Showing papers in "Journal of the American Ceramic Society in 1983"

Ba(Zn1/3Ta2/3)O3 Ceramics with Low Dielectric Loss at Microwave Frequencies

Abstract: The dielectric properties at microwave frequencies of Ba(Zn1/3Ta2/3)O3 ceramics prepared by sintering were investigated. These ceramics had lower density but higher loss quality than ceramics hot-pressed at 1400°C. Loss quality was greatly improved by prolonged sintering. The Q of the ceramics measured by the dielectric resonator method was 14 000 at 12 GHz. The ceramics were investigated by X-ray diffraction analysis. It was found that Q improvement corresponds with increased Zn and Ta ordered structures in the ceramics.

A Molecular Mechanism for Stress Corrosion in Vitreous Silica

Abstract: The mechanical strength of most glasses and ceramics decreases with time under static loading in an ambient environment. This strength loss is associated with slow growth of preexisting surface flaws due to stress corrosion by water from the surrounding environment. We studied stress corrosion in vitreous silica exposed to water and several nonaqueous environments; environments which enhance stress-corrosion crack growth in silica contain active groups with electron donor sites on one end and proton donor sites at the other. These results suggest a detailed chemical model for the interaction of the environment with mechanically strained bonds in the solid at the tip of a crack. The proposed model for stress-corrosion crack growth also has implications for the long-term strength behavior of a wide variety of brittle materials.

Formation of Ultrafine Tetragonal ZrO2 Powder Under Hydrothermal Conditions

Abstract: Ultrafine tetragonal ZrO2 powder was prepared by hydrothermal treatment at 100 MPa of amorphous hydrous zirconia with distilled water and LiCl and KBr solutions. The resulting powder consisted of well-crystallized particles; at 200°C, the particle size was 16 nm and at 500°C, 30 nm. Under hydrothermal conditions tetragonal ZrO2 appears to crystallize topotactically on nuclei in the amorphous hydrous zirconia.

Revised Phase Diagram of the System ZrO2‐CeO2 Below 1400°C

Abstract: The phase diagram of the system ZrO2-CeO2 was reinvestigated using hydrothermal techniques. Cubic, tetragonal, and monoclinic solid solutions are present in this system. The tetragonal solid solution decomposes to monoclinic and cubic solid solutions by a eutectoid reaction at 1050°±50°C. The solubility limits of the tetragonal and cubic solid solutions are about 18 and 70 mol% CeO2, respectively, at 1400°C, and about 16 and 80 mol% CeO2, respectively, at 1200°C. Solubility limits of the monoclinic and cubic solid solutions are about 1.5 and 88 mol% CeO2 at 1000°C, and 1.5 and 98 mol% CeO2 at 800°C, respectively. The compound Ce2Zr3O10 is not found in this system.

Electronic Conductivity, Seebeck Coefficient, and Defect Structure of La1-xSrxFeO3 (x=0.l, 0.25)

Abstract: The electrical conductivity and Seebeck coefficients of La1-xSrxFeO3 (x=0.1, 0.25) were measured as a function of oxygen partial pressure, PO2, at T=900° to 1300°C. The electrical conduction was p type in the higher PO2 range, and n type in the lower PO2 range. The Seebeck coefficient indicated that the conduction was due to electron hopping between Fe×Fe and FeFe, in the higher PO2 range and electron hopping between Fe′Fe and Fe×Fe in the lower range. The carrier concentrations were calculated from the values of electrical conductivity and Seebeck coefficient. From the PO2 dependences of the carrier concentrations, the defect structure of La1-xSrxFeO3 was determined. It was found that the electrical properties can be described by considering the imperfections SrLa, Vo, FeFe, and Fe′Fe.

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

Abstract: The subsolidus phase relations in the entire system ZrO2-Y2O3 were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: (a) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y2O3 and ∼490°C, (b) cubic zirconia solid solutiow→δ-phase Y4Zr3O12+hexagonalphase Y6ZrO11 at 45 mol% Y2O3 and ∼1325°±25°C, and (c) yttria C-type solid solution→wcubic zirconia solid solution+ hexagonal phase Y6ZrO11 at ∼72 mol% Y2O3 and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y2O3 with ideal formula Y4Zr3O12, and another, a new hexagonal phase, at 75 mol% Y2O3 with formula Y6ZrO11. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C-type solid solution, respectively. The extent of the cubic zirconia and yttria C-type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO2-Y2O3.

Processing‐Related Fracture Origins: II, Agglomerate Motion and Cracklike Internal Surfaces Caused by Differential Sintering

Abstract: Effects of differential sintering kinetics due to differential green density were studied by fabricating agglomerate-powder matrix specimens from two Al2O3/ZrO2 (30 vol%) powders and subjecting them to a cyclic sintering schedule to allow intermittent observations. Low-green-density compacts sintered faster than high-density compacts, although high-green-density compacts reached end-point densities first. Specimens containing low-green-density agglomerates produced circumferential cracklike voids at the agglomerate/matrix interface. Agglomerates with a higher green density than the matrix were subjected to compressive strains by the matrix to produce agglomerate motion when the resulting stress field was non-symmetric. Such agglomerates also produced a variety of cracklike separations. Implications regarding sintering kinetics and end-point densities are discussed. Implications regarding strength of sintered bodies are obvious, viz. strength can be proportional to the square root of the agglomerate size.

Physics of Fracture

Abstract: The underlying physical bases of present-day fracture theory are examined. It is proposed that the atomically sharp crack should be taken as the cornerstone for modeling propagation processes at the fundamental level. lkansmission electron microscopy evidence is presented in support of this contention. Linear continuum fracture mechanics is shown to have intrinsic limitations in its capacity to describe crack-tip phenomena; a more realistic description is provided by lattice statics, incorporating the picture of a crack as a narrow slit terminated by nonlinear linkage bonds. This description establishes a powerful starting point for understanding and predicting the effects of important crack-tip interaction processes.‘pwo such processes, chemically enhanced slow crack growth and process-zone toughening, are discussed in this light. Finally, the nature of strength-controlling flaws in brittle ceramics is considered, with particular reference to the validity of the widely adopted hypothesis that such flaws may be regarded as true micro-cracks.

Infrared-transparent mullite ceramic

Abstract: Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al2O3 yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A12O3 formed precipitates of α-Al2O3 at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO2-Al2O3 phase diagram.

Role of Water Vapor in Crystallite Growth and Tetragonal‐Monoclinic Phase Transformation of ZrO2

Abstract: The role of water vapor in crystallite growth and the tetragonal-to-monoclinic phase transformation of ZrO2 was studied using three specially prepared samples: an ultrafine powder of monoclinic ZrO2 obtained by hydrolysis of ZrOCI2, an aggregated powder of tetragonal ZrO2 obtained by thermal decomposition of Zr(OH)4 under reduced pressure, and an ultrafine powder of tetragonal ZrO2 obtained by thermal decomposition of zirconyl acetate dispersed in caramel. The samples were heat-treated up to 1000°C in dry and wet atmospheres saturated with water vapor at 90°C. It was found that water vapor markedly accelerated crystallite growth for both monoclinic and tetragonal ZrO2 and facilitated the tetragonal-to-monoclinic phase transformation. Water vapor increases surface diffusion and thus enhances crystallite growth and decreases surface energy, which leads to stabilization of the tetragonal phase.

A Modified Indentation Toughness Technique

Abstract: A modified indentation technique for measuring toughness is described. The method retains the elastic/plastic basis of previous contact fracture descriptions but eliminates explicit reference to residual stress parameters in the toughness formulation. Accordingly, improved correlations between indentation data and “conventional” Kc values are obtained, even for materials (e.g. anomalous glasses) with nonideal deformation responses.

Kinetics of Crystallization of ZrF4‐Ba2‐LaF3 Glass by Differential Scanning Calorimetry

Abstract: The kinetics of devitrification in ZrF4-BaF2-LaF3 (62–33–5 mol %) glass were studied by isothermal and nonisothermal methods using differential scanning calorimetry. The crystallization reaction followed the Avrami equation, x=1-exp[—(kt)], where x is the fraction crystallized after time t. The rate con stant k obeyed an Arrhenius equation, k(s−1)=6.69 × 1021 exp The value of the Avrami exponent, n, bas 3.2×0.2, indicating three-dimensional crystal growth at a constant number of nucleation sites. Values of kinetics parame ters obtained from isothermal and nonisothermal techniques were in excellent agreement. The heat of crystallization measured from isothermal peaks was computed to be 38.5×3.5 J/g.

Sintering of PZT Ceramics: I, Atmosphere Control

Abstract: Methods of PbO atmosphere control during the sintering of PZT ceramics are briefly reviewed. The problem of atmosphere control of PZT compositions containing a PbO excess (PbO/(Zr,Ti)O2>1) is discussed. Weight-loss experiments were performed to compare with behavior predicted by reference to the PbO-rich part of the ternary PbO-ZrO2-TiO2 phase diagram. The results indicated that the PbO activity of a PZT composition is a function of the excess PbO content, contrary to earlier reports. Experimental procedures were established in order to control the PbO content within satisfactory limits. The effects of various packing powders on the PbO content of sintered PZT compacts were evaluated.

Mechanical Properties of Mullite

Abstract: The mechanical properties of alkoxy-derived, high-purity, translucent, theoretically dense mullite (3AI2O3.2SiO2) were investigated over the temperature range room temperature to 1500°C. Large agglomerates were found to contribute to the formation of porosity nests which act as strength-controlling flaws at room temperature as well as at high temperatures. Despite the slow crack growth above 1300°C, a slight increase in fracture stress and a large increase in KIc were observed up to 1500°C. These increases are explained by the dominance of energy dissipation through plastic relaxation in the plastic zone over grain-boundary sliding due to the presence of the glassy phase.

The Nonconformist Ion

Abstract: Conference sur le transport protonique dans les solides, cristallins ou amorphes, le transport se faisant par un mecanisme de Grotthuss ou sous une forme oxygenee (H 2 O ou H 3 O + )

Processing‐Related Fracture Origins: I, Observations in Sintered and Isostatically Hot‐Pressed A12O3/ZrO2 Composites

Abstract: Strengths of sintered Al2O3/ZrO2 composites are much lower than those of hot-pressed composites due to large flaws associated with the sintering process. Fractography has shown that cracklike internal surfaces are at the fracture origins of the sintered materials. It is hypothesized that the cracklike internal surfaces are developed as a result of differential sintering of agglomerates relative to their surrounding powder compact. Isostatic hot-pressing of the sintered composites quickly eliminated the cracklike internal surface. Flaws responsible for fracture of the isostatically hot-pressed composites were low-density regions that are not as easily eliminated.

Calcium Segregation to a Magnesium Oxide (100) Surface

Abstract: The segregation of calcium to a (100) cleavage surface of an MgO crystal, with bulk calcium concentration of 200 ppm was measured in situ at T=900° to 1450°C in ultrahigh vacuum, using Auger and low-energy ion-scattering spectroscopies. A measured heat of segregation of approximately -50.3 kJ/mol (-12 kcal/mol) is in favorable agreement with a value of -58.7 kJ/mol (-14 kcal/mol) determined using solute strain energy and surface free energy criteria. The equilibrium value for the calcium segregation between 950° and 1000°C is estimated to correspond to a 20% occupation of the surface cation sites.

Uniformity of Al2O3-ZrO2 Composites by Colloidal Filtration

Abstract: Dispersion states of aqueous composite Al2O3/ZrO2 colloidal suspensions were studied by measuring particle size distribution as a function of pH. Mutual dispersion was achieved at pH values of 2.0 to 3.5. Consolidated composites formed by colloidal filtration reflected the uniformity of the colloidal state. The mean flexural strength (896 MPa) of the sintered compacts was 1.6 times that of bodies consolidated by isostatic pressing.

Thermal Expansion of Alkali Borate Glasses

Abstract: The thermal expansion behavior of lithium, sodium, potassium, and rubidium borate glasses was measured. The results indicate that the “borate anomaly” in the thermal expansion coefficient occurs at ∼20 mol% alkali oxide. A maximum in the glass transformation and dilatometric softening temperatures also occurs at ∼27 mol% alkali oxide. No evidence for phase separation was observed in this study. These effects are related to the structural changes reported for these glasses by Bray and coworkers.

Effect of MgO Solute on the Kinetics of Grain Growth in A12O3

Abstract: The kinetics of grain growth in fully dense Al2O3 with and without MgO solute additions were measured. The MgO was found to retard boundary migration and to rkvelop more uniform microstructures. A grain-growth mechanism involving solute partitioning of segregated ions (calcium and magnesium) between different boundary types is proposed. Implications regarding the sintering of Al2O3 are discussed.

Energy Principle of Elastic-Plastic Fracture and Its Application to the Fracture Mechanics of a Polycrystalline Graphite

Abstract: Utilizing loading-unloading procedures on the basis of nonlinear energy principles, an empirical method for evaluating the nonlinear fracture mechanics parameters, i.e. the nonlinear energy toughness Gc, the crack growth resistance R, the Jc value, and the plastic energy dissipation rate φp, was established. These parameters were experimentally determined for an isotropic polycrystalline graphite enabling the elastic-plastic fracture mechanics of graphite to he addressed. The graphite exhibits a typical elastic-plastic fracture with ×38% of the total fracture energy consumed as plastic energy. It was concluded that the widely used assumption of the applicability of linear elastic fracture mechanics to polycrystalline graphites can lead to erroneous results if the fracture tests are conducted with the usual specimen size. The proposed experimental method for evaluating elastic-plastic fracture parameters is potentially very effective for studying various nonlinear fractures in other ceramic materials.

Sintering of PZT Ceramics: II, Effect of PbO Content on Densification Kinetics

Abstract: The presence of a PbO-rich liquid phase was previously shown to affect the densification of lead zirconate-lead titanate (PZT) ceramics The present work attempts to quantify the role of this liquid phase Increasing amounts of PbO-rich liquid phase is shown to increase the rates of densification in the initial and intermediate stages, but lower the final density This behavior is discussed In the case of PbO-deficient samples, the amount of the deficiency is shown to affect the densification rate at the final stage

Empirical Dependence of Elastic Moduli on Porosity for Ceramic Materials

Abstract: Twenty-seven sets of elastic modulus-porosity data spanning ranges > 20% in porosity were fitted by linear, exponential, Hasselman, and 2/3-power relations. The fits were tested for goodness of fit, for extrapolation to zero-porosity moduli, and for consistency in determining zero-porosity Poisson's ratios and bulk moduli. On the whole, the linear relation appears to give a superior fit. This result is seen as supportive of self-consistent theoretical calculations of moduli in porous media. Nonlinear data sets are attributed to a tendency for the spheroidicity of pores to increase with porosity.

Effect of Microstructure on the Properties of TiB2 Ceramics

Abstract: Samples of a TiB2 ceramic containing 0 to 10 wt% Ni were fabricated by hot-pressing. Several properties, including fracture strength, indentation fracture toughness, and thermal expansion (between 25° and 1000°C) were measured. Resulting data were correlated with sample microstructure and composition.

Investigation of Phase Stability in the System Sic‐AlN

Abstract: Dense samples of several compositions in the system SiC-AIN were fabricated by hot-pressing. The SiC-AIN powder was prepared by carbothermal reduction of an intimate mixture of alumina, silica, and carbon in a nitrogen atmosphere. X-ray diffraction and electron and optical microscopy were used to determine the chemical and microstructural characteristics of the hot-pressed specimens. Materials with bulk compositions between 15 and 75 wt% AIN were found to be nonhomo-geneous when hot-pressed below 2100°C. These materials were determined to be a mixture of SiC-AIN solid solutions with different compositions. The observed compositional variations were distinctly bimodal. The source of the in-homogeneity was the starting SiC-AIN powder. The powders, as well as the hot-pressed samples, consisted of a mixture of small crystals rich in SiC and large AIN-rich crystals. Compositions outside the 15 to 75 wt% AIN region were found to be single phase and to have the wurtzite structure. Hot-pressing SiC-AIN in the intermediate composition range at 2300°C produced an optically and chemically homogeneous material. The precipitation of an SiC-rich phase from a 75 wt% AIN solid solution and the precipitation of an AIN-rich phase from a 47 wt% AIN alloy when annealed at 1700°C are strong indications that a miscibility gas exists in the system SiC-AIN.

A Biaxial-Flexure Test for Evaluating Ceramic Strengths

Abstract: An improved test fixture for biaxial-tension strength testing of ceramics featuring uniform pressure loading of disks was developed and qualified. Biaxial data were obtained for an alumina ceramic, along with comparable uniaxial data from three- and four-point flexure tests. Weibull statistics provided a good description of the size effect on data from the two uniaxial tests, but underestimated the effect of stress biaxiality. The biaxial strengthening observed in the alumina ceramic is consistent with that observed previously in a glass-ceramic.

Investigation of the Phase Transition in ZrTiO4 and ZrTiO4-SnO2 Solid Solutions

Abstract: Continuous phase transitions were found to occur in ZrTiO4 above and below a major discontinuity at 1125°±10°C. The space group of both forms of ZrTiO4 is Pcnb. For specimens quenched from high temperatures, the volume of the unit cell decreases linearly from ∼1450°C to the discontinuity, with the major change occurring in the c axis. The volume decreases considerably at this temperature and continues to decrease with lower-temperature annealing. Substitutional tin in solid solution stabilizes the high-temperature structure type.

Preparation of High‐Silica Glasses from Colloidal Gels: III, Infrared Spectrophotometric Studies

Abstract: The gel process for making silica was studied by infrared spectrophotometry of solid samples in various stages of dehydration. Spectra were recorded in the high-frequency overtone region (2500 to 8000 cm-1) as well as in the region of fundamental absorption (200 to 4000 cm-1), depending on the nature of the sample. Molecular water was distinguished from silanol groups and the effects of hydrogen bonding were observed. Drying and heat treatment of gels at low temperatures ( 800°C). Hydration of siloxane groups on pore surfaces and attachment of water molecules by hydrogen bonding to these surface silanol groups is reversible for heat treatments up to 1025°C. At higher temperatures (>1025°C), active sintering takes place, converting the porous structure into a homogeneous silica network. In this process, surface silanol groups become isolated within the silica network, giving rise to vibrational frequencies characteristic of internal -OH groups. Also, at the higher temperatures silanol groups react to form siloxane groups with loss of H2O. A model is presented for formation of a gel in which hydrogen bonding initiates association of colloidal particles, followed by strengthening through interparticle solid silica precipitation.

Preparation of High-Silica Glasses from Colloidal Gels: I, Preparation for Sintering and Properties of Sintered Glasses

Abstract: A new method for preparing fused silica and high-silica glasses through the sougel process is described. Ordinarily, fused silica is formed by fusion of silica at 1900° to 2000°C. The method described consists of preparation of a porous body by mixing colloidal silica with water, drying, redispersion, molding by casting, followed by gelation, drying, and sintering at 1450° to 1500°C. The sintering is facilitated by addition of 3 to 5 wt% B2O3. The process of dispersion-drying-redispersion (so-called “double process”) results in formation of a two-mode pore structure which permits drying of cast tubes and rods without breakage. The preparation of the porous bodies, study of pore structure, and properties of resulting glasses are described in Part I, the sintering is considered in Part II, and the mechanism of gel formation is interpreted from infrared spectroscopy studies in Part III.