergranular glassy phase of high purity Si02. In contrast, unexposed samples had to be HIP'ed to 2050°C to achieve a density of ∼70 %p Th. These samples consisted of equiaxed β-Si3N4 grains, with localized high density regions where no inter-granular phase (crystalline or glassy) was detected to within 0.66 nm.
Transient Viscous Phase Reaction Sintered (Tvprs) Silicon Oxynitride Ceramics.
Authors: Plucknett Kevin P. , Wilkinson David S.
Published: March 2011

High density silicon oxynitride ceramics were fabricated by reaction-sintering a mixtureof silicon nitride and silica without the use of sintering aids. Precursor silicon nitride compacts were prepared by conventional means after which they were subjected to a lowtemperature oxidation heattreatment (∼1000ºC) producing a composite silicon nitride/silica compact. Oxidized compacts were then reaction-sintered in a nitrogen atmosphere at temperatures between 1400 and 1800ºC using a range of protective ‘powder-bed’ compositions. A ‘powder-bed’ comprising a mixture of boron nitride, silicon nitride and silica was found to be most effective in preventing decomposition and subsequent weight loss. Nearly complete reactive transformation to silicon oxynitride was observed under optimised sintering conditions.
Microstructural Design by Selective Grain Growth of β-Si3N4
Authors: Hirosaki Naoto , Akimune Yoshio , Mitomo Mamoru
Published: March 2011

Raw β-Si3N4 powder was gas-pressure sintered with Y2 O3-Nd2O3additives at > 1700ºC. Graingrowth behavior was investigated in relation to sintering conditions. Selective growth of large grains was accomplished by sintering the powder at high temperatures with small amounts of additives. As a result, in-situ composites were obtained from β-powder.

The desired material properties have been attained by controlling the microstructural design using large grains. Materials with high reliability, having a Weibull modulus of about 50, were fabricated by maintaining a uniform size and distribution of elongated grains. Tough materials, having fracture toughness of, were developed by increasing the diameter of elongated grains. This method was applied to the sintering of refractory grade powder with the aim of lowering sintered material cost. Fairly good mechanical properties have been obtained even with impure powders.
Grain Morphology and Intergranular Structure of Si3N4 Based Ceramics Formed by Hip
Authors: BjÖrklund H. , Falk L. K. L. , WasÉn J. , Adlerborn J. E. , Larker H. T.
Published: March 2011

The microstructures of unreinforced and reinforced Si3N4 ceramics have been characterized by analytical electron microscopy in combination with quantitative microscopy. Special attention was paid to the effect which different additives, viz. metal oxides and SiC- and β-Si3N4-whiskers, have upon matrix morphology and intergranular microstructure. It was demonstrated that SiC-whiskers may suppress Si3N4 grain growth while an addition of β-Si3N4-whiskers results in a coarser Si3N4 microstructure. The different microstructures have been related to the mechanical properties of the ceramics.
Cavity Evolution During Tensile Creep of Si3N4
Authors: Luecke William , Wiederhorn S. M. , Hockey B. J. , Long G. G.
Published: March 2011

We have characterized the evolution of cavities during tensile creep of a Y2O3-hot isostatically pressed Si3N4, using precision density measurements, small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). The cavities are bimodally distributed in size. Lenticular, 200 nm-size cavities are common, and lie primarily on two-grain boundaries. Irregularly shaped 500-1000 nm-size cavities are rare and lie at multi-grain junctions, but comprise approximately half of the total volume fraction of cavities. Although the material shows a continuous decrease in strain rate with strain, the cavity volume fraction evolves linearly with strain. Cavities account for approximately 85% of the total strain at any point during creep.
Mechanical Properties of Silicon Carbide Whisker-Reinforced Silicon Nitride Matrix Composites
Authors: Huang Yong , Le Huirong , Li Jianbao , Zheng Longlie , Wu Jianguang
Published: March 2011

In the present paper, the effect of La2O3-Y2O3-Al2O3 (LYA) and La2O3-Y2O3-MgO (LYM) additives on the mechanical properties of hot pressed SiC whisker-reinforced Si3N4 matrix composites and the relationship between whisker content and additive content were investigated in detail. The results show obvious strengthening in the case of LYA additives containing ≤2.4 wt% Al2O3. When the Al2O3 content of the additive was > 1.2 wt%, the fracture toughness decreased accompanied by decreased interfacial debonding and whisker pull-out. With LYM additives containing 2.4 wt% MgO, both the room temperature strength and toughness increased. A combination of Y2O3 + MgO (YM) with LYA additives produced substantial improvements in strength and toughness, up to 1050 MPa and ∼11 MPa√m, respectively. The Weibull modulus of a SiC whisker-reinforced composite was found to be greater than that of either the unreinforced Si3N4 or Si3N4 containing SiC particles. The experiments also demonstrated that in order to achieve full density the additive content had to be increased with increased whisker content. Preliminary but encouraging results on strength retention at elevated temperatures were also reported.
The Mechanical Properties of a Novel Si3N4-Amorphous Si3N4 Composite
Authors: Reimanis Ivar E. , Petrovic J. J. , Suematsu H. , Mitchell T. E. , Leung O. S.
Published: March 2011

The hardness and fracture toughness of a model two-phase composite consisting of crystalline Si3N4 particles in a matrix of amorphous Si3N4 are examined. The composite is created by heat treating high purity, partially amorphous CVD Si3N4 in N2 for various times and temperatures in order to induce crystallization of the a phase. Microindentation tests at temperatures up to 1200 °C are conducted to evaluate the high temperature hardness and fracture toughness. The role of the microstructure is examined using optical and transmission electron microscopy. Finally, the relationship between the microstructure and the mechanical properties is discussed.
New Grain-Boundary Phases for Nitrogen Ceramics
Authors: Thompson Derek P.
Published: March 2011

The drive for improved refractoriness in nitrogen ceramics, which has motivated the removal of residual glass from grain-boundaries by heat-treatment at sub-solidus temperatures, has resulted in the discovery of a wide range of crystalline oxynitride phases. Generally, these phases are oxygen-rich, and can be classified in the same way as the mineral silicates, in which oxygen atoms are coordinated to a maximum of two silicon atoms, with SiO4 tetrahedra joined at either 0,1,2,3 or 4 corners to other tetrahedra. However, the field of oxynitride crystal chemistry is wider than this, because nitrogen in SiN4 tetrahedra commonly occurs coordinated to three silicon atoms, and an additional range of structures exist with nitrogen in this coordination.

Oxynitride analogues of mineral silicates are well known in neso-, soro- and cyclostructure types; new structures have recently been reported which belong to the pyroxene family (ino-silicates), and information on their preparation and crystal chemistry is reported here. The sialon U-phase (typical composition Ln3Si3Al3O12N2, Ln = La,Ce,Nd,Sm), is an example of an oxynitride with a structure intermediate between layer (phyllo-) and framework (tecto-) types. Sialon W-phase (approximate composition Ln4Si9Al5O30N, Ln = La,Ce,Nd) has not been completely characterized, but appears to have a structure related to the amphibole group of double-chain silicates.

Comments are made on the many oxynitride structures which are still uncharacterized. The suitability of all these oxynitrides as grain-boundary phases in silicon nitride and sialon ceramics is discussed.
Oxynitride Glasses and Glass Ceramics
Authors: Hampshire Stuart
Published: March 2011

Silicon nitride based ceramics contain oxynitride glass phases at the grain boundaries which can impair subsequent high temperature properties. Investigations of bulk glasses in various M-Si-Al-O-N systems have been carried out and it has been shown that up to 15 atomic % N can be incorporated into these oxynitride glasses. Studies have revealed that nitrogen increases the viscosity, hardness and glass transition temperature of the glasses. Heat treatments of the glasses to form crystalline phases have been reported but further improvements are possible if glass-ceramic processes using two-stage heat treatments are introduced. The development of oxynitride glasses and the effects of nitrogen on properties are reviewed and the optimisation of glass-ceramic heat-treatments are reported.
Rheology and Microstructure of Concentrated Zirconia-Alumina Suspensions for Gelcasting Composites
Authors: Bleier A. , Omatete O. O.
Published: March 2011

The relationships among colloidal stability, suspension rheology, and solids loading are elucidated for zirconia-alumina mixtures containing 20 vol% ZrO2, based on solids. The lower colloidal stability of ZrO2 limits the rheological properties of this system. If the zeta potential of ZrO2 is less than 49 mV, high degrees of pseudoplasticity, high yield stress, high viscosity, and long relaxation times characterize the binary suspensions. These effects occur, despite the fact that ZrO2 is the minor ceramic constituent. If the zeta potential of ZrO2 is maintained above 49 mV, suspensions with high solids loading (55 vol%) can be prepared which behave as Newtonian fluids over the 0-to-200 s−1 shear rate range and as an elastic solid at higher rates. As the solids loading of a highly stable binary suspension is increased, the rheological properties change. They evolve from those of a near Newtonian-like fluid with nearly independent particles (40 vol%) to those of a pseudoplastic fluid with a weakly interacting particle network (50 vol%) to those of an elastic-like solid composed of crowded, strongly repulsive particles (55 vol%). Low-shear conditions for suspension-transport and mold-filling operations that ensure a homogeneous arrangement of ZrO2 and A12O3 particles in a gelcast ceramic seem promising.
Characterization of Ceramic Batch Via Capillary Rheometry
Authors: Janas Victor F. , Malarkey Christopher J. , Treacy David R. , Zak Michael E.
Published: March 2011

Rheological testing has been developed to predict the extrudability of ceramic pastes. Inorganic batches containing cellulosic binders were mixed in a twin screw extruder prior to characterization in a capillary rheometer. Various die lengths were used to separate viscosity from wall slip parameters. A power law model was fit to the paste viscosity. The wall shear stress, assuming 100% slip, was a linear function of the wall velocity. The resulting rheological models were used in fluid flow finite element analysis for determination of extrusion pressure, and the evaluation of binders.