bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct Formation of Crystalline Gadolinium‐Doped Ceria Powder via Polymerized Precursor Solution
Authors: Wang Shuqiang , Maeda Kunihiro , Awano Masanobu
Published: July 2010

Synthesis of Ce0.9Gd0.1O1.95 (CGO) powder from a polymeric precursor solution containing a mixture of nitrates, nitric acid, and ethylene glycol was investigated with emphasis on the effect of polymerization of the precursor solution on the crystallization and morphology of the derived solid intermediate and the final oxide powder. It is shown for the first time in this work that the solid intermediate derived from the polymerized solution is present in the form of well‐crystallized cerium‐gadolinium formate solid solution, Ce1−xGdx(HCOO)3, exhibiting anisotropic growth. Further polymerization of the precursor solution resulted in the direct formation of loosely agglomerated nanoscaled CGO oxide powder from the polymerized solution at temperatures as low as 130°C.
Microstructure and Grain‐Boundary Effect on Electrical Properties of Gadolinium‐Doped Ceria
Authors: Zhou Xiao‐Dong , Huebner Wayne , Kosacki Igor , Anderson Harlan U.
Published: July 2010

The microstructural evolution and grain‐boundary influence on electrical properties of Ce0.90Gd0.10O1.95 were studied. The nanoscale powders synthesized from a semibatch reactor exhibited 50% green density and 92% sintering density at 1200°C (∼200°C lower than previous studies). Impedance spectra as a function of temperature and grain size were analyzed. The Ce0.90Gd0.10O1.95 with finest grain size possessed highest overall grain‐boundary resistance; this contribution was eliminated at temperatures >600°C, regardless of grain size. The grain conductivity was independent of grain size and was dependent on temperature with two distinct regimes, indicative of the presence of Gd′Ce−Vo∘∘ complexes that dissociated at a critical temperature of ∼580°C. The activation energy for complex dissociation was ∼0.1 eV; the value for the grain‐boundary was ∼1.2eV, which was size independent.
Creep of Duplex Alumina–Titanium Nitride Polycrystalline Microstructures
Authors: Tai Qiu , Mocellin Alain
Published: July 2010

Compression creep results in an inert atmosphere are presented for fine‐grained Al2O3–TiN composites deformed under a stress range of 10–50 MPa stress and a temperature range of 1250°–1400°C. The effects of TiN content (10–70 vol%) and the preparation route (i.e., reactive versus nonreactive hot pressing) are considered. Al2O3 and TiN both are efficient grain‐growth inhibitors, relative to each other; thus, deformation develops in stable equiaxed microstructures. Except for low TiN contents (10 and 20 vol%), the processing route does not markedly influence the observed strain rates, at least up to a strain of −0.16. In all cases, deformation occurs mainly via interfacial/grain‐boundary sliding, and its rate is enhanced in approximate proportion to the amount of Al2O3/TiN interfacial area. The deformation rate seems to be limited by either some interface reaction or a complex boundary diffusional mechanism that occurs at the Al2O3 grain boundaries.
Characterization and Tribological Investigation of Sol–Gel Titania and Doped Titania Thin Films
Authors: Zhang Wenguang , Liu Weimin , Li Bin , Mai Guangxin
Published: July 2010

Titania (TiO2) and doped TiO2 ceramic thin films were prepared on a glass substrate by a sol–gel and dip‐coating process from specially formulated sols, followed by annealing at 460°C. The morphologies of the original and worn surfaces of the films were analyzed with atomic force microscopy (AFM) and scanning electron microscopy. The chemical compositions of the obtained films were characterized by means of X‐ray photoelectron spectroscopy (XPS). The tribological properties of TiO2 and doped TiO2 thin films sliding against Si3N4 ball were evaluated on a one‐way reciprocating friction and wear tester. The AFM analysis shows that the morphologies of the resulting films are very different in nanoscale, which partly accounts for their tribological properties. XPS analysis reveals that the doped elements exist in different states, such as oxide and silicate, and diffusion took place between the film and the glass substrate. TiO2 films show an excellent ability to reduce friction and resist wear. A friction coefficient as low as 0.18 and a wear life of 2280 sliding passes at 3 N were recorded. Unfortunately, all the doped TiO2 films are inferior to the TiO2 films in friction reduction and wear resistance, primarily because of their differences in structures and chemical compositions caused by the doped elements. The wear of the glass is characteristic of brittle fracture and severe abrasion. The wear of the TiO2 thin film is characteristic of plastic deformation with slight abrasive and fatigue wear. The doped TiO2 thin films show lower plasticity than the TiO2 thin film, which leads to large cracks. The propagation of the cracks caused serious fracture and failure of the films.
Influence of Surface Residual Stress State on Crack Path Evolution in Polycrystalline Alumina
Authors: Fang Yi , Ravi‐Chandar Krishnaswamy , White Kenneth W.
Published: July 2010

The residual stress distribution in polycrystalline alumina is estimated by an object‐oriented finite element method. By combining the microstructural image, individual grain orientation, and the crystal elastic properties, the residual stress distribution under a plane stress assumption is obtained by an analysis cooling of the sample through 1000°C. Furthermore, the residual stresses associated with grain boundary areas are investigated and discussed in the context of the concomitant influence on the observed crack path.
Characteristics of a NaOH‐Activated Blast Furnace Slag Blended with a Fine Particle Silica Waste
Authors: Escalante‐Garcia Jose Ivan , Palacios‐Villanueva Victor M. , Gorokhovsky Alexander V. , Mendoza‐Suárez Guillermo , Fuentes Antonio F.
Published: July 2010

Mortars of blast furnace slag blended with a geothermal silica waste at various replacement levels of 0%, 5%, 10%, 15%, and 20% were cured for up to 90 days. The binder was activated by 6 wt% Na2O equivalent of sodium hydroxide. Lime was added as an activating agent and also to promote pozzolanic reaction with the silica. It was found that the presence of the silica waste increased the reactivity of the cementitious materials, as measured by means of nonevaporable water. The compressive strength was increased in the presence of the silica, except at the highest replacement level of 20%, the optimum silica replacement was that of 5%–10%. The microstructures of blended slag showed less porosity than those of neat slag mortars. The lime was completely consumed after 90 days of hydration.
Thermal Stability, Phase Evolution, and Crystallization in Si‐B‐C‐N Ceramics Derived from a Polyborosilazane Precursor
Authors: Janakiraman Narayanan , Weinmann Markus , Schuhmacher Jörg , Müller Klaus , Bill Joachim , Aldinger Fritz , Singh Paramanand
Published: July 2010

Amorphous Si‐B‐C‐N ceramic powder samples obtained by thermolysis of boron‐modified polysilazane, {B[C2H4Si(H)NH]3}n, were isothermally annealed at different temperatures (1400–1800°C) and hold times (3, 10, 30, and 100 h). A qualitative and semiquantitative analysis of the crystallization behavior of the materials was performed using X‐ray diffraction (XRD). The phase evolution was additionally followed by11B and29Si MAS NMR as well as by FT‐IR spectroscopy in transmission and diffuse reflection (DRIFTS) modes. Bulk chemical analyses of selected samples were performed to determine changes in the chemistry/phase composition of the materials. It was observed that silicon carbide is the first phase to nucleate around 1400–1500°C, whereas silicon nitride nucleates at and above 1700°C. Crystallization accelerates with increasing annealing temperature and proceeds with increasing annealing time. Furthermore, the surface area of the powders strongly influences the thermal stability of silicon nitride and thus controls overall chemical and phase composition of the materials on thermal treatment.
Novel Method for Obtaining Corundum Layers of High Surface Area on Ceramic Supports for High‐Temperature Catalysis
Authors: Souto Alejandro , Guitian Francisco
Published: July 2010

The surface of an aluminosilicate ceramic was transformed to a corundum layer of high specific surface area by heating at 1300°–1450°C in a controlled reducing atmosphere. This procedure selectively reduced and volatilized the silica of the glass and mullite, and the alumina of the mullite formed a layer of corundum crystals with a thickness of ≤20 μm and a specific surface area of ≤16 m2/g. Specific surface area remained stable at 10.5 m2/g after prolonged heating at 1300°C in air, and at 8.5 m2/g at 1450°C. These materials are well suited for use as catalyst supports in applications such as catalytic combustion at temperatures in this range.
Effect of Heating Schedule on the Microstructure and Fracture Toughness of α‐SiAlON—Cause and Solution
Authors: Zenotchkine Misha , Shuba Roman , Chen I‐Wei
Published: July 2010

The effect of heating schedule on microstructure and fracture resistance has been investigated in single‐phase Nd‐, Y‐, and Yb‐α‐SiAlON. Such effect is strongly system dependent, reflecting the strong influence of phase stability on α‐SiAON nucleation and the amount of transient/residual liquid during processing. The addition of 1% of α‐SiAlON seeds to the starting powders nearly completely obliterates such effect, while it simultaneously improves microstructure homogeneity and fracture resistance. SENB toughness of 7 MPa·m1/2 and peak R‐curve toughness of ∼11 MPa·m1/2 have been obtained for seeded Y‐α‐SiAlON ceramics using heating rates from 1°C/min to 25°C/min.
Photoluminescence Probing of the Formation of Titanium Dioxide Sols from a Titanium Peroxide Solution
Authors: Seok Sang Il , Kim Mi Sun , Suh Tae Soo
Published: July 2010

Photoluminescence spectroscopy was used to probe the formation of colloidal TiO2 sols from a titanium peroxide solution, which were prepared by reaction of H2O2 and titanium hydrate gel that was precipitated from acidic TiOCl2 solution neutralized by NH4OH, during aging at 100°C for 0–64 h. Emission spectra revealed broad band luminescence which is interpreted as the superposition of a multiple photoluminescence process involving TiO2 precursors. The maximum peak position of the luminescence band was shifted to a shorter wavelength by the generation of the dominant emission peak as aging time increased, and a peak at ∼390 nm, indicating the anatase phase, was observed after aging for >48 h. However, sols aged for 2 h already were of the anatase phase, according to X‐ray diffraction analysis. It was deduced that the interfacial layers between the particles and the liquid in an aqueous solution caused luminescence quenching or accelerated sink.
Growth of Yttria‐Stabilized Zirconia Thin Films on Textured Silver Substrates by Chemical Vapor Deposition
Authors: Kim Gil‐Sung , Hwang Sang‐Chul , Kim Yoen‐Su , Shin Hyung‐Shik
Published: July 2010

The effect of deposition conditions on the growth of yttria‐stabilized zirconia (YSZ) films on textured silver substrates using the chemical vapor deposition (CVD) process was investigated. The crystalline structure of the YSZ film depended strongly on the deposition conditions, such as substrate temperature and deposition time. YSZ films prepared at 750°C using β‐diketone chelate sources, which had an orientation of c‐axis normal to the textured silver substrate surface. The YSZ surface was dense but not rough, and the YSZ film grew granular‐like. The cross‐sectional image of YSZ film showed the columnar growth feature; the growth rate was ∼20 nm/min.
Raman Spectroscopy of Nanocrystalline Ceria and Zirconia Thin Films
Authors: Kosacki Igor , Petrovsky Vladimir , Anderson Harlan U. , Colomban Philippe
Published: July 2010

The results of Raman‐scattering studies of nanocrystalline CeO2 and ZrO2:16% Y (YSZ) thin films are presented. The relationship between the lattice disorder and the form of the Raman spectra is discussed and correlated with the microstructure. It is shown that the Raman line shape results from phonon confinement and spatial correlation effects and yields information about the material nonstoichiometry level.
Electrical Conductivity of Silver Vanadium Tellurite Glasses
Authors: Moawad Hassan M. M. , Jain Himanshu , El‐Mallawany Raouf , Ramadan Tawfik , El‐Sharbiny Mohamed
Published: July 2010

We have investigated mixed electronic‐ionic conduction in 0.5[xAg2O·(1 −x)V2O5]–0.5TeO2 glasses, where the Ag2O amount varies over a range of 5–40 mol%. The glass samples have been prepared by quenching the melt. The ac conductivity has been measured at frequencies from 10 Hz to 100 kHz and temperatures of 300–425 K. The data indicate that the conduction mechanism changed from being predominantly electronic to ionic for Ag2O contents of >27.5 mol%. This transition is due to the change in glass structure, which affects both electronic‐ and ionic‐transport properties. The electronic dc conductivity results have been analyzed in terms of a small polaron‐hopping model.
Fabrication and Microstructure of Silicon Nitride/Boron Nitride Nanocomposites
Authors: Kusunose Takafumi , Sekino Tohru , Choa Yong Ho , Niihara Koichi
Published: July 2010

A chemical process for fabrication of Si3N4/BN nanocomposite was devised to improve the mechanical properties. Si3N4/BN nanocomposites containing 0 to 30 vol% hexagonal BN (h‐BN) were successfully fabricated by hot‐pressing α‐Si3N4 powders, on which turbostratic BN (t‐BN) with a disordered layer structure was partly coated. The t‐BN coating on α‐Si3N4 particles was prepared by reducing and heating α‐Si3N4 particles covered with a mixture of boric acid and urea. TEM observations of this nanocomposite revealed that the nanosized hexagonal BN (h‐BN) particles were homogeneously dispersed within Si3N4 grains as well as at grain boundaries. As expected from the rules of composites, Young's modulus of both micro‐ and nanocomposites decreased with an increase in h‐BN content, while the fracture strength of the nanocomposites prepared in this work was significantly improved, compared with the conventional microcomposites.
Single‐Step Deposition of Gel‐Derived Lead Zirconate Titanate Films: Critical Thickness and Gel Film to Ceramic Film Conversion
Authors: Kozuka Hiromitsu , Takenaka Shinsuke
Published: July 2010

PZT thin films were prepared on silica‐glass substrates from Pb(CH3COO)2·3H2O–Zr(n‐OC3H7)4–Ti(i‐OC3H7)4–H2O–CH3COOH–CH3OC2H4OH–n‐C3H7OH solutions containing polyvinylpyrrolidone (PVP) via nonrepetitive dip‐coating or spin‐coating. The thickness of the gel films was varied by changing the substrate withdrawal and rotation speeds, and the crack formation was examined after firing the gel films at 700°C. The critical thickness, i.e., the maximum thickness achievable without crack formation, was maximized at a PZT/alkoxide mole ratio of ∼1, where crack‐free PZT films as thick as 2 μm could be obtained via single‐step deposition. Thermal analysis was conducted on gel films deposited on a glass substrate at ∼25°C and then peeled off the substrate. Infrared absorption spectroscopy and X‐ray diffractometry measurements were conducted on films deposited on silicon wafers. These analyses revealed that PVP and CH3COO− were thermally decomposed at 250°–320°C, leaving carbonaceous species, followed by the crystallization of Pb3(CO3)2(OH)2 at ∼350°C. The oxidation of the carbonaceous species occurred at ∼360°–460°C; perovskite PZT then was formed at ∼550°C. The film thickness decreased greatly at 200°–350°C, where thermal decomposition of PVP and CH3COO− occurred. The films fired at 700°C were rather porous and not very transparent. Preheating the gel films at 100° or 200°C resulted in particulate microstructure with gaps between the particles, and, at 350°C, caused crack formation, both of which further decreased the optical transmittance.
Gelling of Alumina Suspensions Using Alginic Acid Salt and Hydroxyaluminum Diacetate
Authors: Studart Andre R. , Pandolfelli Victor C. , Tervoort Elena , Gauckler Ludwig J.
Published: July 2010

This paper proposes a novel direct casting method of alumina suspensions using alginic acid salt and the coagulation agent hydroxyaluminum diacetate (HADA). These two compounds allowed the consolidation of alumina suspensions through a simultaneous time‐delayed physical and chemical gelation process. The physical gel was formed by the gradual release of aluminum and acetate ions from the HADA in water, while the chemical gel originated from the cross‐linking of alginate molecules by the polyvalent aluminum ions. Wet alumina green bodies displayed enhanced mechanical properties with the addition of minimal contents of organic material (<0.1 wt%).
Low‐Temperature Sintering and Dielectric Properties of Ag(Nb,Ta)O3 Composite Ceramics
Authors: Kim Hyo Tae , Shrout Thomas , Randall Clive , Lanagan Michael
Published: July 2010

The sintering behavior and dielectric properties of perovskite Ag(Nb1−xTax)O3 (0 < x < 1) solid solutions and two‐phase composite assemblages were explored. A small amount of CuO (1 wt%) was used for liquid‐phase sintering and led to high densification at temperatures <950°C. The temperature coefficient of capacitance, TCC, was adjusted by varying the Nb:Ta ratio within the solid‐solution series and by creating composite microstructures. Two‐phase assemblages consisting of Ag(Nb3/4Ta1/4)O3 and Ag(Nb1/4Ta3/4)O3 were synthesized to achieve a temperature‐stable dielectric material for high‐frequency applications. The composite dielectric with CuO addition had an average dielectric constant of 390 and a Q×f factor of 410 GHz at 2 GHz, with a stable TCC (0 to −180 ppm/°C) in the temperature range from −20° to +60°C. In addition, process compatibility with a silver conductor was confirmed by high‐frequency ring‐resonator measurements and microstructural characterization. The Ag(Nb1−xTax)O3 solid solutions and composites are promising candidates as embedded capacitors for radio‐frequency/microwave applications.
Phase Relationships in the Si3N4–SiO2–Lu2O3 System
Authors: Hirosaki Naoto , Yamamoto Yoshinobu , Nishimura Toshiyuki , Mitomo Mamoru , Takahashi Junichi , Yamane Hisanori , Shimada Masahiko
Published: July 2010

Phase relationships in the Si3N4–SiO2–Lu2O3 system were investigated at 1850°C in 1 MPa N2. Only J‐phase, Lu4Si2O7N2 (monoclinic, space group P21/c, a= 0.74235(8) nm, b= 1.02649(10) nm, c= 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si3N4–SiO2–Lu2O3 system. The Si3N4/Lu2O3 ratio is 1, corresponding to the M‐phase composition, resulted in a mixture of Lu–J‐phase, β‐Si3N4, and a new phase of Lu3Si5ON9, having orthorhombic symmetry, space group Pbcm (No. 57), with a= 0.49361(5) nm, b= 1.60622(16) nm, and c= 1.05143(11) nm. The new phase is best represented in the new Si3N4–LuN–Lu2O3 system. The phase diagram suggests that Lu4Si2O7N2 is an excellent grain‐boundary phase of silicon nitride ceramics for high‐temperature applications.
Self‐Reinforced α‐SiAlON Ceramics with Improved Damage Tolerance Developed by a New Processing Strategy
Authors: Shen Zhijian , Peng Hong , Pettersson Pernilla , Nygren Mats
Published: July 2010

Alpha‐SiAlON ceramics with a refined self‐reinforced microstructure, i.e., containing acicular grains with dimensions much smaller than those obtained in previous studies, embedded in a matrix consisting of submicrometer‐sized isotropic grains, were prepared by applying a rapid one‐step sintering procedure. To suppress the overabundant formation of α‐SiAlON nuclei, a combination of stabilizing cations, Y + Yb, was used; to encourage formation of acicular α‐SiAlON grains, a small amount of an extra liquid (∼3 vol%) was introduced; to avoid abnormal grain coarsening resulting from dynamic ripening, the final sintering temperature was set to just slightly above the minimum temperature threshold for activating grain growth (1700°C). The fully dense compacts obtained exhibited excellent thermal‐shock resistance, and hardness and fracture toughness values of 20 GPa and 5.1 MPa·m1/2, respectively.
Solid Solubility and Precipitation in a Single‐Crystal Alumina–Zirconia System
Authors: Stough Matthew A. , Hellmann John R.
Published: July 2010

Solid solubility was examined in Zr‐doped sapphire and Al‐doped yttria‐stabilized zirconia (YSZ) single crystals from 1200° to 1600°C. Specimens were fabricated via ion implantation of single crystals, followed by annealing in air. Secondary ion mass spectroscopy (SIMS) was used to quantify solute redistribution during annealing. Comparison of SIMS results with analytical electron microscopy (AEM) revealed an alumina solubility of 0.2–0.3 wt% in zirconia, and a zirconia solubility of 0.004–0.027 wt% in alumina. Direct imaging of zirconia precipitates revealed that tetragonal zirconia precipitates from supersaturated sapphire with the following orientation relationship: (100)tetragonal‖ (0001)sapphire and [01¯1]tetragonal‖ [12¯10]sapphire.
Reaction Synthesis of Aluminum Nitride–Boron Nitride Composites Based on the Nitridation of Aluminum Boride
Authors: Zhang Guo‐Jun , Yang Jian‐Feng , Ando Motohide , Ohji Tatsuki
Published: July 2010

Aluminum nitride–boron nitride (AlN–BN) composites were prepared based on the nitridation of aluminum boride (AlB2). AlN powder was added to change the BN volume fraction in the obtained composites. Thermogravimetry–differential thermal analysis (TG‐DTA), X‐ray diffractometry, and the nitridation ratio were used to investigate the nitridation process of AlB2. At ∼1000°C, a sharp exothermic peak occurred in the DTA curve, corresponding to the rapid nitridation of aluminum in AlB2. On the other hand, the nitridation of the transient phase, Al1.67B22, was very slow when the temperature was <1400°C. However, the nitridation speed obviously accelerated at temperatures >1600°C. The pressure of the nitrogen atmosphere was also an important factor; high nitrogen pressure remarkably promoted nitridation. Treatment at 2000°C was disadvantageous for nitridation, because of the rapid formation of a dense surface layer that inhibited nitrogen diffusion into the specimen interior. Three specimens, with 5 wt% Y2O3 additive and different BN contents, were prepared by pressureless reactive sintering, according to the determined sintering schedule. Electron microscopy (scanning and transmission) observations revealed that the in‐situ‐formed BN flakes were homogeneously and isotropically distributed in the AlN matrix. A schematic mechanism for microstructural formation was developed, based on the results of nitridation and the microstructural features of the obtained composites. The obtained composites, with a low BN content, exhibited a high bending strength, comparable to that of reported hot‐pressed AlN–BN composites.
Physical Properties of Barrier Ribs of Plasma Display Panels: I, Formation of Pores during Sintering of Lead Borosilicate Glass Frits
Authors: Hwang Geon‐Ho , Jeon Hyung‐Jo , Kim Yong‐Seog
Published: July 2010

The effects of chemical compositions on the sintering behavior and physical properties of lead borosilicate glass developed for barrier ribs of plasma display panels (PDP) were investigated. The formation of pores during sintering of the glass was noted, and their formation mechanism was investigated using XPS, TG/DTA, and XRD. A change in the oxidation state of the lead was discussed as a possible cause for the formation of pores in the sintered samples.
Dissolution and Dispersion Behavior of Barium Carbonate in Aqueous Suspensions
Authors: Li Chia‐Chen , Jean Jau‐Ho
Published: July 2010

Dissolution of BaCO3 and its effect on the dispersion behavior of aqueous BaCO3 suspensions at various pH values have been investigated. The amount of leached Ba2+ decreases with increasing pH value, which agrees with thermodynamically calculated results. The dissolution of BaCO3 also causes an increase in pH value of the suspension, but the change decreases with increasing initial pH value. The isoelectric point (IEP) of leached BaCO3 powder is at a pH of ∼10–10.5 and remains unchanged with increasing solids loading. The IEP of BaCO3 shows no significant change with added KCl or K2CO3, but shifts to a higher pH with increasing concentration of added BaCl2.
13C NMR Study of the Solution Chemistry of Barium Titanium Citrate Gels Prepared Using the Pechini Process
Authors: Fang Tsang‐Tse , Wu Ma‐Shine , Tsai Jenq‐Dar
Published: July 2010

The chemistry of the initial precursor solutions of the barium titanate citrate gel prepared using the Pechini process was characterized by carbon nuclear magnetic resonance spectroscopy (13C NMR). Several experiments have been conducted to investigate the effect of the order of adding the species in the initial precursor solutions on synthesis of the barium titanium citrate gel. Based on the data from 13C NMR and photoacoustic Fourier‐transform infrared spectroscopy (PA‐FTIR) in previous work, it was found that the alcoholic and terminal carboxyl ligands of citric acid (CA) chelating with titanium ions are deprotonated. Esterification has a significant influence on the reaction of the species in the solution. Reaction mechanisms of the species in the precursor solution have been detailed. The possible molecular structures of Ti‐CA and a barium titanate mixed‐metal CA complex are proposed.
Potential and Impedance Imaging of Polycrystalline BiFeO3 Ceramics
Authors: Kalinin Sergei V. , Suchomel Matthew R. , Davies Peter K. , Bonnell Dawn A.
Published: July 2010

Electrostatic‐force‐sensitive scanning probe microscopy (SPM) is used to investigate grain boundary behavior in polycrystalline BiFeO3 ceramics. Scanning surface potential microscopy (SSPM) of a laterally biased sample exhibits potential drops due to resistive barriers at the grain boundaries. In this technique, the tips acts as a moving voltage probe detecting local variations of potential associated with the ohmic losses within the grains and at the grain boundaries. An approach for the quantification of grain boundary, grain interior, and contact resistivity from SSPM data is developed. Scanning impedance microscopy (SIM) is used to visualize capacitive barriers at the grain boundaries. In SIM, a dc‐biased tip detects the variations of local potential induced by the lateral ac voltage applied to the sample. Unlike the traditional dc and ac transport measurement, both of these techniques are sensitive to the variation of local potential (SSPM) or local voltage oscillation amplitude and phase (SIM), rather than to current. Therefore, special attention is paid to the relationship between SSPM and SIM images and data obtained from traditional impedance spectroscopy and dc transport measurements. For BiFeO3 ceramics excellent agreement between the local SIM measurements and impedance spectroscopy data are demonstrated.
Low‐Thermal‐Conductivity Rare‐Earth Zirconates for Potential Thermal‐Barrier‐Coating Applications
Authors: Wu Jie , Wei Xuezheng , Padture Nitin P. , Klemens Paul G. , Gell Maurice , García Eugenio , Miranzo Pilar , Osendi Maria I.
Published: July 2010

Rare‐earth zirconates have been identified as a class of low‐thermal‐conductivity ceramics for possible use in thermal barrier coatings (TBCs) for gas‐turbine engine applications. To document and compare the thermal conductivities of important rare‐earth zirconates, we have measured the thermal conductivities of the following hot‐pressed ceramics: (i) Gd2Zr2O7 (pyrochlore phase), (ii) Gd2Zr2O7 (fluorite phase), (iii) Gd2.58Zr1.57O7 (fluorite phase), (iv) Nd2Zr2O7 (pyrochlore phase), and (v) Sm2Zr2O7 (pyrochlore phase). We have also measured the thermal conductivity of pressureless‐sintered 7 wt% yttria‐stabilized zirconia (7YSZ)—the commonly used composition in current TBCs. All rare‐earth zirconates investigated here showed nearly identical thermal conductivities, all of which were ∼30% lower than the thermal conductivity of 7YSZ in the temperature range 25°–700°C. This finding is discussed qualitatively with reference to thermal‐conductivity theory.
Strontium Dialuminate SrAl4O7: Synthesis and Stability
Authors: Capron Mickaël , Douy André
Published: July 2010

Up to now, strontium dialuminate, SrAl4O7 (SA2), could be synthesized only by solidification from the high‐temperature liquid state. We describe its synthesis from a spray‐dried amorphous precursor, and specify its stability domains. Its kinetics of formation is very low. It can be crystallized in the 900–1000°C temperature range either directly with a low heating rate or via two metastable solid solutions—hexagonal strontium monoaluminate (SrAl2O4 (SA)) and γ‐alumina—by annealing at 950–1000°C. As the temperature is raised beyond 1100°C, SA2 becomes metastable, its formation is no longer possible, and the crystallization of Sr4Al14O25 (S4A7) is favored. The latter compound, whose composition is close to that of SA2, is stable up to 1500°C. At higher temperature it decomposes into SA and SA2, which in its turn decomposes into SA and SA6 (SrAl12O19). There is again another stability domain for SA2, restricted to a narrow temperature scale close to its melting point (∼1800°C). The behaviors at crystallization from amorphous precursors at low temperature and from liquid at very high temperature are symmetrical: low heating or cooling rates produce pure SA2 while too rapid kinetics result in mixtures of phases.
Laminar Ceramics Utilizing the Zirconia Tetragonal‐to‐Monoclinic Phase Transformation to Obtain a Threshold Strength
Authors: Pontin Michael G. , Rao Masa P. , Sánchez‐Herencia Antonio J. , Lange Fred F.
Published: July 2010

Ceramic laminates have been fabricated with thin layers, containing a mixture of unstabilized zirconia (MZ‐ZrO2) and alumina (Al2O3), sandwiched between thicker layers of alumina that contain a small fraction of Y2O3‐stabilized tetragonal ZrO2 to inhibit grain growth. The MZ‐ZrO2 undergoes a tetragonal‐to‐monoclinic phase transformation during cooling to produce biaxial compressive stresses in the thin layers. Cracks that extend within the thicker alumina layers can be arrested by the compressive layers to produce a threshold strength, i.e., a strength below which the probability of failure is zero. Laminates composed of Al2O3 layers 315 ± 15 μm thick and Al2O3/MZ‐ZrO2 layers 29 ± 3 μm thick exhibit a threshold strength of 507 ± 36 MPa, regardless of the MZ‐ZrO2 content, for volume fractions ≥0.35. These results, piezospectroscopic stress measurements, and microstructural observations suggest that microcracking produced during the transformation reduces the magnitude of the compressive stresses achieved, which in turn limits the magnitude of the threshold strength.
Activation of Pozzolanic Reaction of Hydrated Portland Cement Fly Ash Pastes in Sulfate Solution
Authors: Lorenzo Mari Paz , Goñi Sara , Guerrero Ana
Published: July 2010

The activation of the pozzolanic reaction of fly ash in portland cement paste immersed in sulfate solution has been studied. Mixtures of two Spanish fly ashes (ASTM class F) with 0%, 15%, and 35% replacement of portland cement by fly ash were immersed in Na2SO4 solution, of 2880 ppm SO42− concentration, for a period of 90 days. The resistance of the different mixtures to the sulfate attack was evaluated using the Koch‐Steinegger test. The results showed that all of the mixtures were sulfate resistant, despite the high Al2O3 content of the fly ash. The diffusion of SO42− and Na+ ions through the pore solution activated the pozzolanic reactivity of the fly ashes, causing microstructural changes, which were characterized by X‐ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). As a result, the flexural strength of the mixtures increased, principally for the fly ash of a lower particle size and 35% of addition.
Distribution and Energies of Grain Boundaries in Magnesia as a Function of Five Degrees of Freedom
Authors: Saylor David M. , Morawiec Adam , Rohrer Gregory S.
Published: July 2010

The multiplicity of distinct grain boundary configurations in polycrystals has made it difficult to determine the relative frequency with which each configuration is adopted. As a result, the physiochemical properties of each boundary and the influence of the distribution of boundaries on macroscopic materials properties are not well understood. Using a semiautomated system, we have measured all five macroscopically observable degrees of freedom of 4.1 × 106 boundary plane segments making up 5.2 × 106μm2 of grain boundary interface area in a magnesia polycrystal. Our observations demonstrate that not all grain boundary configurations occur with the same frequency and that the relative free energies of the different interfacial configurations influence the population distribution. Furthermore, the results indicate that relative grain boundary energies can be estimated based on the free surface energies.
Low‐Temperature Synthesis of Praseodymium‐Doped Ceria Nanopowders
Authors: Wang Yarong , Mori Toshiyuki , Li Ji‐Guang , Ikegami Takayasu
Published: July 2010

Praseodymium‐doped ceria (CeO2) nanopowders have been synthesized via a simple but effective carbonate‐coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare‐earth double carbonates, with a general formula of (NH4)0.16Ce1−xPrx(CO3)1.58·H2O (0 < x≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO2 lattice, because of the oxidation of Pr3+ to smaller Pr4+, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus‐prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.
Infrared Transparent Germanate Glass‐Ceramics
Authors: Bayya Shyam S. , Sanghera Jasbinder S. , Aggarwal Ishwar D. , Wojcik Joshua A.
Published: July 2010

A new germanate glass‐ceramic material in the BaO‐Ga2O3‐GeO2 system is developed for infrared (IR) dome and window applications. This glass‐ceramic material has a grain size of ∼0.2–0.5 μm and is transparent in the 3–5 μm IR region. Glass ceramization results in 40% improvement in hardness, 65% increase in elastic modulus, 116% increase in strength, and 134% increase in fracture toughness over the base glass.
Energy Transfer and Population Inversion in Heavy Metal Oxide Glasses Doped with Tm3+ and Tb3+
Authors: Song Jay Hyok , Heo Jong
Published: July 2010

Emission properties and energy transfer of PbO–Bi2O3–Ga2O3–GeO2 glasses codoped with Tm3+ and Tb3+ ions were investigated. The 1.48‐μm emission due to the Tm3+:3H4→3F4 transition can be used to amplify the S‐band (1460–1530‐nm) signal light. With Tb3+ addition, the lifetime and emission intensity of the Tm3+:3F4 level decreased sharply via the Tm3+:3F4→Tb3+:7F0,1,2 energy transfer. Population densities of the 3F4 and 3H4 levels in Tm3+ calculated from rate equations clearly verified that population inversion in Tm3+ ions became possible with as little as 0.1 mol% of Tb3+ addition.
Low Temperature Coefficient of Resonance Frequency Composition in the System Pb(Zr,Ti)O3—Pb(Mn1/3 Nb2/3)O3
Authors: Priya Shashank , Kim Hyeoung‐Woo , Uchino Kenji
Published: July 2010

Pb(Zr,Ti)O3–Pb(Mn1/3 Nb2/3)O3 (PZT–PMnN) system has been studied for high‐power piezoelectric applications. This study investigates this system to find out the composition with high‐power density piezoelectric characteristics and low tem‐perature coefficient of resonance frequency (TCF). It was found that the composition 0.9PZT–0.1PMnN (Zr/Ti = 0.51/0.49) modified with 6 mol% Sr exhibits a TCF of −8 ppm/°C (−20 to +80°C). Further, the dielectric and piezoelectric properties of this composition are as follows: kp= 0.53; Qm= 800; d33= 274; ε33/ε0= 1290 and tan δ=1.1%, which shows the suitability of this composition for ultrasonic devices used under fluctuating thermal environment.
Shear Strength as a Function of Test Rate for SiCf/BSAS Ceramic Matrix Composite at Elevated Temperature
Authors: Choi Sung R. , Bansal Narottam P.
Published: July 2010

Both interlaminar and in‐plane shear strengths of a unidirectional Hi‐Nicalon™‐fiber‐reinforced barium strontium aluminosilicate (SiCf/BSAS) composite were determined at 1100°C in air as a function of test rate using double‐notch shear test specimens. The composite exhibited a significant effect of test rate on shear strength, regardless of orientation. The shear strength degraded by about 50% as the test rate decreased from 3.3 × 10−1 to 3.3 × 10−5 mm/s. The rate dependency of shear strength was similar to that observed for ultimate tensile strength at 1100°C for the two‐dimensional (2‐D) SiCf/BSAS composite, in which tensile strength decreased by about 60% when the test rate varied from 5 to 0.005 MPa/s. A phenomenological, power‐law slow crack growth model is proposed and formulated to account for the rate dependency of shear strength of the composite. The proposed model has been validated with additional results of both constant stress‐rate and constant stress testing in shear at 1100°C using a 2‐D Nicalon‐fiber‐reinforced crossply magnesium aluminosilicate (SiCf/MAS‐5) ceramic matrix composite.
Compression Deformation Mechanism of Silicon Carbide: I, Fine‐Grained Boron‐ and Carbon‐Doped β‐Silicon Carbide Fabricated by Hot Isostatic Pressing
Authors: Shinoda Yutaka , Yoshida Michiyuki , Akatsu Takashi , Wakai Fumihiro
Published: July 2010

The deformation behavior of boron‐ and carbon‐doped β‐silicon carbide (B,C‐SiC) with an average grain size of 260 ± 18 nm containing 1 wt% boron was investigated by compression testing at elevated temperatures. Extensive grain growth during deformation was observed. The stress–strain curves were compensated for grain growth by assuming power‐law type of dependence on grain size and strain rate. The stress exponent n was ∼1.3 and the grain size exponent p was ∼2.7 at temperatures ranging from 1593° to 1758°C. The apparent activation energy of deformation Qd was ∼760 kJ/mol, which was lower than the activation energy for lattice diffusion of silicon and carbon in SiC and higher than that for grain‐boundary diffusion of carbon in SiC. These results suggest that the deformation mechanism of the fine‐grained B,C‐SiC is grain‐boundary sliding accommodated by the grain‐boundary diffusion.
Mechanical Strength and Microstructure of Zinc Oxide Varistor Ceramics
Authors: Balzer Beate , Hagemeister Michael , Kocher Peter , Gauckler Ludwig J.
Published: July 2010

ZnO‐based varistor ceramics were sintered under various conditions to optimize their mechanical strength. For highest strength, the optimum sintering temperature was 1070°C or below. At higher maximum temperature, the strength decreased because of grain coarsening and the increasingly inhomogeneous distribution of secondary phases thereby induced. Fracture typically started from holes associated with hollow or poorly compacted sprayed granules. All series contained the same type of critical flaws, but, depending on the sintering temperature, the fracture toughness changed, which led to different strengths. At sintering temperatures above 1050°C, the density started to decrease slightly because of swelling attributed to the pressure of gas entrapped in closed pores.
Zirconia–Silica–Carbon Coatings on Ceramic Fibers
Authors: Boakye Emmanuel E. , Hay Randall S. , Petry M. Dennis , Parthasarathy Triplicane A.
Published: July 2010

Precursors for zircon–carbon mixtures were made to coat fibers for ceramic‐matrix composites. Precursors were characterized using XRD, TGA, and DTA. Zircon formed from vanadium‐ or lithium‐doped precursors after heat treatments at ≥900°C in air, but it did not form at 1200°–1400°C in argon when large amounts of carbon were added. Some precursors were used to coat Nextel™ 720 and Hi‐Nicalon™ fibers. The coatings were characterized using SEM and TEM, and coated‐fiber tensile strengths were measured. Although zircon formed in powders, only tetragonal‐zirconia–silica mixed phases formed in fiber coatings at 1200°C in air. Loss of vanadium oxide flux to the fibers may have caused the lack of conversion to zircon. The strengths of the coated fibers were severely degraded after heat treatment at ≥1000°C in air, but not in argon. The coated fibers were compared with zirconia–carbon‐coated fibers made using similar methods. Mechanisms for fiber strength degradation are discussed.
Phase Relations and Thermal Expansion Studies in the Ceria–Yttria System
Authors: Chavan Sandeep V. , Mathews Malayilparampil D. , Tyagi Avesh K.
Published: July 2010

The synthesis, characterization, and bulk and lattice thermal expansions of a series of compounds with general composition Ce1–xYxO2–x/2 (0.0 ≤x≤ 1.0) are reported. The XRD pattern of each product was refined to learn the solid solubility limit and the homogeneity range. The solid solubility limit of YO1.5 in CeO2 lattice, under the conditions of slow cooling from 1400°C, is represented as Ce0.55Y0.45O1.775 (i.e., 45 mol% of YO1.5). The subsequent compositions were biphase. There was no solubility of CeO2 into the lattice of YO1.5. The bulk thermal expansion measurements from ambient to 1123 K, as investigated using a dilatometer, revealed that the αl (293–1123 K) values, within the homogeneity range, decreased on increased Y3+ content. A similar trend was observed for average lattice thermal expansion coefficient, αa (293–1473 K), as investigated using high‐temperature XRD. No ordered phases were obtained in this system under the used conditions. These studies on Ce1–xYxO2–x/2 (0.0 ≤x≤ 1.0) system can be used to simulate the phase relation and thermal expansion behavior of Pu1–xYxO2–x/2 (0.0 ≤x≤ 1.0), because CeO2 is widely used as a surrogate material for PuO2.
Low‐Temperature Hydrothermal Synthesis of Phase‐Pure (Ba,Sr)TiO3 Perovskite using EDTA
Authors: Gersten Bonnie L. , Lencka Malgorzata M. , Riman Richard E.
Published: July 2010

A thermodynamic model was developed to describe the stability of (Ba,Sr)TiO3 (BST) solid solutions in the Ba–Sr–Ti–K–(EDTA)–H2O (EDTA = ethylenediaminetetraacetic acid) system. Phase diagrams were computed to identify the range of conditions suitable for making phase‐pure BST. Hydrothermal experiments were performed to validate the thermodynamic model. The model was found to be more useful when an ideal solid solution was used to estimate the energetics for the BST phase instead of experimental thermodynamic data. In addition, EDTA was found to promote stable conditions for BST formation. When attempting to prepare Ba0.50Sr0.50TiO3 without EDTA, BaTiO3‐rich and SrTiO3‐rich phases precipitated separately, at 70°–160°C. However, in the presence of EDTA, a phase‐pure Ba 0.55Sr0.45TiO3 solid solution was obtained at 90°–120°C. EDTA is effective because it prevents phase heterogeneities from forming and equalizes the adsorption affinity of strontium and barium species.
Combustion Synthesis and Characterization of Nanocrystalline Tin and Tin Oxide (SnOx, x= 0–2) Particles
Authors: Hall D. L. , Wang A. A. , Joy K. T. , Miller T. A. , Wooldridge M. S.
Published: July 2010

Nanocrystalline SnOx particles (x= 0–2) were synthesized using tetramethyltin (Sn(CH3)4) vapor as the particle precursor reactant in hydrogen/oxygen/argon (H2/O2/Ar) flames. The particle composition and morphology were characterized using X‐ray diffractometry, transmission electron microscopy, and nitrogen (N2) surface adsorption. By controlling the concentration of oxygen in the reactant gases and the flame temperatures, metallic tin (Sn), tin monoxide (romarchite SnO), and/or tin dioxide (cassiterite SnO2) were generated. The crystalline powders consisted of both discrete primary particles and agglomerates, with average primary particle sizes of 23–24 nm for SnO2 and 69 nm for Sn (based on specific surface area measurements of bulk powders collected in the exhaust region of the flame). The compositional results were interpreted using equilibrium and detailed chemical kinetics models.
Incorporation of Chromium(III) and Chromium(VI) Oxides in a Simulated Basaltic, Industrial Waste Glass‐Ceramic
Authors: Huang Deng , Drummond Charles H. , Wang Jue , Blume Russell D.
Published: July 2010

Chromium, an EPA listed toxic element concentrated in many industrial wastes, was stabilized using waste vitrification. Cr2O3 and CrO3 were loaded into a simulated basaltic base composition, vitrified, and cooled at various rates. Chromium incorporation mechanisms, vitrification processability, effect of initial Cr oxidation state, and product performance were investigated. At 1500°C, Cr2O3 has a low solubility limit (0.54 wt%) in the base composition, and crystallized as Cr‐rich primary spinel (Mg,Fe)(Fe,Al,Cr)2O4. On cooling, Cr‐depleted secondary spinel and augite (Na,Ca)(Mg,Fe2+,Al)(Si,Al)2O6 crystallized. Cr(VI) was converted into Cr(III) on vitrification. The apparent viscosity of the melts was estimated using the Bottinga–Weill model as corrected by Roscoe's equation. The end products showed Cr2O3 loading capacities as high as 16.7 wt% without exceeding the toxicity‐leaching limit defined by EPA. The annealed products had Vickers's hardness of about 800 kg·F/mm2 and can be classified as medium‐grade abrasives.
Nickel–Alumina Functionally Graded Materials by Electrophoretic Deposition
Authors: Nagarajan Nandakumar , Nicholson Patrick S.
Published: July 2010

Electrophoretic deposition has been used to synthesize nickel–alumina, functionally graded materials from NiO and alumina suspensions in ethanol. Suspension stability and the kinetics of deposition were studied to determine optimum conditions. Deposition starts with an alumina suspension into which a stream of NiO suspension is injected at various flow rates to obtain the desired profiles. The latter were controlled by varying the deposition current density and component flow rate. The green bodies obtained were sintered in Ar/H2 atmosphere to reduce the NiO to nickel. Various gradation profiles were obtained illustrating the facility of EPD to synthesize continuously graded materials. NiO was used as the precursor for nickel to alleviate settling and rough columnar deposit problems.
Nanostructure and Micromechanical Properties of Silica/Silicon Oxycarbide Porous Composites
Authors: Flores Araceli , Martos César , Sánchez‐Cortés Santiago , Rubio Fausto , Rubio Juan , Oteo José L.
Published: July 2010

The microhardness–nanostructure correlation of a series of silica/silicon oxycarbide porous composites has been investigated, as a function of pyrolysis temperature, Tp. The pyrolyzed products have been studied by means of scanning electron microscopy, mercury porosimetry, chemical analysis, solid‐state 29Si‐NMR, X‐ray diffraction, Raman spectroscopy, and microindentation hardness. Two distinct regimes are found for the microhardness behavior with Tp. In the low‐temperature regime (1000°C ≤Tp < 1300°C), the material response to indentation seems to be dominated by the large amount of pores present in the samples. In this Tp range, low microhardness values, H, are found (<110 MPa). Above Tp= 1300°C, a conspicuous H increase is observed. In this high‐temperature regime (Tp= 1300–1500°C), microhardness values are shown to notably increase with increasing pyrolysis temperature. The H behavior at Tp= 1300–1500°C is discussed in terms of (i) the volume fraction of pores and the average pore size, (ii) the bond density of the oxycarbide network, and (iii) the occurrence of a nanocrystalline SiC phase.
Nanoindentation Characterization of Submicro‐ and Nano‐Sized Liquid‐Phase‐Sintered SiC Ceramics
Authors: Guicciardi Stefano , Sciti Diletta , Melandri Cesare , Bellosi Alida
Published: July 2010

Submicro‐ and nano‐sized liquid‐phase‐sintered SiC ceramics were mechanically tested by nanoindentation in the peak load range 5–400 mN. The submicro‐sized sample showed a marked indentation size effect which the nano‐sized samples did not exhibit. The relevance of indentation depth with respect to the microstructural scale has been outlined. In the investigated grain‐size range, the hardness dependence on the grain size could be described by a load‐dependent inverse Hall–Petch relation. Young's modulus was less microstructure‐ and load‐dependent. Because of the very fine microstructure, the nano‐sized SiC materials gave lower elastic values than the submicro‐sized SiC ceramic.