bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal Stability of Lanthanum Zirconate Plasma‐Sprayed Coating
Authors: Cao X. Q. , Vassen R. , Jungen W. , Schwartz S. , Tietz F. , Stöver D.
Published: July 2010

Lanthanum zirconate (La2Zr2O7, LZ) is a newly proposed material for thermal barrier coatings (TBCs). The thermal stability of LZ coating was studied in this work by long‐term annealing and thermal cycling. After long‐term annealing at 1400°C or thermal cycling, both LZ powder and plasma‐sprayed coating still kept the pyrochlore structure, and a preferred crystal growth direction in the coating was observed by X‐ray diffraction. A considerable amount of La2O3 in the powder was evaporated in the plasma flame, resulting in a nonstoichiometric coating. Additionally, compared with the standard TBC material yttria‐stabilized zirconia (YSZ), LZ coating has a lower thermal expansion coefficient, which leads to higher stress levels in a TBC system.
Chemical Expansivity of Electrochemical Ceramics
Authors: Adler Stuart B.
Published: July 2010

To better understand thermal strain in electrochemical ceramics, the temperature and oxidation‐state dependence of lattice volume in La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) were measured. Large values in the apparent thermal expansion coefficient at high temperature (>50 ppm/°C) were caused by changes in oxygen content, not increases in thermal expansivity. This material can be described using an improved thermodynamic formalism that incorporates a new physical property, the chemical expansivity. Our approach opens new avenues for modeling stress and strain in materials, probing defect structure, and analyzing transport and kinetic properties.
Effect of Ultrasonication on the Microstructure and Tensile Elongation of Zirconia‐Dispersed Alumina Ceramics Prepared by Colloidal Processing
Authors: Suzuki Tohru S. , Sakka Yoshio , Nakano Keishi , Hiraga Keijiro
Published: July 2010

To obtain dense, fine‐grained ceramics, fine particles and advanced powder processing, such as colloidal processing, are needed. Al2O3 and ZrO2 particles are dispersed in colloidal suspensions by electrosteric repulsion because of polyelectrolyte absorbed on their surfaces. However, additional redispersion treatment such as ultrasonication is required to obtain dispersed suspensions because fine particles tend to agglomerate. The results demonstrate that ultrasonication is effective in improving particle dispersion in suspensions and producing a homogeneous fine microstructure of sintered materials. Superplastic tensile ductility is improved by ultrasonication in preparing suspensions because of the dense and homogeneous fine microstructure.
Effect of Particle Volume Fraction on the Gelation Behavior of the Temperature Induced Forming (TIF) Aqueous Alumina Suspensions
Authors: Yang Yunpeng , Sigmund Wolfgang M.
Published: July 2010

Stable aqueous Al2O3 suspensions that can be flocculated by increasing temperature were prepared using tri‐ammonium citrate and poly(acrylic acid) (PAA). Effects of particle volume fraction on the rheological properties of Al2O3 suspensions were investigated. The modified Dougherty–Krieger equations were used to describe the volume fraction dependence of relative viscosity for both the PAA‐addition and PAA‐free suspensions. An equation using percolation model was proposed to analyze the temperature and volume fraction dependence of the relative viscosity of the suspensions. The calculated results were in good agreement with the experimental data.
Point Defect Concentrations in Barium Titanate Revisited
Authors: Tsur Yoed , Randall Clive A.
Published: July 2010

An analytic link between the oxygen partial pressure and the concentrations of the point defects for given temperatures and A/B conditions is presented for perovskites. A clear distinction between three different conditions is made. These are the sintering conditions, an intermediate metastable state, and a low‐temperature metastable state. The analytical solution for a metastable state resulting from nonequilibrated metal vacancies permits a more accurate and self‐consistent approach to calculating the equilibrium constants from conductivity–P(O2) data. One of the reasons for the higher accuracy is that there is no need to divide the existence regime into subregimes with different approximations to the electroneutrality equation (Brouwer approximation). An excellent fit of the experimental conductivity data to a single function with only two adjustable parameters over all conductivity–P(O2) space is obtained. The relative importance of frozen‐in metal vacancies and foreign acceptors is discussed for BaTiO3.
Phase Equilibria in the Lead–Magnesium–Niobium–Oxygen System at 1000°C
Authors: Lucas Pierre , Petuskey William T.
Published: July 2010

Phase equilibria in the PbO–MgO–Nb2O5 system have been determined at 1000°C and 1 atm. A solid‐solution range of the cubic pyrochlore phase was observed and the extent of that domain was determined. This solid solution equilibrates with the surrounding ternary and binary compounds to form a series of diphasic areas. The compatibility relations between all phases were determined except for a small domain where at least six lead niobate phases were stable within a very short range of compositions. The formation of pyrochlore impurities during the processing of Pb(Mg1/3Nb2/3)O3 perovskite is discussed in relation to the phase diagram.
Characterization of Oxidized Polymer‐Derived SiBCN Fibers
Authors: Cinibulk Michael K. , Parthasarathy Triplicane A.
Published: July 2010

The oxidation behavior of a developmental amorphous SiBCN fiber was investigated. Fibers were heat‐treated in stagnant laboratory air at temperatures of 1300°–1500°C for 1 or 2 h. The oxidized SiBCN fibers contained three distinct concentric layers, each increasing in oxygen concentration from the core to the outer surface. The unreacted fiber core retained its amorphous nature. The first oxidation layer next to the core consisted of a mixture of amorphous SiBCNO and turbostratic BN, which evolved into a more oxygen‐rich glass with hexagonal and turbostratic BN grains dispersed throughout nearer the surface. The second layer consisted of essentially pure silica glass with no detectable B, C, or N present. The outermost layer in the fiber oxidized at 1500°C had devitrified to cristobalite. The fiber suffered significant strength degradation after oxidation.
Fracture Toughness of Amorphous Precursor‐Derived Ceramics in the Silicon‐Carbon‐Nitrogen System
Authors: Bauer Arndt , Christ Martin , Zimmermann Andre , Aldinger Fritz
Published: July 2010

The toughness of amorphous precursor‐derived ceramics in the Si‐C‐N system is investigated. Crack‐growth data are obtained from DCB specimens, whereas the crack‐tip toughness is determined from the crack‐tip profile of indentation cracks. For amorphous Si‐C‐N ceramics that have been fabricated via the powder route, an R‐curve effect is observed. The initial values of the rising R‐curve are consistent with the estimated crack‐tip toughness.
Nanoscale Densification Creep in Polymer‐Derived Silicon Carbonitrides at 1350°C
Authors: Shah Sandeep R. , Raj Rishi
Published: July 2010

The measurement of axial and radial strains during uniaxial compression creep of SiCN shows the deformation to be entirely volumetric (as opposed to shear). Phenomenologically, the densification strain rate shows a good fit to an exponential stress dependence. This result is explained by the large volume of the diffusing molecular units in the oligomeric amorphous structure of SiCN, which causes the driving force to become nonlinear in stress. The size of the diffusing unit is estimated to be 1.2 nm.
Mechanical Properties of Silicon Oxycarbide Ceramic Foams
Authors: Colombo Paolo , Hellmann John R. , Shelleman David L.
Published: July 2010

The mechanical properties of ceramic foams obtained through a novel process that uses the direct foaming and pyrolysis of preceramic polymer/polyurethane solutions were investigated. The elastic modulus, flexural strength, and compressive strengths were obtained for foams in the as‐pyrolyzed condition; values up to 7.1 GPa, 13 MPa, and 11 MPa, respectively, were obtained. The strength of the foam was virtually unchanged at temperatures up to 1200°C in air; however, long‐term exposure at 1200°C led to a moderate degradation in strength, which was attributed to the evolution of intrastrut porosity during the oxidation of residual free carbon, as well as devitrification of the foams struts.
Effect of Solids Loading and Dispersant Concentration on Strain Mismatch and Deformation of Slip‐Cast Green Bodies
Authors: Yates John D. , Lombardo Stephen J.
Published: July 2010

A method has been developed to identify low levels of strain mismatch present in bodies in the green state. High‐aspect‐ratio beams of alumina were slip cast at 35–50 vol% solids over a range of poly(acrylic acid) concentrations. After sintering in an unconstrained manner, the beams deform into an arc. The magnitude and direction of curvature was observed to depend on both the solids loading and the concentration of polyelectrolyte. Model calculations indicate that strain mismatch within the beams of 1%, which corresponds to green density variations of up to 2.5%, would account for the observed degree of bending. The direction of bending was rationalized in terms of the competing effects of cake compressibility and particle velocity.
Electrical Properties of Boron Nitride Matrix Composites: III, Observations near the Percolation Threshold in BN–B4C Composites
Authors: Gerhardt Rosario A. , Runyan Julie , Sana Catherine , McLachlan David S. , Ruh Robert
Published: July 2010

In this third paper of the series, we discuss the electrical resistivity of BN–B4C composites with compositions ranging from 0% to 100% B4C. After establishing the response of samples whose compositions lie far away from the percolation region, where effective medium models apply, we focus attention on samples with compositions at or near the percolation threshold (∼60% BN–40% B4C). The large differences in electrical properties among samples with the same nominal composition can be explained by invoking a connectivity parameter. Since the difference in the electrical resistivity of BN and B4C is about 9 orders of magnitude, the degree of connectivity of the two components at the percolation threshold determines the resultant composite resistivity. Connectivity in these composites was quantified by taking BN peak height ratios in X‐ray diffraction patterns of all samples containing 60% BN–40% B4C. The degree of preferred orientation of the BN platelets can be correlated with systematic increases in the electrical resistivity of the composites.
Strength‐Limiting Surface Damage in Single‐Crystal Sapphire Imaged by X‐ray Topography
Authors: Black David
Published: July 2010

X‐ray topography was used in an effort to understand the relationship between microstructure and fracture strength in single‐crystal sapphire bend bars. The bars were indistinguishable using standard optical characterization techniques, but X‐ray diffraction topography revealed that they could be separated into two distinct types. The first type had an oriented microstructure consisting of a pattern of linear features parallel to the long axis of the bars. This microstructure was attributed to fabrication damage. The second type was well‐polished, nearly damage‐free sapphire. Fracture strength data showed that the damaged bars had strengths less than 70% of the bars without damage.
Structural Control of Elastic Constants of Mullite in Comparison to Sillimanite
Authors: Hildmann Bernd , Ledbetter Hassel , Kim Sudook , Schneider Hartmut
Published: July 2010

Nine elastic stiffness coefficients, cij, of a mullite single crystal (2Al2O3·SiO2) are measured using acoustic resonance spectroscopy. The obtained values are similar to those of the structurally related aluminosilicate phase sillimanite (Al2O3·SiO2). Characteristic elastic properties of the two minerals are interpreted with the help of their crystal structures and atomic force constants for sillimanite. The high longitudinal stiffness coefficients, c33, of mullite (∼352 GPa) and sillimanite (∼388 GPa) are caused by continuous “stiff” load‐bearing tetrahedral chains parallel to c‐axis, while the “soft” octahedral chains have minor direct influence. They stabilize the tetrahedal chains against tilting. The lower c33 value of mullite in comparison to the sillimanite value may be caused by a weakening of the load‐bearing tetrahedral chains which are parallel to c‐axis because of partial replacement of silicon by the weaker‐bonded aluminum. The longitudinal stiffness coefficients perpendicular to c‐axis are significantly lower, because of sequences of alternating “soft” octahedral and “stiff” tetrahedral units. Within the plane (001), the compliant octahedra exhibit stiffness‐controlling influence with coefficients parallel to b‐axis (c22∼ 233 GPa) being somewhat lower than parallel to a‐axis (c11∼ 291 GPa). This is explained with the occurrence of compliant octahedral Al(1)–O(D) bonds, which are more effective parallel to b‐axis rather than to a‐axis. Because octahedra are unaffected by the aluminum to silicon substitution, c11 and c22 coefficients of mullite and sillimanite are very similar. Shear stiffness coefficients of mullite increase from c55 (∼77 GPa) to c66 (∼80 GPa) to c44 (∼110 GPa), indicating increasing resistance against shear deformation within the planes (010), (001), and (100). The lattice plane of the highest shear stiffness (100) is built up of an oxygen‐oxygen network, diagonally braced along 〈011〉 (“Jägerzaun”). This network with short oxygen–oxygen distances can be sheared by compression and elongation along oxygen–oxygen interaction lines only which is rather unlikely. Because of the lack of such networks in the planes (010) and (001), bending and deformation of structural units become easier, and consequently c55 and c66 are <c44. All three shear stiffness coefficients of mullite are slightly lower than those of sillimanite because of the reduction of the mean tetrahedral bond strength in mullite caused by partial substitution of silicon by aluminum.
Phase Equilibria in the Hafnia–Yttria–Lanthana System
Authors: Andrievskaya Elena R. , Lopato Lidiya M.
Published: July 2010

The HfO2–Y2O3–La2O3 system was studied in the wide range of temperatures (1250°–2800°C) and concentrations by methods of X‐ray analysis at 20°C, petrography, differential thermal analysis in helium at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X‐ray analysis. The complete phase diagram was constructed. The liquidus and solidus projections, crystallization paths for the alloys, isothermal (1250°, 1600°, and 1900°C) and polythermal sections are presented. The structure of the boundary binary systems defines the phase equilibria in the ternary system. No ternary compounds were found. Ternary solid‐solution regions were determined based on constituent oxides and intermediate phases.
Silicon Nitride/Molybdenum Disilicide Composite with Superior Long‐Term Oxidation Resistance at 1500°C
Authors: Klemm Hagen , Schubert Christian
Published: July 2010

The long‐term high‐temperature cyclic oxidation (100 cycles, 104 h, 1500°C) of a Si3N4 material and a Si3N4/MoSi2 composite, both fabricated with Y2O3 as a sintering additive, was studied. Both materials exhibited similar oxidation rates because of surface SiO2 formation described by an almost parabolic law and a total weight gain of 3–4 mg/cm2 after 104 h. As a consequence of oxidation processes in the bulk, microstructural damage was found in the Si3N4 material. These effects were not observed in the composite. The remarkable microstructural stability observed offers the high potential of Si3N4/MoSi2 composites for long‐term structural applications at elevated temperatures up to 1500°C.
Self‐Lubrication of Chlorine‐Implanted Titanium Nitride Coating
Authors: Aizawa Tatsuhiko , Akhadejdamrong Thananan , Iwamoto Chihiro , Ikuhara Yuichi , Mitsuo Atsushi
Published: July 2010

Chlorine implantation into TiN coatings decreases the wear loss and the friction coefficient. Even by low‐dose chlorine implantation, the wear volume is decreased by three orders of magnitude or more, and the friction coefficient becomes <0.1. This self‐lubrication mechanism is related to the presence and mobility of implanted chlorine atoms inside the columnar TiN microstructure. According to observations of chlorine‐implanted TiN coatings using high‐resolution transmission electron microscopy, the chlorine atoms are present in the damaged region, where TiN is composed of nanosized grains. From these data, a self‐lubrication mechanism is proposed with chlorine catalyzing the oxidation of titanium and leading to the formation of some tribological reaction product.
Theoretical Prediction of the Structure and Properties of Cubic Spinel Nitrides
Authors: Ching Wai‐Yim , Mo Shang‐Di , Ouyang Lizhi , Rulis Paul , Tanaka Isao , Yoshiya Masato
Published: July 2010

The structure and properties of cubic spinel nitrides were investigated based on first‐principles theoretical calculations. The lattice constants, bulk moduli, band structures, electronic bonding, and lattice stability of thirty‐nine single and double nitrides were studied. The single spinel nitrides of the form c‐A3N4 (where A is a Group IVA element), except c‐Hf3N4, are all semiconductors with band gaps ranging from an indirect gap of 0.07 eV in c‐Ti3N4 to a direct gap of 3.45 eV in c‐Si3N4. For double nitrides of the form c‐AB2N4 (where A and B are Group IVA (Ti, Zr, Hf) or IVB (C, Si, Ge, Sn) elements), both metallic and insulating band structures are possible. The stability of the double spinel nitrides, relative to single nitrides, is dependent on the optimal cation radii and polyhedral volumes at the tetrahedral A sites and the octahedral B sites. Of the thirty‐two double nitrides, only nine are predicted to be energetically favorable. Among the potentially stable phases, the most interesting ones are c‐CSi2N4 (which has an exceptionally strong covalent bonding and large bulk modulus), c‐SiGe2N4 (which has an energetically favorable direct band gap of 1.85 eV), and c‐SiTi2N4 (which is metallic).
Fabrication of CrN/Cr2N Bulk Composites and Their Mechanical Properties
Authors: Aizawa Tatsuhiko , Kuwahara Hideyuki , Tamura Masayuki
Published: July 2010

Bulk CrN/Cr2N billet with almost full density was successfully fabricated using spark‐plasma sintering. Elastic moduli, hardness, and fracture toughness were evaluated to investigate the effect of sintering conditions on the mechanical properties. Ball‐on‐disk testing was performed to understand the wear and friction behavior of this bulk CrN/Cr2N composite solid. The wear map was constructed as a function of the sliding velocity and the applied pressure, to categorize the wear mechanism of this nitride composite.
Elastic Moduli and Hardness of Cubic Silicon Nitride
Authors: Zerr Andreas , Kempf Markus , Schwarz Marcus , Kroke Edwin , Göken Mathias , Riedel Ralf
Published: July 2010

The bulk modulus B0= 290(5) GPa and its first pressure derivative B′0= 4.9(6) were obtained for c‐Si3N4 from volume versus pressure dependence. Measurements were performed under quasi‐hydrostatic conditions in a diamond anvil cell to 53 GPa using synchrotron radiation and energy dispersive X‐ray powder diffraction. This combined with nanoindentation measurements determined the shear modulus G0 of c‐Si3N4 to be 148(16) GPa. The Vickers microhardness HV(0.5) for dense, oxygen‐free c‐Si3N4 was estimated to be between 30 and 43 GPa. Both the elastic moduli and microhardness of c‐Si3N4 exceed those of the hexagonal counterparts, α‐ and β‐phases.
Rapid Synthesis of Crystalline Spinel Tin Nitride by a Solid‐State Metathesis Reaction
Authors: Shemkunas Michael P. , Wolf George H. , Leinenweber Kurt , Petuskey William T.
Published: July 2010

Spinel tin nitride (Sn3N4) was prepared by high‐pressure solid‐state metathesis reactions. Tin tetraiodide was reacted with lithium nitride and ammonium chloride in a piston–cylinder apparatus at 623 K and 2.5 GPa. Powder X‐ray diffraction combined with Rietveld refinement confirmed the spinel structure, with a lattice constant of a= 9.0144(1) Å. Raman spectroscopy revealed five active vibrational modes at 160, 252, 416, 524, and 622 cm−1, which are consistent with the spinel structure. Mid‐infrared spectroscopy exhibited only a Sn–N stretching mode at 547 cm−1. Transmission electron microscopy was used to reveal particle size and morphology. Electron probe microanalysis was used to verify the elemental composition.
Carbon Nitride‐Related Nanomaterials from Chemical Vapor Deposition: Structure and Properties
Authors: Wang E. G.
Published: July 2010

Nanoscale‐sized carbon nitride‐related materials exhibit a wealth of interesting structural, electronic, and optical property behaviors. Chemical vapor deposition technology allows almost unlimited freedom to produce films with compositions and structures approaching the nanometer scale among light elements. Aligned polymerized carbon nitride (CN) nanobells have been grown on a large scale and provide excellent field electron emission properties, as described by a side‐emission mechanism. Separation of single CN nanobells and fabrication of heterojunctions between CN nanobells and pure carbon nanotubes have been achieved. Boron carbonitride (BCN) nanofibers with controlled orientation and composition have been synthesized; these nanofibers show strong blue‐violet photoluminescense at room temperature. Recent progress also has been made on nitrogen‐containing diamond, CN, and BCN films. The purpose of this paper is to survey the work that has been conducted and to detail the level of understanding that has been attained in the research on nitride‐related materials.
Surface Decomposition of Strontium‐Doped Soft PbZrO3–PbTiO3
Authors: Zheng Hong , Reaney Ian M. , Lee William E. , Jones Nik , Thomas Howard
Published: July 2010

Sintering temperature has a pronounced effect on perovskite phase stability at the surface of Pb0.88Sr0.12Zr0.54Ti0.44Sb0.02O3 (PSZT) soft piezoelectric ceramics (d33≈ 600 pC/N). After sintering 4 h at 1070°C, XRD reveals only perovskite PSZT peaks in the bulk and at the surface. As sintering temperature increases, XRD from the ceramic surface reveals a second‐phase peak at ∼27° (2θ), 0.316 nm (d‐spacing). After 4 h at 1280°C, further second‐phase peaks are observed, confirming it to be monoclinic ZrO2, accompanied by a strong increase in the degree of tetragonality of the perovskite phase. These observations are consistent with decomposition of the PSZT to ZrO2 and tetragonal PZT (PbZrO3–PbTiO3) associated with PbO loss. SEM and cross‐sectional TEM indicated that surface decomposition had progressed ∼0.5 mm into the sample after 4 h at 1280°C.
Defects and Charge Transport near the Hematite (0001) Surface: An Atomistic Study of Oxygen Vacancies
Authors: Warschkow Oliver , Ellis Donald E. , Hwang Jinha , Mansourian‐Hadavi Negar , Mason Thomas O.
Published: July 2010

Atomistic calculations were performed on a slab model of the (0001) surface of hematite as well as the bulk structure. In particular, the energetics of oxygen vacancies near the surface was studied. Atomistic modeling was used to establish the defect energies in the bulk versus distance from the surface. Transition state calculations were performed to compute barriers for several pathways of migration of oxygen vacancies in the bulk and at varying depths relative to the surface. We find energy barriers of several transitions considerably lowered closer to the surface. Considerations of literature data for electrical conductivity and Seebeck coefficient on bulk versus thin‐film hematite suggest high populations of point defects near surfaces, in agreement with our predictions.
Characterization of the Free‐Carbon Phase in Precursor‐Derived Si‐C‐N Ceramics: I, Spectroscopic Methods
Authors: Trassl Stephan , Motz Günter , Rössler Ernst , Ziegler Günter
Published: July 2010

Polyvinylsilazane, as a precursor for Si‐C‐N ceramics, was prepared by ammonolysis of functionalized chlorosilanes. Pyrolysis under inert atmospheres at Tp= 1000°C led to an amorphous Si‐C‐N‐(H) ceramic. Further heat treatment caused the transformation to the thermodynamically stable crystalline phase assemblage. The structural changes, especially those of the excess carbon, were studied by characterizing the solid intermediates via solid‐state magic angle spinning nuclear magnetic resonance spectroscopy. Moreover, Raman spectroscopy, electron spin resonance spectroscopy, and chemical analysis were used. Based on these methods, a comprehensive picture of the formation and behavior of the free‐carbon phase present in polymer‐derived ceramics was obtained.
Analytical Electron Microscopy Study of Green Ceramics Formed from Aqueous Suspensions Using the Hydrolysis‐Assisted Solidification Process
Authors: Novak Saša , Dražič Goran
Published: July 2010

During the hydrolysis‐assisted solidification (HAS) of aqueous ceramic suspensions, aluminum hydroxides are formed that bind the ceramic particles into a stiff wet body. Transmission electron microscopy investigations of HAS‐processed Al2O3 and ZrO2 green parts after drying revealed that the secondary phase is amorphous and distributed uniformly around the host ceramic particles. The estimated thickness of this layer was 3–5 nm. Moreover, areas of a few tens of nanometers in size were found at three‐particle junctions that contained an amorphous phase and individual nanocrystals of boehmite.
Nanosized Zinc‐Oxide Particles Derived from Mechanical Activation of Zn5(NO3)2(OH)8·2H2O in Sodium Chloride
Authors: Fah Chong Phui , Xue Junmin , Wang John
Published: July 2010

Nanosized ZnO particles are successfully synthesized via mechanical activation of a zinc nitrate hydroxide hydrate (Zn5(NO3)2(OH)8·2H2O) precursor in NaCl matrix for 15 h. The ZnO particles obtained are in the nanosize range of ∼20 nm, with a well‐established hexagonal morphology. They compare favorably with those derived from conventional calcination of the precursor. The decomposition of Zn5(NO3)2(OH)8·2H2O precursor and formation of nanocrystalline ZnO cannot be completed by mechanical activation in the absence of NaCl, which acts as both an effective dispersing matrix and drying agent although it remains chemically inert during mechanical activation. The powder derived from calcination at 400°C does not possess powder characteristics comparable to that of the powder derived from the mechanical activation in NaCl, because of the extensive particle coarsening and aggregation at the calcination temperature.
Local Flow Behavior of Ceramic Slurries in Tape Casting, as Investigated by Laser–Doppler Velocimetry
Authors: Schmidt Michael , Münstedt Helmut , Svec Michael , Roosen Andreas , Betz Thomas , Koppe Franz
Published: July 2010

The local flow behavior in a tape‐casting unit for processing ceramic slurries was investigated by laser–Doppler velocimetry (LDV). The most remarkable feature of the LDV system used was its high spatial resolution (24 μm) and temporal resolution (5 μs). A translucent Newtonian model fluid which, in the relevant processing window, at higher shear rates, exhibits a rheological behavior similar to that of a typical ceramic slurry, was chosen to determine the velocity distribution in the flow channel (i.e., in the tape‐casting unit). The flow behavior was measured directly in several particular regions of the flow channel, which substantially consisted of a double doctor‐blade assembly. The velocity distribution obtained below the forming doctor blade showed the strong influence of the casting speed on the flow behavior of the model fluid. Furthermore, the measured flow behavior was analyzed, based on the assumptions of fluid dynamics. Below the doctor blade, the flow behavior was considered to be a combination of pressure flow and drag (Couette) flow. The flow rates from the applied hydrostatic pressure and external drag forces were shown to be additive. A comparison between the measured velocity profiles and the model calculations showed excellent agreement. A flow reversal (secondary flow), never before measured quantitatively, was detected between the two inserted rectangular blades. Apparently, secondary flows influenced the surface properties of the cast green tapes.
Synthesis and Characterization of a New Sillenite Compound—Bi12(B0.5P0.5)O20
Authors: Valant Matjaz , Suvorov Danilo
Published: July 2010

A new sillenite compound, Bi12(B0.5P0.5)O20, was synthesized using a solid‐state reaction method. The stoichiometry was confirmed by XRD analyses, microstructural investigations, and quantitative elemental analysis. An investigation of the dielectric properties at frequencies from 100 Hz to 1 MHz revealed a broad, highly frequency‐dispersive, relaxor‐like dielectric anomaly, which appeared in the temperature range of −80°–100°C. The permittivity, Q×f value, and temperature coefficient of the resonant frequency, measured at ∼5.5 GHz, were determined to be 37.4, 850 GHz, and −19 ppm/K, respectively.
Provenance and Technology Investigation of Agia Sophia Bricks, Istanbul, Turkey
Authors: Moropoulou Antonia , Cakmak Ahmet , Polikreti Kyriaki
Published: July 2010

Bricks from the Agia Sophia in Istanbul, Turkey, were investigated to better formulate a plan for restoration. According to an ancient text, these bricks are extremely light and were manufactured on the island of Rhodes. Results of neutron activation analysis, grouped by multivariate statistics, show that the brick samples do not correlate with clays thought to be local and used in contemporary churches in Istanbul. The bricks are similar to those used in the Great Basilica of Rhodes. However, the Agia Sophia bricks are lighter (45% porosity) than the bricks from the other churches (35% porosity). The Agia Sophia bricks are made of a noncalcareous, fine paste with quartz temper, fired at low temperature (∼750°C). The exceptional characteristic is their homogeneity and small pore‐size distribution (0.3–0.8 μm). The tensile strength of the dome bricks is unexpectedly high (up to 1.3 MPa), even though the bricks have high porosity.
Andalusite‐Based High‐Alumina Castables
Authors: Rebouillat Lionel , Rigaud Michel
Published: July 2010

Andalusite is easily converted to mullite and silica on heating. A better understanding of the mullitization mechanisms provides new information on use of this mineral in refractory castables. By using specific particle‐size distributions for andalusite‐based high‐alumina castables, the primary mullite formation can be effectively enhanced by a secondary mullite reaction within castables matrices. The influence of ultrafine andalusite grains on thermomechanical properties of specimens is underlined by testing hot modulus of rupture in combination with mineralogical and microscopic analyses. The results demonstrate that andalusite has great promise as a component of high‐alumina no‐cement‐ or ultra‐low‐cement‐containing castables.
Tensile Creep Behavior and Thermal Stability of Orthogonal Three‐Dimensional Woven TyrannoTM ZMI Fiber/Silicon‐Titanium‐Carbon‐Oxygen Matrix Composites
Authors: Ogasawara Toshio , Ishikawa Takashi , Ohsawa Yutaka , Ochi Yasuo , Zhu Shijie
Published: July 2010

This article presents experimental results for tensile creep behavior of orthogonal three‐dimensional woven Tyranno™ ZMI fiber/Si‐Ti‐C‐O ceramic matrix composites at 1300°–1450°C in air. The composite contained Tyranno ZMI (56% silicon, 1% zirconium, 34% carbon, and 9% oxygen) fibers with a BN coating layer to improve interface properties, and it exhibited excellent tensile properties at elevated temperature in air. For creep stresses between 60 and 140 MPa, the creep rate decreased continuously with time, with no apparent steady‐state regime observed at 1300°–1450°C. Under the test conditions, the microstructure of the Tyranno ZMI fiber and Si‐Ti‐C‐O matrix was unstable, resulting in weight loss and SiC grain growth. As a result, the viscosity of the fiber and matrix increased, because increased viscosity caused a creep rate that continuously decreased, which made steady‐state creep impossible under these conditions.
Investigation of Electroplating‐Induced Degradation of PMZNT Relaxor Ferroelectrics
Authors: Cao Jiang Li , Li Long Tu , Wang Yong Li , Gui Zhi Lun
Published: July 2010

Resistance degradation of lead magnesium niobate‐based (PMZNT) ceramics during nickel electroplating was investigated using resistivity measurement and auger electron spectroscopy (AES) analysis. It was found that the resistivity of the ceramics remained nearly unchanged at the initial stage of the electroplating, and, as electroplating time increased, the resistivity decreased rapidly. An AES depth profile analysis of a degraded specimen showed oxygen content in the surface to be lower than the theoretical value, and the relationship of oxygen content and detection depth was obtained. The results indicated that Pb2+ and Nb5+ were reduced to lower valence states and oxygen in the lattice was lost, thereby leading to a generation of oxygen vacancies. Hence, the conductivity of the dielectrics was enhanced. The results also indicated that only the outermost surface had suffered the reducing effect induced by hydrogen generated during nickel electroplating, whereas the bulk material remained impervious.
Transparent Hydroxyapatite Ceramics through Gelcasting and Low‐Temperature Sintering
Authors: Varma Harikrishna , Vijayan Sekhara Pillai , Babu Sivadasan Suresh
Published: July 2010

Transparent hydroxyapatite (HAP) was prepared by sintering gel‐cast powder compacts at 1000°C for 2 h; the resultant HAP material was studied using X‐ray diffractometry, transmission electron microscopy, scanning electron microscopy, and microhardness measurement. Nanoscale HAP crystallites were prepared using a precipitation method that involved calcium nitrate and ammonium dihydrogen orthophosphate solutions; the preparation was conducted at a temperature of 0°C. The precipitate was gel‐cast and sintered at 1000°C in the form of a transparent ceramic that had a uniform grain size of 250 μm. The maximum Vickers microhardness obtained for a sample sintered at 1000°C was 6.57 GPa. The sintering behavior of gel‐cast samples prepared from high‐temperature‐precipitated HAP was compared with that of material prepared at 0°C.
Kinetics and Products of Molybdenum Disilicide Powder Oxidation
Authors: Zhu Yuntian T. , Shu Li , Butt Darryl P.
Published: July 2010

In this study, we investigated the kinetics and products of the oxidation of MoSi2 powder with an average particle size of 1.6 μm at 900°, 1000°, and 1100°C, using a small sample size of 0.5 g. Such a small sample size allowed us to minimize the effect of oxygen transportation through the powder volume, while maintaining a good relative weighing accuracy. X‐ray diffraction of oxidized samples indicated the formation of Mo5Si3 and Mo metal. Analysis of the oxidation kinetics suggested that gaseous MoO3 formed initially and amorphous SiO2 film later. The oxidation kinetics and products observed in this study differ from those reported in an early study, in which a larger sample size was used.
Direct Synthesis and Hydration of Calcium Aluminosulfate (Ca4Al6O16S)
Authors: Song Jong‐Taek , Young J. Francis
Published: July 2010

Calcium aluminosulfate (Ca4Al6O16S or C4A3S̄) was prepared by direct synthesis from calcium and aluminum nitrates, and aluminum sulfate. CaAl4O7(CA2) formed as an intermediate at 900°C, and C4A3S̄ was the main phase after calcination at 1100°C. The specific surface areas after calcination at 1100° and 1300°C were ∼2.5 and 1 m2/g, respectively. Hydration was investigated using XRD, DSC, SEM, conduction calorimetry, and solid‐state 27Al MAS‐NMR spectroscopy. Calorimetry showed that the induction period was longer than that of a sample prepared using conventional solid‐state sintering, and this was attributed to the formation of amorphous coatings. Crystalline hydration products, principally calcium monoaluminosulfate hydrate and aluminum hydroxide, appeared subsequently. Although the induction period was very long, complete hydration occurred as early as 3 d in the sample calcined at 1100°C and was 91% complete in the sample calcined at 1300°C.
Crystalline Structure and Dielectric Properties of Li1+x−yNb1−x−3yTix+4yO3M‐Phase Solid Solutions
Authors: Borisevich Albina Y. , Davies Peter K.
Published: July 2010

The structure and dielectric properties of the so‐called M‐phase solid solutions in the Li2O–Nb2O5–TiO2system have been investigated. Detailed studies of the lattice parameters of these phases agree well with structure models based on intergrowths of LiNbO3slabs with a titanium‐rich corundum‐type layer. The relative permittivity ranges from ∼80 to ∼55; microwave quality factors increase with the titanium content, reaching values of Q×f= 9000 at 6 GHz. The temperature coefficient of the resonant frequency changes sign within the solid solution region, thus permitting tunability to a zero value. All the compositions can be sintered to high density at temperatures ≤1100°C.
Dispersion and Upconversion Fluorescence of Nd3+ Ions in Zinc Chloride‐Based Glasses
Authors: Wada Noriyuki , Kojima Kazuo
Published: July 2010

Nd3+‐ion‐doped ZnCl2‐based glasses were prepared via the melt‐quenching method. The Nd3+‐ion upconversion excitation mechanism and the ability to disperse Nd3+ ions into ZnCl2‐based glasses were investigated using absorption and upconversion fluorescence spectra of Nd3+ ions. The ZnCl2‐based glass that had an average cationic radius equal to the ionic radius of Nd3+ (98 pm) had the greatest ability to disperse Nd3+ ions. The 20KCl·25BaCl2·55ZnCl2glass, which had the average cationic radius nearest to 98 pm, dispersed Nd3+ ions the most, and it had the strongest upconversion fluorescence.
Effects of Thermal Aging on the Mechanical Properties of a Porous‐Matrix Ceramic Composite
Authors: Carelli Eric A. V. , Fujita Hiroki , Yang James Y. , Zok Frank W.
Published: July 2010

The present article focuses on changes in the mechanical properties of an all‐oxide fiber‐reinforced composite following long‐term exposure (1000 h) at temperatures of 1000–1200°C in air. The composite of interest derives its damage tolerance from a highly porous matrix, precluding the need for an interphase at the fiber–matrix boundary. The key issue involves the stability of the porosity against densification and the associated implications for long‐term durability of the composite at elevated temperatures. For this purpose, comparisons are made in the tensile properties and fracture characteristics of a 2D woven fiber composite both along the fiber direction and at 45° to the fiber axes before and after the aging treatments. Additionally, changes in the state of the matrix are probed through measurements of matrix hardness by Vickers indentation and through the determination of the matrix Young's modulus, using the measured composite moduli coupled with classical laminate theory. The study reveals that, despite evidence of some strengthening of the matrix and the fiber–matrix interfaces during aging, the key tensile properties in the 0°/90° orientation, including strength and failure strain, are unchanged. This strengthening is manifested to a more significant extent in the composite properties in the ±45° orientation, wherein the modulus and the tensile strength each exhibit a twofold increase after the 1200°C aging treatment. It also results in a change in the failure mechanism, from one involving predominantly matrix damage and interply delamination to one which is dominated by fiber fracture. Additionally, salient changes in the mechanical response beyond the maximum load suggest the existence of an optimum matrix strength at which the fracture energy in the ±45° orientation attains a maximum. The implications for long‐term durability of this class of composite are discussed.
Effect of Lead Zirconate Titanate Seeds on PtxPb Formation during the Pyrolysis of Lead Zirconate Titanate Thin Films
Authors: Wu Aiying , Vilarinho Paula M. , Salvado Isabel M. Miranda , Baptista João L. , Zhou Zhaoxia , Reaney I. M. , Ramos Ana R. , da Silva Maria F.
Published: July 2010

Lead zirconate titanate (PZT) thin films were prepared on platinized silicon substrates by dip‐coating using a modified diol‐based sol–gel route without and with up to 5 mol% PZT nanometric seeds dispersed in the precursor sol. A metastable intermetallic PtxPb phase formed at the early stages of heat treatment. XRD, TEM, and RBS revealed that the thickness and stoichiometry of the PtxPb layer varied with the concentration of seeds and heat treatment of the films. The relation of the PtxPb layer to the final crystalline texture of the PZT thin films is reported and discussed.
Anomalous Grain Growth in Donor‐Doped Barium Titanate with Excess Barium Oxide
Authors: Drofenik Miha , Makovec Darko , Zajc Igor , Langhammer Hans T.
Published: July 2010

Grain growth and semiconductivity of donor‐doped BaTiO3ceramics with an excess of BaO and additions of SiO2or B2O3were studied. The microstructures and electrical measurements on sintered samples revealed that their electrical properties are related to the microstructure development of the sintered samples. Samples heated with an excess of BaO developed a normal microstructure during sintering, as a consequence of normal grain growth (NGG), and were yellow and insulating. In contrast, samples with an excess of BaO and an addition of SiO2or B2O3exhibited anomalous grain growth (AGG) and were dark blue and semiconducting after sintering. When some BaTiO3seed grains were embedded in a sample of donor‐doped BaTiO3with an excess of BaO (without SiO2or B2O3), AGG was observed, i.e., some seed grains grew into large grains and were blue and semiconducting. An explanation is given for why AGG is responsible for the oxygen release and the formation of semiconducting grains in donor‐doped BaTiO3and not NGG.
Step‐Energy Barriers and Particle Shape Changes during Coarsening
Authors: Sheldon Brian W. , Rankin Janet
Published: July 2010

Models of particle shape changes usually do not account for the step‐energy barrier associated with adding or removing additional atomic planes from a faceted surface. However, the step‐energy barrier can be a substantial limitation when the free energy changes that drive particle shape changes are relatively low. A good example of this is particle coarsening. The analysis presented in this article describes dislocation‐free particles with surfaces that have faceted and nonfaceted regions. When the chemical potential differences responsible for shape changes are too small to overcome the step‐energy barrier, atomic layers cannot be added or removed from the facets. Even with this constraint, it is possible to add or remove atoms from the particle surface; however, this can cause the particle shape to differ substantially from the traditional equilibrium shape.
Properties of 0–3 Lead Zirconate Titanate–Polymer Composites Prepared in a Centrifuge
Authors: Nhuapeng Wim , Tunkasiri Tawee
Published: July 2010

The purpose of this work was to present the electrical and piezoelectric properties of the 0‐3 piezoceramic–polymer composites prepared by spinning the lead zirconate titanate (PZT) powders with polyester resin in a centrifuge. PZT powders with average sizes of 55 and 160 μm were used and mixed in the resin with different volumetric percentages. The dielectric and piezoelectric properties such as permittivity, loss angle, electromechanical coupling factor, and piezoelectric coefficient were measured. The mechanical quality factor was calculated. The acoustic impedance was accessed by the echo‐shift method. The results were analyzed and fit to mechanical models. Distribution of the ceramic particles in the polyester‐resin phase was examined by scanning electron microscopy. Smaller‐ceramic‐particle composites seemed to form denser samples. Most of the properties showed linearly varying as the volumetric percentage of the ceramic phase. The fabrication using the centrifuging techniques resulted in more homogeneity of the ceramic and polymer phases, and the fabricated samples could be loaded up to 65% or more with the ceramic powders.
Dependence of Silicon Carbide Product Morphology on the Degree of Mechanical Activation
Authors: Shaw Leon L. , Yang Zhenguo , Ren Ruiming
Published: July 2010

Mechanical activation before carbothermic reduction can substantially enhance the formation of SiC from SiO2and carbon mixtures. However, the morphology (e.g., particles or whiskers) of SiC formed from mechanically activated SiO2and carbon mixtures is dependent of the degree of mechanical activation and the condition of the subsequent carbothermic reduction. These phenomena are investigated and rationalized based on the increased reactivity of the reactants and SiC formation mechanisms.
Effect of Manganese Ion Diffusion on the Microstructural Evolution of Lead Lanthanum Zirconate Titanate Ceramic
Authors: Choi Jong‐Jin , Kim Sang‐Wook , Kim Hyoun‐Ee
Published: July 2010

Microstructural evolution of lead lanthanum zirconate titanate (PLZT) ceramics caused by diffusion of the Mn ion was observed. Specimens with layered structures were fabricated by copressing a PLZT powder (9/65/35) doped with Mn and same PLZT powder without the dopant. When the copressed specimen was sintered at 1200°C in air, the Mn ion diffused out of the doped region. The region originally containing the Mn ions was totally free of pores while all other regions remained porous. The formation of lattice vacancies, as a result of Mn diffusion, was attributed to the enhanced material transport and the resultant rapid densification.
Magnesium Hydroxide Dehydroxylation/Carbonation Reaction Processes: Implications for Carbon Dioxide Mineral Sequestration
Authors: Béarat Hamdallah , McKelvy Michael J. , Chizmeshya Andrew V. G. , Sharma Renu , Carpenter Ray W.
Published: July 2010

Gas‐phase magnesium hydroxide carbonation processes were investigated at high CO2 pressures to better understand the reaction mechanisms involved. Carbon and hydrogen elemental analysis, secondary ion mass spectrometry, ion beam analysis, X‐ray diffraction, and thermogravimetric analysis were used to follow dehydroxylation/rehydroxylation/carbonation reaction processes. Dehydroxylation is found to generally precede carbonation as a distinct but interrelated process. Above the minimum CO2 pressure for brucite carbonation, both carbonation and dehydroxylation reactivity decrease with increasing CO2 pressure. Low‐temperature dehydroxylation before carbonation can form porous intermediate materials with enhanced carbonation reactivity at reduced (e.g., ambient) temperature and pressure. Control of dehydroxylation/rehydroxylation reactions before and/or during carbonation can substantially enhance carbonation reactivity.
Ink‐Jet Printing of Binders for Ceramic Components
Authors: Moon Jooho , Grau Jason E. , Knezevic Vedran , Cima Michael J. , Sachs Emanuel M.
Published: July 2010

Layered manufacturing methods for fabricating ceramic components can involve selective deposition of binder using ink‐jet printing. Selection of a proper binder plays a critical role in fabricating parts with good surface finish, dimensional accuracy, and high resolution. Several polymeric solution‐phase binders were investigated in terms of their physical properties, printing performance, and binder‐powder bed interaction. It was observed that the molecular weight should be <15 000 for the binder to be penetrated into dense powder compacts. Binder infiltration kinetics and printed line width were also significantly influenced by powder‐bed characteristics, such as surface roughness and pore size, as well as the physical properties of the binder, such as viscosity and surface tension.