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Showing papers in "Journal of the American Ceramic Society in 2004"


Journal ArticleDOI
TL;DR: In this article, a review on the science of the t-m transformation in ZrO2 and its application in the modeling of transformation-toughhening behavior, while also summarizing the microstructural control needed to use the benefits in ZRO2-toughened ceramics.
Abstract: The recognition of the potential for enhanced fracture toughness that can be derived from controlled, stress-activated tetragonal (t) to monoclinic (m) transformation in ZrO2-based ceramics ushered in a new era in the development of the mechanical properties of engineering ceramics and provided a major impetus for broader-ranging research into the toughening mechanisms available to enhance the fracture properties of brittle-matrix materials. ZrO2-based systems have remained a major focal point for research as developments in understanding of the crystallography of the t→m transformation have led to more-complete descriptions of the origins of transformation toughening and definition of the features required of a transformation-toughening system. In parallel, there have been significant advances in the design and control of microstructure required to optimize mechanical properties in materials developed commercially. This review concentrates on the science of the t→m transformation in ZrO2 and its application in the modeling of transformation-toughening behavior, while also summarizing the microstructural control needed to use the benefits in ZrO2-toughened ceramics.

1,552 citations


Journal ArticleDOI
TL;DR: Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot-zone components made entirely from ceramics as mentioned in this paper, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties.
Abstract: Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot-zone components made entirely from ceramics. The ceramic engine programs have had only partial success, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties, which in many respects is more advanced than of more widely used technical ceramics. This review examines from the historical standpoint the development of silicon nitride and of its processing into a range of high-grade ceramic materials. The development of understanding of microstructure–property relationships in the silicon nitride materials is also surveyed. Because silicon nitride has close relationships with the SiAlON group of materials, it is impossible to discuss the one without some reference to the other, and a brief mention of the development of the SiAlONs is included for completeness.

1,253 citations


Journal ArticleDOI
TL;DR: In this article, a review of colloidal processing of ceramics is presented with an emphasis on interparticle forces, suspension rheology, consolidation techniques, and drying behavior.
Abstract: Colloidal processing of ceramics is reviewed with an emphasis on interparticle forces, suspension rheology, consolidation techniques, and drying behavior. Particular attention is given to the scientific concepts that underpin the fabrication of particulate-derived ceramic components. The complex interplay between suspension stability and its structural evolution during colloidal processing is highlighted.

1,211 citations


Journal ArticleDOI
TL;DR: In this paper, BaZrO3 and La2Zr2O7 powders were optimized for application as powders for plasma spraying and thermal properties of these materials were determined.
Abstract: Zirconates with high melting points were investigated for application as materials for thermal barrier coatings at operating temperatures >1300°C. SrZrO3, BaZrO3, and La2Zr2O7 powders were synthesized and sintered to compacts with various levels of porosity. The sintering results indicated a promising low-sintering activity of the investigated materials. Thermal properties of these dense materials were determined. Thermal expansion coefficients were slightly lower than those of Y2O3-stabilized ZrO2 (YSZ); thermal conductivities of SrZrO3 and BaZrO3 were comparable or slightly higher than those of YSZ. La2Zr2O7 had a lower thermal conductivity. SrZrO3 was not suitable for application as a thermal barrier coating because of a phase transition at temperatures between 700° and 800°C. Mechanical properties (hardness, fracture toughness, and Young’s modulus) of dense BaZrO3 and La2Zr2O7 samples were determined by indentation techniques and showed lower hardness and Young’s modulus compared to YSZ. BaZrO3 and La2Zr2O7 powders were optimized for application as powders for plasma spraying. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner at 1200°C showed a massive attack of the BaZrO3 coating, with loss of BaO. On the other hand, the La2Zr2O7 coating showed excellent thermal stability and thermalshock behavior.

1,074 citations


Journal ArticleDOI
TL;DR: In this paper, the mid-, near-, and far-infrared (IR) spectra of synthetic, single-phase calcium silicate hydrates (C-S-H) with Ca/Si ratios (C/S) of 0.41-1.2 were analyzed.
Abstract: The mid-, near-, and far-infrared (IR) spectra of synthetic, single-phase calcium silicate hydrates (C-S-H) with Ca/Si ratios (C/S) of 0.41–1.85, 1.4 nm tobermorite, 1.1 nm tobermorite, and jennite confirm the similarity of the structure of these phases and provide important new insight into their H2O and OH environments. The main mid-IR bands occur at 950–1100, 810–830, 660–670, and 440–450 cm−1, consistent with single silicate chain structures. For the C-S-H samples, the mid-IR bands change systematically with increasing C/S ratio, consistent with decreasing silicate polymerization and with an increasing content of jennite-like structural environments of C/S ratios >1.2. The 950–1100 cm−1 group of bands due to Si-O stretching shifts first to lower wave number due to decreasing polymerization and then to higher wave numbers, possibly reflecting an increase in jennite-like structural environments. Because IR spectroscopy is a local structural probe, the spatial distribution of the jennite-like domains cannot be determined from these data. A shoulder at ∼1200 cm−1 due to Si-O stretching vibrations in Q3 sites occurs only at C/S lessthan equal to 0.7. The 660–670 cm−1 band due to Si-O-Si bending broadens and decreases in intensity for samples with C/S > 0.88, consistent with depolymerization and decreased structural order. In the near-IR region, the combination band at 4567 cm−1 due to Si-OH stretching plus O-H stretching decreases in intensity and is absent at C/S greater than ∼1.2, indicating the absence of Si-OH linkages at C/S ratios greater than this. The primary Si-OH band at 3740 cm-1 decreases in a similar way. In the far-IR region, C-S-H samples with C/S ratio greater than ∼1.3 have increased absorption intensity at ∼300 cm−1, indicating the presence of CaOH environments, even though portlandite cannot be detected by X-ray diffraction for C/S ratios <1.5. These results, in combination with our previous NMR and Raman spectroscopic studies of the same samples, provide the basis for a more complete structural model for this type of C-S-H, which is described.

1,034 citations


Journal ArticleDOI
TL;DR: In this article, the crystal structure of compounds with the general formula AB 2 X 4, which crystallize with the same atomic structure as the mineral spinel, MgAl 2 O 4, is reviewed.
Abstract: This paper reviews the crystal structure of compounds with the general formula AB 2 X 4 , which crystallize with the same atomic structure as the mineral spinel, MgAl 2 O 4 . Three degrees of freedom associated with the detailed atomic arrangements of spinels are considered here: (i) the lattice parameter, a; (ii) the anion parameter, u; and (iii) the cation inversion parameter, i. Oxide spinels are used as examples to explore the interrelationships between these parameters.

914 citations


Journal ArticleDOI
TL;DR: In this article, a comprehensive survey of the historical development of the science and technology of ferrite materials as well as applications of the ferrites is presented, with a forecast of the future of ferrites in terms of their chemistry.
Abstract: Ferrites—ceramic ferromagnetic materials—have been considered as highly important electronic materials for more than half a century. During this time, the characteristics of commercial ferrite materials, both soft and hard ferrites, have come to approach theoretical values. The quality of commercial ferrites has been improved through accumulated scientific knowledge and advanced technology. This article provides a comprehensive survey of the historical development of the science and technology of ferrite materials as well as applications of the ferrites. The article also offers a forecast of the future of ferrites in terms of their chemistry.

892 citations


Journal ArticleDOI
TL;DR: In this paper, Zirconium diboride and ZrB 2 ceramics containing 10, 20, and 30 vol% SiC particulates were prepared from commercially available powders by hot pressing.
Abstract: Zirconium diboride (ZrB 2 ) and ZrB 2 ceramics containing 10, 20, and 30 vol% SiC particulates were prepared from commercially available powders by hot pressing. Four-point bend strength, fracture toughness, elastic modulus, and hardness were measured. Modulus and hardness did not vary significantly with SiC content. In contrast, strength and toughness increased as SiC content increased. Strength increased from 565 MPa for ZrB 2 to >1000 MPa for samples containing 20 or 30 vol% SiC. The increase in strength was attributed to a decrease in grain size and the presence of WC.

731 citations


Journal ArticleDOI
TL;DR: In this article, the isothermal tetragonal-to-monoclinic transformation of a 3Y-TZP ceramic was investigated from 70° to 130°C in water and in steam by X-ray diffraction and optical interferometer techniques.
Abstract: The isothermal tetragonal-to-monoclinic transformation of a 3Y-TZP ceramic is investigated from 70° to 130°C in water and in steam by X-ray diffraction and optical interferometer techniques. Aging kinetics followed by X-ray diffraction are fitted by the Mehl-Avrami-Johnson law, suggesting nucleation and growth to be the key mechanisms for transformation. Optical interferometer observations of highly polished samples effectively reveal a nucleation and growth micromechanism for tetragonal-to-monoclinic transformation. A model based on surface change analysis is developed that fits closely to the X-ray diffraction results.

610 citations


Journal ArticleDOI
TL;DR: In this paper, the thermal conductivities of hot-pressed rare-earth zirconates have been investigated for thermal barrier coatings (TBCs) for gas-turbine engine applications.
Abstract: 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) Gd 2 Zr 2 O 7 (pyrochlore phase), (ii) Gd 2 Zr 2 O 7 (fluorite phase), (iii) Gd 2.58 Zr 1.57 O 7 (fluorite phase), (iv) Nd 2 Zr 2 O 7 (pyrochlore phase), and (v) Sm 2 Zr 2 O 7 (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.

596 citations


Journal ArticleDOI
TL;DR: In this article, the authors fabricated polycrystalline bulk samples of Ti 3 Al 1.1 C 1.8 by reactively hot isostatically pressing a mixture of titanium, graphite, and Al 4 C 3 powders at a pressure of 70 MPa and temperature of 1400°C for 16 h.
Abstract: Polycrystalline bulk samples of Ti 3 Al 1.1 C 1.8 have been fabricated by reactively hot isostatically pressing a mixture of titanium, graphite, and Al 4 C 3 powders at a pressure of 70 MPa and temperature of 1400°C for 16 h. The hot isostatically pressed samples are predominantly single phase (containing ∼4 vol% Al 2 O 3 ), fully dense, and have a grain size of ∼25 μm. This carbide is similar to Ti 3 SiC 2 , with which it is isostructural, and has an unusual combination of properties. It is relatively soft (Vickers hardness of ∼3.5 GPa) and elastically stiff (Young's modulus of 297 GPa and shear modulus of 124 GPa); yet, it is lightweight (density of 4.2 g/cm 3 ) and easily machinable. The room-temperature electrical resistivity is 0.35 ± 0.03 μΩ.m and decreases linearly as the temperature decreases. The temperature coefficient of resistivity is 0.0031 K -1 . The coefficient of thermal expansion, in the temperature range of 25°-1200°C, is 9.0 (± 0.2) x 10 -6 K -1 . The room-temperature compressive and flexural strengths are 560 ± 20 and 375 ± 15 MPa, respectively. In contrast to flexure, where the failure is brittle, the failure in compression is noncatastrophic and is accompanied by some plasticity. The origin of that plasticity is believed to be the formation of a shear band that is oriented at an angle of ∼45° to the applied load. Ti 3 Al 1.1 C 1.8 also is a highly damage-tolerant material; a 10-kg-load Vickers indentation made in a bar 1.5 mm thick reduces the postindentation flexural strength by ∼7%. This material also is quite resistant to thermal shock. At temperatures of >1000°C, the deformation in compression is accompanied by significant plasticity and very respectable ultimate compressive stresses (200 MPa at 1200°C).

Journal ArticleDOI
TL;DR: In this article, two types of nanocrystalline glass-ceramics are studied: transparent glass and high-modulus glass with precisely engineered surfaces, and the key crystalline phases include β-quartz solid solutions, characterized by low-thermalexpansion behavior; spinel, with high hardness and elastic modulus; and mullite, which shows unique chromium-luminescence behavior.
Abstract: Future applications for glass-ceramics are likely to capitalize on designed-in, highly specialized properties for the transmission, display, and storage of information. Glass-ceramics with microstructures comprised of uniformly dispersed crystals <100 nm in size offer promise for many potential new applications as well as provide unique attributes for many current products. This paper focuses on two types of nanocrystalline glass-ceramics: transparent glass-ceramics and tough, high-modulus glass-ceramics with precisely engineered surfaces. Transparent glass-ceramics are formed from certain aluminosilicate glasses capable of efficient crystal nucleation and slow growth. The key crystalline phases include β-quartz solid solutions, characterized by low-thermal-expansion behavior; spinel, with high hardness and elastic modulus; and mullite, which shows unique chromium-luminescence behavior.

Journal ArticleDOI
TL;DR: In this paper, the authors compared the experimentally observed stability range of the pyrochlore structure to A and B cation size and disorder, and found that disorder enthalpies decrease dramatically with increasing Bcation size.
Abstract: Contour plots of disorder reaction energies across a wide variety of A2B2O7 pyrochlore oxides have been produced, using atomistic simulation calculations based on energy-minimization techniques. The cations studied range from Lu3+ to La3+ on the A site and from Ti4+ to Ce4+ on the B site. The present results relate the experimentally observed stability range of the pyrochlore structure to A and B cation size and disorder. Disorder enthalpies decrease dramatically with increasing B cation size, while the rate of increase with A cation size depends markedly on the particular B cation. The association of anion Frenkel and cation antisite disorders is found to be of importance.

Journal ArticleDOI
TL;DR: In this paper, a biomimetic coating was applied on titanium and porous-tantalum implants to facilitate rapid bone formation around the implant, reducing therewith the patient's recovery time after surgery.
Abstract: The combination of the high mechanical strength of metals with the osteoconductive properties of calcium phosphates make hydroxyapatite coatings on titanium implants widely used in orthopedic surgery However, the most popular coating method, plasma spraying, exhibits some important drawbacks: the inability to cover porous implants and to incorporate biologically active agents, delamination, and particle release The aim of this study was to elaborate a dense, strong, and thick calcium-phosphate coating on titanium and porous-tantalum implants using a two-step biomimetic procedure In the first step, the implants were soaked in a solution that was 5 times more concentrated than regular simulated body fluid (SBF-A solution) A thin but uniform amorphous calcium-phosphate coating was deposited on the metal Then, the implants were immersed in the SBF-B solution, which had a similar composition as the SBF-A solution, but with decreased contents of crystal growth inhibitors (ie, Mg2+ and HCO3−) This resulted in the fast precipitation of a 30 μm thick crystalline calcium-phosphate coating The pH of the SBF-B solution and the thickness of the crystalline coating layer were studied as a function of time The Fourier transform infrared spectra and X-ray diffraction patterns showed that this new coating closely resembles bone mineral Our biomimetic coating should facilitate rapid bone formation around the implant, reducing therewith the patient's recovery time after surgery

Journal ArticleDOI
TL;DR: In this paper, the impedance of cubic perovskite BaZr0.9Y0.1O3-δ has been systematically investigated in dry and wet atmospheres at high and low oxygen partial pressures.
Abstract: The impedance of the cubic perovskite BaZr0.9Y0.1O3-δ has been systematically investigated in dry and wet atmospheres at high and low oxygen partial pressures. In the grain interior, conductivity contributions from oxygen ions, electron holes, and protons can be identified. Below 300°C, proton conduction dominates and increases linearly with the frozen-in proton concentration. The proton mobility, with an activation energy of 0.44 ± 0.01 eV is among the highest ever reported for a perovskite-type oxide proton conductor. For dry oxygen atmos-pheres, electron hole conduction dominates with an activation energy of ∼0.9 eV. At temperatures <500°C, the grain-boundary conductivity can be separated and increases upon incorporation of protons. The high electrical conductivity and chemical stability make acceptor-doped barium zirconate a good choice for application as a high-temperature proton conductor.

Journal ArticleDOI
TL;DR: In this article, the fundamental principles for green body formation are discussed; a distinction is made between physical and chemical gels based on the nature of the induced gelation; and an overview of the properties of dense suspensions is presented, with a focus on the factors controlling the maximum solids loading.
Abstract: Novel approaches for optimized powder processing of advanced ceramics are reviewed with an emphasis on direct-casting methods and solid freeform fabrication techniques. The fundamental principles for green body formation are discussed; a distinction is made between physical and chemical gels based on the nature of the induced gelation. An overview of the properties of dense suspensions is presented, with a focus on the factors controlling the maximum solids loading. Recent work on direct measurements of interparticle forces in ceramic systems is presented and related to rheological properties.

Journal ArticleDOI
TL;DR: In this paper, reaction products formed during alkaline activation, with 8M NaOH solutions, of fly ashes have been characterized with 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy.
Abstract: Reaction products formed during alkaline activation, with 8M NaOH solutions, of fly ashes have been characterized with 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. In particular, the influence of curing conditions (time and temperature of reaction) has been analyzed. NMR results show the formation of amorphous tecto-silicates in which the amount of aluminum decreases in the two consecutive formed phases. The Si/Al ratio of zeolite precursor obtained at 85°C changes from 0.95 to 1.86 when curing time is increased from 5 h to 1 week. The evolution of the mechanical properties of prepared cements has also been discussed in terms of the phases formed and texture and morphology of samples.

Journal ArticleDOI
TL;DR: Amorphous solid electrolytes in the Li2S-P2S system were prepared successfully from a mixture of crystalline Li 2S and P2S5, using a mechanical milling technique as mentioned in this paper.
Abstract: Amorphous solid electrolytes in the Li2S–P2S5 system were prepared successfully from a mixture of crystalline Li2S and P2S5, using a mechanical milling technique. The amorphous-forming region was extended to higher Li2S compositions by mechanical milling, compared with melt quenching. The pelletized samples of the 75Li2S·25P2S5 (on a mole-percent basis) amorphous powders obtained by mechanical milling for 20 h exhibited high conductivity (2 × 10−4 S/cm at room temperature) and an activation energy for conduction of 34 kJ/mol. The lithium-ion transport number of the amorphous powders was almost unity.

Journal ArticleDOI
TL;DR: In this paper, the second part of a two-part study, the authors report on the mechanical behavior of Ti3SiC2 in simple compression and flexure tests, and compared the results with those of coarse-grained (100-200 μm) Ti3 SiC2.
Abstract: In this article, the second part of a two-part study, we report on the mechanical behavior of Ti3SiC2. In particular, we have evaluated the mechanical response of fine-grained (3–5 μm) Ti3SiC2 in simple compression and flexure tests, and we have compared the results with those of coarse-grained (100–200 μm) Ti3SiC2. These tests have been conducted in the 25°–1300°C temperature range. At ambient temperature, the fine- and coarse-grained microstructures exhibit excellent damage-tolerant properties. In both cases, failure is brittle up to ∼1200°C. At 1300°C, both microstructures exhibit plastic deformation (>20%) in flexure and compression. The fine-grained material exhibits higher strength compared with the coarse-grained material at all temperatures. Although the coarse-grained material is not susceptible to thermal shock (up to 1400°C), the fine-grained material thermally shocks gradually between 750° and 1000°C. The results presented herein provide evidence for two important aspects of the mechanical behavior of Ti3SiC2: (i) inelastic deformation entails basal slip and damage formation in the form of voids, grain-boundary cracks, kinking, and delamination of individual grains, and (ii) the initiation of damage does not result in catastrophic failure, because Ti3SiC2 can confine the spatial extent of the damage.

Journal ArticleDOI
TL;DR: In this article, the photoluminescence spectra of europium-, terbium-, and praseodymium-doped Ca-α-SiAlON ceramics were reported.
Abstract: Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-α-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-α-SiAlON displayed a single broad emission band peaking at λ = 550-590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-α-SiAlONs were assigned to the allowed transition of Eu 2+ from the lowest crystal field component of 4f 6 5d to 8 S 7/2 (4f 7 ) ground-state level. The emission spectra of terbium- and praseodymium-doped Ca-α-SiAlON ceramics both consisted of several sharp lines, which were attributed to the 5 D 4 → 7 F j (j = 3, 4, 5, 6) transitions of Tb 3+ and 3 P o → 3 H j (j = 3, 4, 5) transitions of Pr 3+ , respectively. In particular, the terbium-doped α-SiAlON ceramics showed a strong green emission among these phosphors.

Journal ArticleDOI
TL;DR: The power-law dependence of the parabolic oxidation rate on the partial pressure of water vapor at all temperatures of the study indicated that the molecular species was not the sole rate-limiting oxidant as discussed by the authors.
Abstract: Chemically vapor deposited silicon carbide (CVD SiC) was oxidized at temperatures of 1000°-1400°C in H2O/O2 gas mixtures with compositions of 10-90 vol% water vapor at a total pressure of 1 atm. Additional experiments were conducted in H2O/argon mixtures at a temperature of 1100°C. Experiments were designed to minimize impurity and volatility effects, so that only intrinsic water-vapor effects were observed. The oxidation kinetics increased as the water-vapor content increased. The parabolic oxidation rates in the range of 10-90 vol% water vapor (the balance being oxygen) were approximately one order of magnitude higher than the rates that were observed in dry oxygen for temperatures of 1200°-1400°C. The power-law dependence of the parabolic oxidation rate on the partial pressure of water vapor at all temperatures of the study indicated that the molecular species was not the sole rate-limiting oxidant. The determination of an activation energy for diffusion was complicated by variations in the oxidation mechanism and oxide-scale morphology with the partial pressure of water vapor and the temperature.

Journal ArticleDOI
TL;DR: In this article, an improved thermodynamic formalism that incorporates a new physical property, the chemical expansivity, was proposed for modeling stress and strain in materials, probing defect structure, and analyzing transport and kinetic properties.
Abstract: 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.

Journal ArticleDOI
TL;DR: In this article, metakaolin (MK) and (MK + Ca(OH) 2 ) mixes were activated with 5M and 12M NaOH solutions and cured at 45°C.
Abstract: The alkali activation of metakaolin (MK) leads to the production of high-mechanical-performance network-structure materials. Adding calcium hydroxide (Ca(OH) 2 ) to the raw MK produces a somewhat different reaction: a network structure and C-S-H gel form. In the present study, MK and (MK + Ca(OH) 2 ) mixes were activated with 5M and 12M NaOH solutions and cured at 45°C. A 5M concentration, in the absence of Ca(OH) 2 , did not produce MK activation within the test time. An activator concentration of 12M resulted in complete activation and the formation of a network structure. When Ca(OH) 2 was present in the raw mix, a small amount of C-S-H gel formed.

Journal ArticleDOI
TL;DR: In this paper, a study of the complex permittivity in bismuth titanate was conducted to reveal the nature of an anomaly in the real part of the permittivities, which occurs below the Curie temperature.
Abstract: A study of the complex permittivity in bismuth titanate was conducted to reveal the nature of an anomaly in the real part of the permittivity, which occurs below the Curie temperature. This anomaly is frequency dependent and is caused by a combination of two relaxation phenomena that appear in the imaginary part of the permittivity. One of the relaxations showed classic characteristics of an ion-jump process. Niobium doping suppressed this relaxation and eliminated the nonferroelectric anomaly in permittivity. Niobium is proposed to affect the ion-jump relaxation through a decrease in the concentration and possibly the mobility of oxygen vacancies.

Journal ArticleDOI
TL;DR: In this article, the reaction path and microstructure evolution during the reactive hot isostatic pressing of Ti3SiC2, starting with titanium, SiC, and graphite powders, are reported.
Abstract: In this article, the first part of a two-part study, we report the reaction path and microstructure evolution during the reactive hot isostatic pressing of Ti3SiC2, starting with titanium, SiC, and graphite powders. A series of interrupted hot isostatic press runs have been conducted as a function of temperature (1200°–1600°C) and time (0–24 h). Based on X-ray diffractometry and scanning electron microscopy, at 1200°C, the intermediate phases are TiCx and Ti5Si3Cx. Fully dense, essentially single-phase samples are fabricated in the 1450°–1700°C temperature range. The time-temperature processing envelope for fabricating microstructures with small (3–5 μm), large (∼200 μm), and duplex grains, in which large (100–200 μm) Ti3SiC2 grains are embedded in a much finer matrix, is delineated. The microstructure evolution is, to a large extent, determined by (i) the presence of unreacted phases, mainly TiCx, which inhibits grain growth; (ii) a large anisotropy in growth rates along the c and a directions (at 1450°C, growth normal to the basal planes is about an order of magnitude smaller than that parallel to these planes; at 1600°C, the ratio is 4); and (iii) the impingement of grains. Ti3SiC2 is thermally stable under vacuum and argon atmosphere at temperatures as high as 1600°C for as long as 24 h. The influence of grain size on the mechanical properties is discussed in the second part of this study.

Journal ArticleDOI
TL;DR: In this paper, the thermal stability of Lanthanum zirconate (LZ) was studied by long-term annealing and thermal cycling, and a preferred crystal growth direction in the coating was observed by X-ray diffraction.
Abstract: Lanthanum zirconate (La 2 Zr 2 O 7 , 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 La 2 O 3 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.

Journal ArticleDOI
TL;DR: In this paper, the authors used spectral or parametric optical properties of materials, combined with knowledge of the configuration of the materials, to determine the long-range van der Waals forces.
Abstract: The London dispersion forces, along with the Debye and Keesom forces, constitute the long-range van der Waals forces. London's and Hamaker's work on the point-to-point dispersion interaction and Lifshitz's development of the continuum theory of dispersion are the foundations of our understanding of dispersion forces. Dispersion forces are present for all materials and are intrinsically related to the optical properties and the underlying interband electronic structures of materials. The force law scaling constant of the dispersion force, known as the Hamaker constant, can be determined from spectral or parametric optical properties of materials, combined with knowledge of the configuration of the materials. With recent access to new experimental and ab initio tools for determination of optical properties of materials, dispersion force research has new opportunities for detailed studies. Opportunities include development of improved index approximations and parametric representations of the optical properties for estimation of Hamaker constants. Expanded databases of London dispersion spectra of materials will permit accurate estimation of both nonretarded and retarded dispersion forces in complex configurations. Development of solutions for generalized multilayer configurations of materials are needed for the treatment of more-complex problems, such as graded interfaces. Dispersion forces can play a critical role in materials applications. Typically, they are a component with other forces in a force balance, and it is this balance that dictates the resulting behavior. The ubiquitous nature of the London dispersion forces makes them a factor in a wide spectrum of problems; they have been in evidence since the pioneering work of Young and Laplace on wetting, contact angles, and surface energies. Additional applications include the interparticle forces that can be measured by direct techniques, such as atomic force microscopy. London dispersion forces are important in both adhesion and in sintering, where the detailed shape at the crack tip and at the sintering neck can be controlled by the dispersion forces. Dispersion forces have an important role in the properties of numerous ceramics that contain intergranular films, and here the opportunity exists for the development of an integrated understanding of intergranular films that encompasses dispersion forces, segregation, multilayer adsorption, and structure. The intrinsic length scale at which there is a transition from the continuum perspective (dispersion forces) to the atomistic perspective (encompassing interatomic bonds) is critical in many materials problems, and the relationship of dispersion forces and intergranular films may represent an important opportunity to probe this topic. The London dispersion force is retarded at large separations, where the transit time of the electromagnetic interaction must be considered explicitly. Novel phenomena, such as equilibrium surficial films and bimodal wetting/dewetting, can result in materials systems when the characteristic wavelengths of the interatomic bonds and the physical interlayer thicknesses lead to a change in the sign of the dispersion force. Use of these novel phenomena in future materials applications provides interesting opportunities in materials design.

Journal ArticleDOI
TL;DR: In this article, the authors measured the excess enthalpies of nanocrystalline tetragonal, monoclinic, and amorphous zirconia, as a function of area from nitrogen adsorption (BET).
Abstract: The synthesis of nanocrystalline powders of zirconia often produces the tetragonal phase, which for coarse-grained powders is stable only at high temperatures and transforms into the monoclinic form on cooling. This stability reversal has been suggested to be due to differences in the surface energies of the monoclinic and tetragonal polymorphs. In the present study, we have used high-temperature oxide melt solution calorimetry to test this hypothesis directly. We measured the excess enthalpies of nanocrystalline tetragonal, monoclinic, and amorphous zirconia. Monoclinic ZrO 2 was found to have the largest surface enthalpy and amorphous zirconia the smallest. Stability crossovers with increasing surface area between monoclinic, tetragonal, and amorphous zirconia were confirmed. The surface enthalpy of amorphous zirconia was estimated to be 0.5 J/m 2 . The linear fit of excess enthalpies for nanocrystalline zirconia, as a function of area from nitrogen adsorption (BET) gave apparent surface enthalpies of 6.4 and 2.1 J/m , for the monoclinic and tetragonal polymorphs, respectively. Due to aggregation, the surface areas calculated from crystallite size are larger than those measured by BET. The fit of enthalpy versus calculated total interface/ surface area gave surface enthalpies of 4.2 J/m 2 for the monoclinic form and 0.9 J/m 2 for the tetragonal polymorph. From solution calorimetry, the enthalpy of the monoclinic to tetragonal phase transition for ZrO 2 was estimated to be 10 ± 1 kJ/mol and amorphization enthalpy to be 34±2 kJ/mol.

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TL;DR: In this paper, a review of crack deflecting coatings for fiber-reinforced ceramic composites is presented, and the implications on the design of coatings and of composite systems using alternative coatings are discussed.
Abstract: Fiber-reinforced ceramic composites achieve high toughness through distributed damage mechanisms. These mechanisms are dependent on matrix cracks deflecting into fiber/matrix interfacial debonding cracks. Oxidation resistance of the fiber coatings often used to enable crack deflection is an important limitation for long-term use in many applications. Research on alternative, mostly oxide, coatings for oxide and non-oxide composites is reviewed. Processing issues, such as fiber coatings and fiber strength degradation, are discussed. Mechanics work related to design of crack deflecting coatings is also reviewed, and implications on the design of coatings and of composite systems using alternative coatings are discussed. Potential topics for further research are identified.

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TL;DR: In this article, for the first time in the field of piezoelectric materials, cement-based 0-3 PZT composites were fabricated by the normal mixing and spreading method.
Abstract: To meet the requirements of development for smart or intelligent structures in civil engineering, new functional materials that have good compatibility with civil engineering structural materials are needed. In this study, for the first time in the field of piezoelectric materials, cement-based 0-3 piezoelectric (PZT) composites were fabricated by the normal mixing and spreading method. The new materials have very good compatibility with portland cement concrete. The cement-based 0-3 piezoelectric composites were shown to have a slightly higher piezoelectric factor and electromechanical coefficient than those of 0-3 PZT/polymer composites with a similar content of PZT particles; thus, they are adequate for sensor application. There is potential for the application of cement-based 0-3 PZT composites in civil engineering because of their better piezoelectric properties and good compatibility with portland cement concrete.