Journal of the American Ceramic Society

Butt, Darryl P.; Wantuck, Paul J.; Sappey, Andrew D.

doi: 10.1111/j.1151-2916.1994.tb09737.xpmid: N/A

Laser diagnostic methods are developed to study the behavior of laser‐ablated zirconium carbide (ZrC). The optical emission spectra from the Zr‐C plasma plume is measured from 200 to 500 nm. Emission from zirconium atoms dominated the observed spectra. From the spectra, the average plasma temperature was estimated to be 10500 ± 1500 K. Plume formation is characterized using a CCD camera to capture emission at delay times (with respect to the ablation laser pulse) ranging from 1 μs to 1 ms and using planar laser‐induced fluorescence to map Zr atom distributions.

Dadras, Parviz; Mehrotra, Gopal M.

doi: 10.1111/j.1151-2916.1994.tb09738.xpmid: N/A

Joining of carbon‐carbon (C‐C) composites by graphite formation, using manganese, magnesium, and aluminum interlayers, has been investigated. The process involved the formation of a metal carbide by chemical reaction between the metal interlayer and the composite, followed by the decomposition of the carbide and evaporation of the metal at elevated temperatures. The maximum bonding temperature in these experiments was 2200°C. Bonding of composite specimens occurred when manganese or a powder mixture of aluminum and graphite was used as interlayers. Attempts to join C‐C pieces using a magnesium interlayer were unsuccessful. The double notch shear strengths of the joints produced using Mn interlayers were very low and ranged from 0.15 to 1.61 MPa at test temperatures of 1200° and 1400°C. The interlayer, after completion of the joining operation, consisted, in most cases, only of graphite. The joints produced with aluminum plus graphite interlayers were even weaker, with strength values of 0.11 MPa or less. The presence of aluminum could be detected in some of these joints, suggesting incomplete dissociation of Al4C3 at the maximum bonding temperature of 2150°C.

Evans, Anthony G.; Domergue, J.‐M.; Vagaggini, E.

doi: 10.1111/j.1151-2916.1994.tb09739.xpmid: N/A

A methodology for the straightforward and consistent evaluation of the constituent properties of ceramic‐matrix composites (CMCs) is summarized, based on analyses from the literature. The results provide a constitutive law capable of simulating the stresshain behavior of these materials. The approach is illustrated using data for two CMCs: SiC/CAS and SiC/SiC. The constituent properties are also used as input to mechanics procedures that characterize stress redistribution and predict the effect of strain concentrations on macroscopic performance.

Shter, Gennady E.; Grader, Gideon S.

doi: 10.1111/j.1151-2916.1994.tb09740.xpmid: N/A

A process for synthesis of ultrafine YBa2Cu3O7–x powder by oxalate coprecipitation from nearly saturated solutions of the metal acetates and a 2‐propanol solution of oxalic acid was developed. The coprecipitation was complete within 5 min in an ice bath at 0–2°C. The final stoichiometry was Y:Ba:Cu = 1:1.994:2.991, while the particle size and surface area in the homogeneous coprecipitated powder were 0.1–0.2 pm and 24.9 m2.g−1, respectively. Because of the uniformity and particle size of the coprecipitated material, reactive YBCO powder with a surface area of 1.7 m2.g−1 can be obtained at 780°C in about 12 h.

Christoffersen, Roy; Davies, Peter K.; Wei, Xiaohan; Negas, Taki

doi: 10.1111/j.1151-2916.1994.tb09741.xpmid: N/A

The crystal structure and microwave dielectric properties of (Zr1–xSnx)TiO4 ceramics with x ranging from 0.0 to 0.15 have been investigated to characterize the effect of Sn substitution on cation ordering and the effect of ordering on dielectric loss. The crystal structure of the samples was investigated using high‐resolution transmission electron microscopy and electron diffraction. Although Sn substitution inhibits ordering, the effect is progressive and ordering still proceeds to an observable degree in samples with x= 0.15. The ordered structure of all samples is of the incommensurate modular type previously observed for Sn‐free zirconium titanate compositions. In this structure individual (100) cation layers are observed to switch abruptly from Zr‐rich to Ti‐rich occupancy, indicating that ordering proceeds by the segregation of Zr and Ti into elongate two‐dimensional domains on (100). The domains get smaller as the Sn content of the samples increases and in Zr0.5Sn0.15TiO4 the length scale of the cation correlations is approximately 25–50 Å. We propose that the decrease in the driving energy for the coarsening of the domains during the ordering transformation is due to the preferential segregation of Sn to the Zr‐Ti domain boundaries. Whereas the long‐range cation ordering increases the dielectric loss of Sn‐free ceramics by 30%, it results in a negligible change in the loss properties of Zr0.91Sn0.09TiO4. It is possible that the hypothesized effect of Sn in stabilizing the Zr‐Ti boundaries may also affect the contribution of these sites to the dielectric loss of the ordered structures.

Tsuzuku, Koichiro; Fujimoto, Masayuki

doi: 10.1111/j.1151-2916.1994.tb09742.xpmid: N/A

Matrix grains of Pb‐relaxor ceramics with chemical inhomogeneity, showing the so‐called core‐shell structure already reported for BaTiO3‐based X7R ceramics, were obtained by a controlled sintering process in a MgO‐excess and PbO‐excess PMN‐PMW‐PT system. Multilayer ceramic capacitors exhibiting the X7T specifications were then fabricated from the 8 mol% MgO excess and 4 mol% PbO excess powder. Their chemical inhomogeneity was stable, showing a decrease of capacitance of only 2.9% after an 800‐h aging test.

Booth, Andrew S.; Roberts, Steven G.

doi: 10.1111/j.1151-2916.1994.tb09743.xpmid: N/A

Four‐point bend tests were performed on precracked single‐crystal MgO specimens at different temperatures and strain rates. A large amount of stable crack growth before fracture occurred; etching revealed dislocations in the crack advance region. We believe that dislocation sources near the crack tip emit loops, producing shielding and antishielding dislocations; the latter promote crack advance by a repeated microcleavage mechanism. “Warm‐prestressing” experiments, which improve the room‐temperature fracture toughness by dislocation shielding, were performed. There was a prestress below which no “warmprestressing effect” (WPSE) was exhibited and the magnitude of the WPSE increased with increasing prestressing temperature.

Brown, David D.; Green, David J.

doi: 10.1111/j.1151-2916.1994.tb09744.xpmid: N/A

An investigation was made into the source of strut cracking during the fabrication process of open cell ceramics that are produced by coating a polymeric foam. Several sources for the stress that produces these cracks were considered, viz., differential drying, thermal expansion mismatch between the polymer and the green ceramic coating, and the gas pressure produced by pyrolysis of the organic skeleton. Thermogravimetric analysis of the polymeric foam was used to estimate the gas evolution rate associated with the pyrolysis process, but this was found to be very low compared to the pressures required to cause strut damage. SEM observations on samples taken by interrupting the fabrication procedure showed the cracks were not produced during drying but rather at a temperature near the melting/decomposition point of the polymer and prior to pyrolysis. It was then deduced that the differential thermal expansion between the polymer and the ceramic coating was the source of the stress. The strut cracking is observed to occur primarily in the region of the highly curved strut edges of the polymer foam, at which the ceramic coating is often rather thin. Techniques to change the processing procedure to overcome the strut cracking are discussed.

Hay, Randall S.

doi: 10.1111/j.1151-2916.1994.tb09745.xpmid: N/A

The kinetics of the reaction of directionally solidified eutec‐tics of yttrium‐aluminum perovskite and alumina to yttrium‐aluminum garnet were measured between 1150° and 1550°C for times as long as 20 000 min. Deformation from the 11 vol% increase was observed. Samples were characterized by optical microscopy, SEM, and extensively by TEM. Surface displacements from self‐stress relaxation were measured by atomic force microscopy. YAG nucleated at yttrium silicate impurities as 20‐nm‐thick plates. For nucleation to occur, the 20‐GPa self‐stress must relax. YAG growth was parabolic with a 550‐kJ/mol activation energy. Self‐stress relaxation caused creep polygonization in YAG, forming short‐circuit diffusion pathways that may have increased the growth rate. The maximum flow stress in YAG was estimated to be 1–3 GPa and decreased as the reaction progressed. The inferred flow stresses should not have significantly decreased the reaction driving force. Eventually, off‐axis creep of alumina dominated self‐stress relaxation because its flow stress was much lower than that of YAG.

Peng, Chien H.; Desu, Seshu B.

doi: 10.1111/j.1151-2916.1994.tb09746.xpmid: N/A

Lead zirconate titanates (Pb (ZrxTi1–x)O3 or PZT) are well‐known materials with useful ferroelectric properties for nonvolatile memory applications. The device applications usually require processes which form the ferroelectric perovskite phase at low temperatures. Understanding the various aspects of structure development of PZT films is the key to develop low‐temperature processes. An effective, versatile, and nondestructive optical method was developed for the study of the structure development in PZT films. Also, models for the structure development were proposed and were verified by this optical method. Using this method, the characteristic temperatures at which both pyrochlore phase and perovskite phase are initiated and completed were identified. The pyrochloreperovskite transformation was initiated at lower temperature and completed faster for the films on the titanium‐rich side than those on the zirconium‐rich side. In addition, the volume fractions of the perovskite phase were obtained as a function of annealing temperature by this optical method.