bluuebunny/crossref_metadata_2025 huggingface dataset:
Designing Cofired Multilayer Electrostrictive Actuators for Reliability
Authors: Winzer Stephen R. , Shankar Natarajan , Ritter Andrew P.
Published: March 2005

Multilayer electrostrictive actuators are large capacitors with the added complexity of having to perform mechanically as well as electrically. High operating fields and severe operating conditions make it necessary to predict and limit stress levels generated in the device. A constitutive three‐dimensional model for electrostrictors has been developed and implemented as a design tool for actuators. The effects of electrode geometry and transition‐layer thickness on stress levels have been modeled and failure in high‐stress regions has been correlated with the presence of flaws (pores and delaminations) in the devices. [Key words: capacitors, electrical properties, stress, multilayer, modeling.]
Review of Mechanically Related Failures of Ceramic Capacitors and Capacitor Materials
Authors: Freiman Stephen W. , Pohanka Robert C.
Published: March 2005

This paper reviews the brittle fracture behavior of dielectric ceramics such as barium titanate, and describes some of the relationships between defects such as cracks and electrical degradation and failure of multilayer capacitors. Stresses arising from the ferroelectric phase transformation in these dielectric materials are shown to play a part as a driving force for crack growth. In addition, possible contributions to failure from stresses arising from thermal excursions in the capacitor are discussed. Low‐voltage failures arising from a short between the electrodes in multilayer capacitors are shown to be related to the growth of cracks in the dielectric. A technique for predicting the onset of these types of failures based upon fracture mechanics techniques is described. Possible effects of the electric field itself in promoting or retarding the growth of cracks are discussed.
Improving Thermal Shock Test Reliability
Authors: Power Oliver Kenneth
Published: March 2005

The failure rate required of surface mount multilayer ceramic capacitors is so low that testing at use conditions will not detect important shifts in reliability performance. Multilayer ceramic capacitors manufactured from various dielectrics were tested at multiple temperatures to develop a sensitive test and to understand failure mechanisms occurring during wave solder. Test temperatures of the solder wave ranged from 240° to 450°C. Effects of preheat, flux type, and machine differences, as well as various termination and plating parameters, were also evaluated. Results from these production experiments were used to develop test conditions to screen commercial parts to 1 or 2 orders of magnitude lower failure rates and to study production processes and materials to further improve the reliability of ceramic chip capacitors. This paper documents the steps in developing an accelerated thermal shock test.
Characterization of the Microstructure of Ceramics Used in Multilayer Ceramic Capacitors by Means of the Scanning Laser Acoustic Microscope
Authors: Kessler Lawrence W. , Semmens Janet E.
Published: March 2005

Analytical data obtained by the scanning laser acoustic microscope (SLAM) on multilayer ceramic capacitors can be correlated to microstructural elastic properties of the dielectric and electrode materials, to the quality of bonding between the layers, and to construction variables, such as materials thicknesses, number of layers, etc. These data can be used to build up predictive models of high‐quality and poor‐quality components, which is very important for nondestructive screening of high‐reliability parts. Although SLAM screening of multilayer capacitors (such as specified in MIL‐C‐123) is quite straight forward and in current use for quality control, dilemmas can occur in which batches of nominally identical parts demonstrate widely different ultrasonic transmission and widely different SLAM images. These differences correspond to elastic property differences which may affect reliability. In this paper the authors discuss materials characterization aspects of acoustic microscopy as it pertains to multilayer ceramic capacitors. [Key words: capacitors, multilayer, ultrasonics, electrical properties, lasers.]
Structural and Mechanical Property Changes in Toughened Magnesia‐Partially‐Stabilized Zirconia at Low Temperatures
Authors: Marshall David B. , Jarnes Michael R. , Porter John R.
Published: March 2005

The mechanical properties of high‐toughness magnesia‐partially‐stabilized zirconia were found to be dramatically altered by a single cooling cycle between room temperature and − 196°C. Raman spectroscopy and X‐ray diffraction were used to correlate the changes in mechanical properties with structural changes that occur at temperatures below ∼− 100°C. Most of the tetragonal precipitates that are responsible for toughening transformed to an orthorhombic phase with unit‐cell volume intermediate between those of the tetragonal and monoclinic phases. The orthorhombic phase was stable with heating to 300°C, but it transformed back to the tetragonal structure when heated to 400°C. Surprisingly, the orthorhombic phase was not readily transformable by stress, with the consequence that, after the cooling cycle, most of the high‐toughness properties of the original tetragonal‐containing material were lost.
Grain‐Boundary Migration in Nonstoichiometric Solid Solutions of Magnesium Aluminate Spinel: I, Grain Growth Studies
Authors: Chiang Yet‐Ming , Kingery W. David
Published: March 2005

The grain‐boundary mobility in magnesium aluminate spinel (MgO · nAl2O3) of magnesia‐rich (n < 1) and alumina‐rich (n > 1) compositions has been measured from normal grain growth in dense, hot‐pressed samples. Over the temperature range 1200° to 1800°C, the mobility in magnesia‐rich compositions is found to be greater than that in alumina‐rich compositions by a factor of 102 to 103. Within the alumina‐rich field, the mobility varies by less than a factor of 10 over the composition range 1 > n > 1.56. Nearly stoichiometric spinels (1 < n < 1.07) form a variety of sources and starting materials exhibit boundary mobilities within a factor of 5 at fixed temperature, showing an impurity tolerance which has not been found in other ionic solids.
Oxidation Resistance Evaluation of Silicon Carbide Ceramics with Various Additives
Authors: Maeda Minoru , Nakamura Kazuo , Yamada Mamoru
Published: March 2005

Five silicon carbide ceramics with various additives were evaluated for oxidation resistance at 1300°C in flowing dry and wet air. In the dry atmosphere, the oxidation of the five samples was diffusion‐controlled, and in wet atmosphere they exhibited a linear relation beween weight gain by oxidation and water vapor content. Water vapor in the atmosphere strongly accelerated oxidation. The influence of oxidation on room‐temperature strength was complex, but the samples were not as affected by oxidation.
Spray‐Drying Ceramic Powders
Authors: Lukasiewicz Stanley J.
Published: March 2005

Spray‐drying is a convenient process for producing a granulated, flowable powder of uniform bulk density. In this review the effect of feed slurry solids content, viscosity, binder type, and atomization technique on the granule character are discussed. Also, mechanisms of hollow granule formation and binder migration are reviewed.
Thermodynamics of Densification: I, Sintering of Simple Particle Arrays, Equilibrium Configurations, Pore Stability, and Shrinkage
Authors: Kellett Bruce J. , Lange F. F.
Published: March 2005

Equilibrium configurations for linear and closed arrays (rings and regular polyhedra containing a single pore) of identical particles (cylinders or spheres) were determined by minimizing the array's surface and grain‐boundary energies with the assumption that each particle conserves its mass. The change in free energy between the initial and equilibrium configuration increases with dihedral angle (i.e., the equilibrium angle). More significantly, it is shown that pores will shrink to an equilibrium size if the number (n) of coordinating particles is greater than a critical value. The critical pore coordination number (nc) increases with the dihedral angle. Only pores withnncare thermodynamically unstable during sintering. It is also shown that any mass‐transport mechanism can lead to pore shrinkage while a connecting path to the pore surface remains open. The effective sintering “stress” (i.e., driving force) increases with the dihedral angle and decreases to zero as the equilibrium configuration is reached. Sintering stresses increase with decreasing coordination number. It is also shown that the shrinkage strain for closed arrays increases with the pore coordination number. Rearrangement phenomena within a powder compact are discussed with regard to resultant sintering forces on nonsymmetrically coordinated particles.
Microstructural Studies of the Interfacial Zone of a SiC‐Fiber‐Reinforced Lithium Aluminum Silicate Glass‐Ceramic
Authors: Bischoff Ewald , Rühle Manfred , Sbaizero Orfeo , Evans Anthony G.
Published: March 2005

Transmission electron microscopy studies have been conducted on interfaces in a lithium aluminum silicate/SiC‐fiberreinforced composite. In the as‐processed state, interphases of amorphous C and carbides of Nb have been confirmed, with circumferential thermal debonds evident in the C layer. After heat treatment in air at 800°C, the C is found to be replaced by amorphous SiO2, and the carbides of Nb replaced by oxides. The SiO2 thickens with exposure time and typically contains circumferential separations. Some Mg and Al diffusion also accompanies the heat‐treatment process and eventually leads to the formation of MgO and Mg silicates in the interfacial zone.
Calculations and Measurements of the Spatial Piezoelectric Response and of the d15 Parameter of PZT5A Ceramic
Authors: Kahn Manfred , Ingel Robert , Lewis David
Published: March 2005

In the analytical section of this paper the magnitudes of the polarization vectors (Pi) of lead zirconate titanate are shown as they vary with the angle of an applied uniaxial stress. This is done by applying a directional cosine matrix to the stress tensor σjk in Pi=dijkσjk. In addition the polarization in the direction of a rotating stress is derived from rotating the dijk tensor. Two‐dimensional as well as three‐dimensional isometric plots of the results are shown. The experimental section describes the preparation of disks poled uniformly at 90° to their thickness and the use of the Berlincourt d33 meter to measure d333 as a function of angle. Very good agreement with the calculated results was obtained. The effect of electrodes on the measurement of d33 and the use of a d31 parameter to permit a d15 determination are also demonstrated. The d333 measurements provided ad15 value with a resolution of ± 8%. Its average value was within 5% of d15 values derived with other techniques.
Indentation Fracture Toughness of Sintered Silicon Carbide in the Palmqvist Crack Regime
Authors: Li Zhuang , Ghosh Asish , Kobayashi Albert S. , Bradt Richard C.
Published: March 2005

The fracture toughness of a sintered dense α‐SiC was estimated by the Vickers indentation microfracture method in the low‐load Palmqvist crack regime. It was observed that the use of simultaneously obtained Vickers hardnesses does not yield reliable fracture toughness values, nor does application of the median‐crack‐derived equations. It is necessary to utilize a load‐independent, crack‐free hardness value with this toughness estimation method. Although several of the curvefitting equations yield similar toughnesses, it is concluded for the Palmqvist crack system in this α‐SiC that the Niihara‐Morena‐Hasselman equation is the only one which yields fracture toughness values in agreement with conventional measurement techniques.
Specimen Size Effect of the Thermal Diffusivity/Conductivity of Aluminum Nitride
Authors: Hasselman D. P. H. , Merkel Gregory A.
Published: March 2005

It has been generally found that the thermal diffusivity/conductivity of AIN measured by the laser‐flash technique decreases with decreasing specimen size. The results of this study indicate that much of this effect can be attributed to the relatively large temperature rises for thin specimens, especially for high‐energy laser pulses, and results from the combination of the noninearity of the commonly used type of IR detector and the strongly negative temperature dependence of the thermal diffusivity of AIN. The experimental results indicate that for reliable data, the specimen temperature rise at thermal equilirium should be kept to less than 1°C by using specimens with a thickness near 3 mm in combination with keeping the pulse energy to a reasonable minimum by attenuation of the laser beam by passing it through an aqueous solution of CuSO4.
Temperature‐Gradient‐Driven Diffusion in Rapid‐Rate Sintering
Authors: Young Robert M. , McPherson Reginald
Published: March 2005

The vacancy diffusion flux, driven by the neck growth curvature (1 /ρ) which contributes to conventional sintering of a powder compact, is theoretically compared with that arising from thermal diffusion, driven by the temperature gradient (grad T/T) during the final stage of sintering. Under conditions experienced in ultrarapid sintering, the thermally driven vacancy flux can exceed the curvature‐driven flux by a factor of 1000. This effect may explain the phenomenon of enhanced densification rates observed in plasma sintering.
Use of Crack Branching Data for Measuring Near‐Surface Residual Stresses in Tempered Glass
Authors: Conway Joseph C. , Mecholsky John J.
Published: March 2005

An analytical procedure based on fracture mechanics is used to obtain the amount of residual stress in glass from measurements on the fracture surface. The technique utilizes the measurement of microcrack branching distances, known as the mirror — mist boundary, which occur at a critical crack branching stress intensity (Km) value. This procedurre shows that σArm1/2YF(θ) =σRr1/2m+Ψ0, where σA is the applied stress, rm is the microcrack branching radius, σR is the residual stress, YF (θ) is the crack‐border correction factor, and Ψ0 is a material constant based on Km. Thus, the equation is that of a straight line with the slope equal to the magnitude of the residual stress. Data for tempered glass from the literature are used to demonstrate the applicability of the technique.
Basic Phosphate‐Bonded Castables from Dolomitic‐Magnesite Clinkers
Authors: Allaire† Claude , Rigaud Michel A. , Dallaire Serge
Published: March 2005

Various basic refractory concretes, using dolomitic‐magnesite clinkers, have been developed using phosphate binders and silica‐rich additives. Three aggregate compositions with CaO/SiO2 ratios varying from 2.01 to 2.31 wt% have been used, with two phosphate binders, differing by their chain length, and two silica additives, one colloidal, one crystalline. Maximum strengths, after curing, drying, and firing at various temperatures, have been attained by controlling the value of the CaO/(P2O5+ SiO2) weight ratio of the concrete mixes close to unity. The value of this ratio has been found to be dependent upon the reactivity of the binder and the silica additive used.
Preparation and Properties of Barium Ferrite Using Hot‐Rolled Mill Scale
Authors: Chien Yung‐Tsen , Pan Hong‐Chih , Ko Yung‐Chao
Published: March 2005

Commercial‐quality barium ferrite was made on a laboratory scale by the solid‐state reaction of raw hot‐rolled mill scale and barium carbonate. The quality of the isotropic magnets reached coercivityHc1750 Oe, remanenceBr2200 G, and maximum energy product (BH)max1.0 × 106G · Oe and that of the anisotropic magnets formed by dry powder die pressingHc2000 Oe,Br3500 G, and (BH)max2.6 × 106G · Oe. The amount of the additives was in the range 0.4 to 0.8 wt%. The processing parameters were reported. The mechanism of ferrite formation was proposed.
Liquid‐Phase Sintering of Aluminum Nitride by Europium Oxide Additives
Authors: VanDamme Nobuko S. , Richard Sarah M. , Winzer Stephen R.
Published: March 2005

Europium oxide has been investigated as a sintering aid and dopant for AIN. Pressureless sintering was carried out with 0 to 9 wt% Eu2O3 additives, and dense sintered specimens were obtained using 1 to 4 wt% Eu2O3. With increasing Eu2O3 content, two additional phases were observed in the X‐ray diffraction patterns. The lattice parameters a and c of AIN in the wurtzite structure changed slightly and non‐monotonically with Eu2O3 content and showed their minimum value in a 4 wt% Eu2O3 sample. The Raman and photoluminescence spectra of sintered specimens were measured. These experimental results and the sintering mechanism in the system were discussed from the standpoint of the effects of oxygen, europium, and stress.
Alumina and Alumina/Zirconia Multilayer Composites Obtained by Slip Casting
Authors: Requena Joaquin , Moreno Rodrigo , Moya Jose S.
Published: March 2005

The slip casting technique has been revealed as a powerful method to obtain multilayer composites close to theoretical density. From zeta potential and viscosity measurements of Al2O3 and Al2O3/ZrO2 (4 vol% ZrO2) suspensions, the corditions for the preparation of multilayer composites by slip casting have been determined. A microstructural analysis of the different layers by scanning electron microscopy is also reported.
Influences of Zirconia and Silicon Nucleating Agents on the Devitrification of Li2O · Al2O3· 6SiO2Glasses
Authors: Hsu Jen‐Yan , Speyer Robert F.
Published: March 2005

The effects of Si and ZrO2dopants on the crystallization and phase transformation process in Li2O · Al2O3· 6SiO2glasses were investigated using differential thermal analysis, X‐ray powder diffractometry (XRD), and high‐resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase‐separated regions in the as‐quenched or heat‐treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β‐quartz(ss) (solid solution), which transformed to β‐spodumene(ss) at higher temperature. Low‐temperature surface crystallization in these glasses occurred as low as 760°C. ZrO2has limited solubility in this glass system. Small ZrO2crystallites (·5 nm) in the as‐quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β‐quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO2. The transformation from β‐quartz(ss) to β‐spodumene(ss) was increasingly inhibited with ZrO2additions. The nucleating efficiency of Si was significantly greater than that of ZrO2in this glass system.
Theory of Drying
Authors: Scherer George W.
Published: March 2005

This review examines the stages of drying, with the emphasis on the constant rate period (CRP), when the pores are full of liquid. It is during the CRP that most of the shrinkage occurs and the drying stresses rise to a maximum. We examine the forces that produce shrinkage and the mechanisms responsible for transport of liquid. By analyzing the interplay of fluid flow and shrinkage of the solid network, it is possible to calculate the pressure distribution in the liquid in the pores. The tension in the liquid is found to be greatest near the drying surface, resulting in greater compressive stresses on the network in that region. This produces differential shrinkage of the solid, which is the cause of cracking during drying. The probability of fracture is related to the size of the body, the rate of evaporation, and the strength of the network. A variety of strategies for avoiding fracture during drying are discussed.
Rare‐Earth Aluminosilicate Glasses
Authors: Shelby James E. , Kohli Jeffrey T.
Published: March 2005

Rare‐earth aluminosilicate glasses of the general formula 20R2O3· 20Al2O3· 60SiO2 have been formed for 10 of the 14 possible rare‐earth oxides. Two series of “mixed‐rare‐earth” glasses were also formed (Nd/Er and Nd/Y). These glasses exhibit exceptionally high glass transformation temperatures, moderate thermal expansion coefficients, and refractive indices of approximately 1.65. The glass transformation temperature and thermal expansion coefficients vary linearly with the field strength of the rare‐earth ion. No evidence of a “mixed‐rare‐earth effect” was observed. MAS‐NMR indicates that the aluminum ions are tetrahedrally coordinated in at least some of these glasses.
Elevated‐Temperature‐Delayed Failure of Alumina Reinforced with 20 vol% Silicon Carbide Whiskers
Authors: Becher Paul F. , Angelini Peter , Warwick W. H. , Tiegs Terry N.
Published: March 2005

Alumina composites reinforced with 20 vol% SiC whiskers were exposed to applied stresses in four‐point flexure at temperatures of 1000°, 1100°, and 1200°C in air for periods of up to 14 weeks. At 1000° and 1100°C, an “apparent” fatigue limit was established at stresses of ∼ 75% of the fast fracture strength. However, after long‐term (>6 weeks) tests at 1100°C, some evidence of crack generation as a result of creep cavitation was detected. At 1200°C applied stresses as low as 38% of the 1200°C fracture strength were sufficient to promote creep deformation and accompanying cavitation and crack generation and growth resulting in failures in times of <250 h.
Oxygen‐Sensing Properties of Spinel‐Type Oxides for Stoichiometric Air/Fuel Combustion Control
Authors: Shimizu Yasuhiro , Kusano Shuji , Kuwayama Hiroyuki , Tanaka Katsuyuki , Egashira Makoto
Published: March 2005

The temperature dependence of electrical resistance of several spinel‐type oxides was measured in exhaust gases of air‐methane combustion, to evaluate the potential of the oxides as an air/fuel stoichiometric oxygen sensor. The results are discussed in terms of sensitivity and response time at the stoichiometric combustion point. It was found that the sensitivity as well as the response time is markedly affected by point defect structures of the oxide specimens. The response time is also related to the catalytic activity of the oxides for complete oxidation of methane. Among the specimens studied, the ZnCr2O4+ Cr2O3 (1.0:0.1 molar ratio) is the most promising as the oxygen sensor, because it exhibits the largest resistance changes of more than 3 orders of magnitude and shortest response times of about 0.5 s when the combustion mixture is switched between the air‐rich versus fuel‐rich composition.
In Situ NMR Study of Dislocation Jump Distance During Creep of Pure Sodium Chloride Single Crystals
Authors: Murty K. Linga , Begert Dietmar , Munter Rolf , Kanert Otmar
Published: March 2005

Nuclear magnetic resonance pulse techniques were used in situ during creep of single crystals of NaCl to evaluate the contribution of mobile dislocations to spin relaxation. 23Na spin‐lattice relaxation rates were measured in the rotating frame (T1p) during compression creep of single crystals of NaCl along the [110] direction at 473 K at an applied stress of 20 MPa. The relaxation rates were evaluated from the spin‐echo height following π/2, locking, and 64° pulse sequence. We describe here the experimental setup and methodology in evaluating the dislocation jump distance from NMR‐creep experiments. The height of the free induction decay decreased as soon as the load was applied followed by a gradual increase until the steady state was reached, at which point a saturation value was observed corresponding to the constant steady‐state creep rate. The mean jump distance of the mobile dislocations, evaluated from the ratio of the signal heights without deformation and during creep, decreased with time/strain, reaching a constant value during the steady‐state creep regime. The results are compared with the dislocation‐dislocation spacing, subgrain size, as well as the jump distance predicted from creep models.
Indentation Behavior of Ion‐Exchanged Glasses
Authors: Tandon Rajan , Green David J.
Published: March 2005

Indentation fracture mechanics was extended to include the effect of a thin layer of residual stress on the identation strength of brittle materials. The proposed theory was used to predict the residual stress values for an ion‐exchanged glass. For flaws placed before the exchange, considerable strengthening was observed, but the value of the surface stress predicted was considerably underestimated. For flaws placed after the exchange, there was no strengthening and the value of the surface stress was predicted to be zero. The failure of the indentation analysis indicates that it has to be modified for accurate stress determination. Thin layers of residual stress were found to retard the initiation of surface damage, but their influence on the strength after damage initiation was minimal.
Oxidation of Silicon Carbide‐Reinforced Oxide‐Matrix Composites at 1375° to 1575°C
Authors: Luthra Krishan L. , Park Hee Dong
Published: March 2005

Oxidation studies were conducted on Al2O3‐SiC and mullite‐SiC composites at 1375° to 1575°C in O2 and in Ar‐1% O2. The composites were prepared by hot‐pressing mixtures of Al2O3 or mullite and SiC powders. The reaction products contained alumina, mullite, an aluminosilicate liquid, and gas bubbles. The parabolic rate constants were about 3 orders of magnitude higher than those expected for the oxidation of SiC. Higher rates are caused by higher oxygen permeabilities through the reaction products than through pure silica. Our results suggest that oxygen permeabilities are comparable in the three condensed phases observed in the reaction products.
Measurement of Infrared Spectra of Dense Ceramics by Specular Reflectance Spectroscopy
Authors: Bliss Mary , Walden Barbara L. , White William B.
Published: March 2005

It is shown that high‐quality IR specular reflectance spectra can be obtained from polished surfaces of dense ceramics. The reflectance spectra can be deconvoluted by the Kramers‐Kronig transformation to yield absorption spectra or the real and imaginary parts of the dielectric function. IR line shapes and peak wavenumbers are comparable to single‐crystal data providing that the particle size is small compared with the IR wavelengths of interest. For noncubic structures, the spectra are a superposition of polarization components. Modes separated by less than a bandwidth are merged into a single band, thus limiting the resolution of the measurement procedure. Grain size effects were modeled using the spectra of single‐crystal and polycrystalline quartz as an example.
Thermal Reaction of Aluminum Alkoxide in Glycols
Authors: Inoue Masashi , Kominami Hiroshi , Inui Tomoyuki
Published: March 2005

Thermal reaction of aluminum isopropoxide in glycols at 300°C yielded the glycol derivatives of boehmite. The physical properties of the products and the aluminas obtained by calcination strongly depended on the crystallite size of the boehmite derivatives, which seems to be controlled by the heterolytic cleavage of the C‐O bond of HO(CH2)nOAI = formed by alkoxide exchange between aluminum alkoxide and glycol. The product had a unique honeycomb texture, which was preserved on calcination to form a transition alumina.
Surface Forces and Their Action in Ceramic Materials
Authors: Horn Roger G.
Published: March 2005

A descriptive account is given of the surface forces acting between two solids. Different contributions to the force are outlined, with particular attention paid to the underlying mechanisms, and how they are affected by the nature of the medium between the surfaces. This is followed by a discussion of the areas of ceramic science and engineering in which surface forces play a role.
Metal Reference Line Technique for Obtaining Dihedral Angles from Surface Thermal Grooves
Authors: Handwerker Carol A. , Dynys Joseph M. , Cannon Rowland M. , Coble Robert L.
Published: March 2005

A metal reference line (MRL) technique is described for the measurement of surface–grain‐boundary dihedral angles, Ψs, from thermal grooves at a sample surface using scanning electron microscopy (SEM). Metal lines deposited onto a thermally grooved surface using photolithography conform to the contours of the grain‐boundary groove and provide a high‐contrast reference line for measuring Ψs by SEM. Measurements of Ψs from optical interferometry and calculated from groove dimensions using surface diffusion models of thermal grooving are compared with the metal reference line measurements from the same thermally grooved surface of MgO‐doped Al2O3. Distributions of Ψs are found to shift to lower angles and approach the true Ψs value as the resolution of the technique increases, with the MRL technique having the highest resolution, a median angle of 113°± 1° and a distribution of angles from 90°± 5° to 139°± 3°.
Fabricating Nanocrystalline Diamond Ceramics by a Shock Compaction Method
Authors: Kondo Ken‐ichi , Sawai Sumiichi
Published: March 2005

Nanocrystalline diamond ceramics—namely, polycrystalline diamond compact consisting of ultrafine grains (one to hundreds of nanometers)—have been produced without any sintering additives via a shock compaction method using commercial diamond powders which consist of monocrystalline or shock‐synthesized polycrystalline particles less than 1 μ in diameter. The method exploits plastic deformation of diamond and requires a precise spatial and timely partitioning of shock‐induced thermal energy. Although much yet remains to be done for practical applications, the superior advantages of using ultrafine powders are supported by the experimental results and the theoretical modeling and considerations which are applicable to advanced ceramics.
Measurement and Prediction of the Dynamic Temperature Distributions in Soda‐Lime Glass Plates
Authors: Field Robert E. , Viskanta Raymond
Published: March 2005

Experimental measurement of the dynamic internal temperature distribution in soda‐lime glass plates using thermocouples fused in the glass has been carried out. Experimentally measured temperatures are compared to predictions obtained from the solution of the transient energy equation where the internal radiative transfer has been accounted for using rigorous radiative transfer theory. A discussion of the experimental method, the process used to fuse the thermocouples in the test plates and the rigorous formulation of the energy equation, for semitransparent materials is given. Predicted and measured instantaneous surface temperatures are compared and very good agreement is obtained. It is also concluded that the empriical equation for the phonon conductivity used in the analysis underpredicts the conductivity. A 20% increase in phonon conductivity is shown to significantly improve the agreement between measured and predicted centerline to surface temperature differences.
Interfacial Characterization of Silicon Carbide Fiber/Lithia‐Alumina‐Silica Glass Matrix Composites
Authors: Homeny Joseph , VanValzah Janet R. , Kelly Mark A.
Published: March 2005

The development of the carbon‐rich interphase in Nicalon SiC fiber/Li2O‐Al2O3–SiO2 glass matrix composites was examined as a function of processing parameters with the use of high‐resolution scanning electron microscopy and Auger electron spetroscopy. Specifically, hot‐pressing temperatures (1000°, 1100°, and 1200°C) and times (15, 30, 60, and 240 min) were systematically varied in such a manner so as to fabricate dense composites suitable for evaluation of reaction kinetics. Carbon‐rich interphase thickness, which ranged from 1400 to 5400 Å (140 to 540 nm), was observed to increase with either increasing times at constant temperature or increasing temperatures at constant time. The kinetics of formation of the carbon‐rich interphase followed a diffusion‐controlled model, with an activation energy of 25.4 kcal/mol.
Equibiaxial Stress Effect on Fracture Strength of Indented Soda‐Lime Glass in Water Environment: An Examination for Dependence on Crack‐Tip Blunting
Authors: Ikeda Kiyohiko , Igaki Hisashi , Tanigawa Yoshinobu , Tagashira Koichiro
Published: March 2005

Equibiaxial stress effects were observed in constant stressing rate tests of indented soda‐lime glass in a water environment. To discuss whether the equibiaxial stress effects on fracture strenght in a water environment were caused by the blunting of crack tips by chemical reaction between the glass and moisture, the specimens with controlled surface flaws soaked in hot water to make the crack tip blunt were fractured under both uniaxial and equibiaxial tensile stresses under both vacuum and air environments. The biaxial strengthening observed under restricted subcritical crack growth indicates that equibiaxial stress effects in a water environment are caused by the rounding of the initial crack tip.
Hardness of Polycrystalline Tungsten and Molybdenum Oxides at Elevated Temperatures
Authors: Lee Minyoung , Flom Donald G.
Published: March 2005

Vickers hardness of WO3, W18O49, and MoO2 is reported for temperatures up to 800°C. Polycrystalline samples of the oxides were prepared by hot‐pressing, and hardness was determined using a Vickers hardness tester modified for high‐temperature applications. The hardness of a heavily deformed tungsten rod was also measured as a reference.
Fracture Strength of α‐ and β‐SiAION Measured by Biaxial and Four‐Point Bending
Authors: Breder Kristin , Andersson Tomas , Schölin Kent
Published: March 2005

The fracture strength of an α‐ and β‐SiAION was measured by biaxial and four‐point bending. By utilizing Weibull statistics a close prediction could be made of the four‐point values from the biaxial values. At a fracture strength of 460 MPa, fracture was initiated by linking of surface and subsurface flaws.
In‐Situ Measurement of Silica‐Gel Coating on Particles of Alumina
Authors: Agrawal Dinesh C. , Raj Rishi , Cohen Claude
Published: March 2005

Time‐dependent growth of a silica layer on alumina particles suspended in a polymeric silica solution has been measured by monitoring the changes in the zeta potential of the alumina particles. There is an incubation period in the process, suggesting that the silica film forms by a nucleation‐and‐growth mechanism. The coated alumina particles are found to flocculate as their zeta potential approaches that of silica near its isoelectric point.
Prediction of Crack Paths in Particulate Composites Using Electrical Analog
Authors: Penugonda Madhusudhan R. , Virkar Anil V. , Shetty Dinesh K.
Published: March 2005

An electrical analog technique was used to simulate crack paths in the vicinity of elastic inhomogeneities in ceramic composites. The technique makes use of the analogy that exists between electrical potential differences between clamped ends of a conducting foil with electrical inhomogeneities (second phases of different electrical conductivity) and the load‐point displacements of a mechanically loaded plate with elastic inhomogeneities (second phases of different elastic modulus). In crack problems, this analogy is exact for mode III loading. The specific problems investigated were the interaction between a crack and a well‐bonded second phase of higher elastic modulus (simulating a fiber or a second‐phase particle) and the interaction between a crack and a pore. Crack paths were incrementally determined by maximizing the end‐point potential differences (corresponding to the maximum energy release rate criterion in the mechanical system) with respect to the direction of crack growth. Comparable mechanical tests were carried out using precracked Plexiglas* plates with either circular holes or elastic inclusions. Good agreements were obtained between the electrically simulated crack paths and the crack paths observed in the mechanical tests despite the fact that the mechanical tests were carried out in mode I loading and the analogy is not exact.
Silicon Carbide Whisker/Alumina Matrix Composites: Effect of Whisker Surface Treatment on Fracture Toughness
Authors: Homeny Joseph , Vaughn Wallace L. , Ferber Mattison K.
Published: March 2005

The fracture toughness of a 30 vol% SiC whisker/Al2O3 matrix composite was evaluated as a function of whisker surface chemistry. Two types of SiC whiskers (Silar‐SC‐9 and Tateho‐SCW‐1‐S) were investigated. Modification of the whisker surface chemistry was achieved by subjecting the whiskers to thermal treatments under controlled atmospheres. Whisker surface chemistry, as determined by X‐ray photoelectron spectroscopy, was correlated to the fracture toughness of the composites.
Thermal Diffusivity/Conductivity of Alumina—Silicon Carbide Composites
Authors: McCluskey Philip H. , Williams Robert K. , Graves Ron S. , Tiegs Terry N.
Published: March 2005

Thermal diffusivity and conductivity values for several Al2O3‐SiC whisker composites were determined. The thermal diffusivity values spanned the range from 373 to 1473 K, and thermal conductivity data wre obtained between 305 and 365 K. The thermal diffusivity decreased with increasing temperature and increased with SiC‐whisker content. An estimate of the thermal conductivity of the whiskers was obtained from the direct thermal conductivity measurements, but attempts to derive whisker conductivity values from the thermal diffusivity data were not successful because the laser flash method lacks the required accuracy and precision. Specimens were subjected to two different thermal quench experiments to investigate the effect of thermal history on diffusivity. In the most severe case, multiple 1073‐ to 373‐K quenches, radial cracks were observed in the test specimens; however, there was no change in diffusivity. The lack of sensitivity to thermal cycling appears to be related to the sample size.
Long Ceramic‐Lined Pipes Produced by a Centrifugal‐Thermit Process
Authors: Odawara Osamu
Published: March 2005

Longer than 3‐m ceramic‐lined pipes can be produced by means of a thermit‐reaction‐induced process under the influence of a centrifugal force (“centrifugal‐thermit process”). By igniting only a part of the reactant, the thermit reaction rapidly proceeds to provide a ceramic lining of homogeneous quality in the direction of the pipe length. As a result of the residual stress measurement of each layer forming the present ceramic‐lined pipe, it is confirmed that the intermediate metal layer produced plays an important role in balancing the stress between the ceramic and outer pipe layers.
Dielectric Properties of Fluxed Barium Titanate Ceramics with Zirconia Additions
Authors: Armstrong Timothy R. , Young Karrie A. , Buchanan Relva C.
Published: March 2005

BaTiO3 compacts, when fluxed with <2 vol% of a complex borate glass phase, were sintered to near theoretical density at temperatures <1175°C in 2 h. Microstructural analysis showed a uniform grain size <1.0 μm with 0.75 wt% ZrO2 added to the flux phase as a grain growth inhibitor. TEM analysis revealed a microcrystalline grain‐boundary phase with the ZrO2 resident along the grain boundaries. These samples displayed an essentially flat dielectric profile, low dissipation factors (<2%) over the range 25° to 125°C, a near linear dependence (≅±15%) between 25° and −55°C, and significantly increased voltage stability. X‐ray diffraction analyss of these small‐grained materials indicated a suppression of the tetragonal structure toward a more cubic modification.
Temperature Dependence of Interfacial Shear Strength in SiC‐Fiber‐Reinforced Reaction‐Bonded Silicon Nitride
Authors: Morscher Gregory , Pirouz Pirouz , Heuer Arthur H.
Published: March 2005

The interfacial shear strength of AVCO SCS‐6 SiC‐fiber‐reinforced reaction‐bonded Si3N4 (RBSN) composites was studied as a function of temperature. Fiber “push‐through” experiments were conducted with a diamond indenter and a high‐temperature microhardness tester. The interfacial shear strength was variable and depended mostly on interfacial bonding at low temperatures (5 to 18 MPa at room temperature) and frictional forces at high temperatures (12 to 32 MPa at 1300°C). The frictional component is attributed to the surface roughness of the fibers. The interfacial shear strength increased with temperature, because of the relief of residual stresses arising from the thermal expansion mismatch between fiber and matrix. Because of the composite nature of these fibers, a number of interfaces were tested in each experiment. The interface which debonded and slid was not always the same. Interfacial fracture took place either between the two outermost carbon layers of the SCS‐6 fibers, or between the SiC core and the innermost of the two outer carbon layers. The outermost carbon layer of the fiber always stayed bonded to the Si3N4 matrix.
Degradation of Aluminum Nitride Powder in an Aqueous Environmet
Authors: Bowen Paul , Highfield James G. , Mocellin Alain , Ring Terry A.
Published: March 2005

The degradation of A1N powder in excess H2O at room temperature for up to 24 h was investigated. Samples were characterized by various techniques (IR; XRD; SEM; XPS; C, H, N analysis; surface area, particle size, and weight change measurements). The reaction rate was found to be significant, with 80% of the A1N being consumed in 24 h. The initial reaction product was found to be a porous, amorphous, hydrated alumina with stoichiometry near AlOOH. After ∽16 h a crystalline phase, bayerite Al(OH)3, was detected which became the predominant phase after 24‐h contact. The kinetics of the A1N consumption were found to be first order and the reaction rate linear. The kinetic data fitted an unreacted core model with a porous product layer where the surface chemical reaction controlled the overall kinetics.
Formation and Pore Structure of Boron Nitride Aerogels
Authors: Lindquist David A. , Borek Theodore T. , Kramer Stephen J. , Narula Chaitanya K. , Johnston Gregory , Schaeffer Riley , Smith Douglas M. , Paine Robert T.
Published: March 2005

Gels containing a poly(borazinyl amine) and tetrahydrofuran were processed by CO2 supercritical drying techniques followed by pyrolysis. The resulting BN ceramic aerogels are highly porous, and the microstructure, porosity, and surface area characteristics have been examined. The aerogels show excellent thermal stability exhibiting surface areas in excess of 350 m2/g and porosities greater than 0.8 even when heated in argon at 1500°C for 8 h. By removing solvent via evaporation before supercritical drying, the mean pore radius can be varied between 3.6 and 10 nm.
Measurement of Phase Abundance in Magnesia‐Partially‐Stabilized Zirconia by Rietveld Analysis of X‐ray Diffraction Data
Authors: Hill Roderick J. , Reichert Bernard E.
Published: March 2005

The relative abundance of the cubic (c), tetragonal (t), monoclinic (m), and orthorhombic (o) polymorphs of ZrO2, and the δ phase, Mg2Zr5O12, present in samples of 3.4‐wt%‐magnesia‐partially‐stabilized zirconia have been determined by Rietveld analysis of X‐ray powder diffraction data. The samples studied correspond to the as‐fired (AF), and subetectoid‐aged maximum‐strength (MS) and thermalshock (TS) states, with their surfaces in the ground or polished condition. The polymorph abundances of the bulk and near‐surface regions are discussed in relation to the type of surface treatment. Grinding produces significant quantities of bothm‐ ando‐ZrO2in the near‐surface regions of all samples. Themcontent increases from about 5 wt% in the bulk, to 10, 24, and 33 wt% in AF, MS, and TS material, respectively, while theocontent increases from trace amounts to about 11 wt% in all samples. Themandophases both increase at the expense oft‐ZrO2, and the transformation is accompanied by significant lattice distortion and/or crystal size reduction. Thus, measurement of only the ‘ground‐surface‐monoclinic’ content does not give an accurate indication of the total amount of transformablet‐ZrO2in ceramics of this kind. Polishing removes some of the ground‐surfacem‐ZrO2in MS and TS, and all of them‐ZrO2in AF material. Theo‐ZrO2produced by grinding also declines substantially in AF and MS, but is not removed by polishing of TS. As a result, the bulk composition cannot be guaranteed, in the general case, to be accessible by X‐ray analysis of polished surfaces.
Electrical Properties of Individual Zinc Oxide Grain Boundaries Determined by Spatially Resolved Tunneling Spectroscopy
Authors: Rohrer Gregory S. , Bonnell Dawn A.
Published: March 2005

Scanning tunneling microscopy (STM) and spatially resolved tunneling spectroscopy (TS) were used to observe correlations between the geometric structure and electrical properties of polycrystalline ZnO surfaces under ultrahigh vacuum. Constant current images revealed crystallographic features at a range of length scales, including facets which are hundreds of nanometers long and monoatomic steps ≅0.5 nm (≅5 Å) in height. Tunneling spectroscopy was used to identify individual ZnO grains, grain boundaries, and surface impurities. Areas of reduced conductivity which extend 5 to 40 nm (50 to 400 Å) on either side of the grain boundaries are attributed to associated space charge regions. This paper demonstrates that, when used together, STM and TS are powerful techniques for the study of the structure and electrical properties of single interfaces and grain boundaries.
High‐Temperature Creep of Yttria‐Stabilized Zirconia Single Crystals
Authors: Martinez‐Fernandez Julian , Jimenez‐Melendo Manolo , Dominguez‐Rodriguez Arturo , Heuer Arthur H.
Published: March 2005

Creep of 9.4‐mol%‐Y2O3‐stabilized cubic ZrO2 has been studied between 1300° and 1550°C. Conventional power‐law creep (stress exponent n∼ 4.5) is found at the higher temperatures, with an activation energy (∼6 eV) corresponding to cation diffusion. Transition to a different creep mechanism occurs at the lower temperatures, as indicated by higher values of the stress exponent (n∼ 7) and an activation energy (∼7.5 eV) higher than that for cation self‐diffusion. The lower‐temperature behavior is caused by a competition between cross‐slip‐controlled and recovery‐controlled creep. Consideration of all the creep and diffusion data now available suggests that the rate‐controlling high‐temperature mass transport in Y2O3‐stabilized ZrO2 can be described by D= 10−3 exp(‐5.0 eV/kT) m2·s−1.
Structure of Alumina Grain Boundaries Prepared with and without a Thin Amorphous Intergranular Film
Authors: Susnitzky David W. , Carter C. Barry
Published: March 2005

The presence of a thin amorphous intergranular film along grain boundaries in alumina is expected to affect the properties of the interface and hence those of the material. In the present study, two types of grain boundaries have been formed in hot‐pressed alumina bicrystals. In one case, the surfaces of the sintered crystals were kept as clean as possible, while in the other a thin layer of SiO2 was intentionally deposited onto the surface of one crystal. The distribution of SiO2 along the resulting grain boundary was then monitored by transmission electron microscopy and compared with the morphological features of the interface. In the special cases chosen here, the glass receded into large pores which grew into the alumina itself. However, the presence of the glassy phase during the early stages of sintering clearly did influence the characteristics of the resulting grain boundaries.
Thermal Expansion of the Low‐Temperature Form of BaB2O4
Authors: Guiqin Dai , Wei Lin , An Zheng , Qingzhen Huang , Jingkui Liang
Published: March 2005

Thermal expansion of the low‐temperature form of BaB2O4 (β‐BaB2O4) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high‐temperature X‐ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B, and C in the expansion formula Lt=L0(1 +At+Bt2+Ct3) are given as follows: a axis, A= 1.535 × 10−7, B= 6.047 × 10−9, C= ‐1.261 × 10−12; c axis, A= 3.256 × 10−5, B= 1.341 × 10−8, C= ‐1.954 × 10−12; and cell volume V, A= 3.107 × 10−5, B= 3.406 × 10−8, C= ‐1.197 × 10−11. Based on αt= (dLt/dt)/L0, the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a, c, and cell volume V.