Grain-Boundary Resistivity of Stabilized Zirconia Films

The influence of grain-boundary density on the resistivity of grains and grain boundaries in stabilized zirconia films, made by a modified doctor blade method using Al₂O₃ as a sintering agent, was investigated at 300° to 500°C. Only cubic zirconia was detected by X-ray diffraction. However, electron probe microanalysis showed that the Al concentration was preferentially enriched near grain boundaries. The resistance per square centimeter of grain-boundary surface decreased with increasing grain-boundary density in the range 170 to 310 cm^-1 .     

Creep Deformation and the Grain-Boundary Resistivity of Tetragonal Zirconia Polycrystalline Materials

The grain-boundary resistivity of tetragonal zirconia polycrystals, which had undergone creep with different applied compressive loads and at different temperatures, has been measured with impedance spectroscopy. A stress exponent of unity was determined from strain rate versus stress data. The grain-boundary resistivity decreased significantly with increasing stress at a constant creep temperature indicating squeezing out of the glassy phase from interfaces between grains. This, however, had no effect on the activation energy for the grain-boundary resistivity.     

Inhomogeneity of Grain-Boundary Resistivity in Calcia-Stabilized Zirconia

Impedance spectroscopy with millimeter-scale electrodes was used to determine the local grain-boundary resistivity (ρ_gb) of a 1-mol%-Al₂O₃-doped CaO-stabilized ZrO₂ (CSZ) specimen. A much higher ρ_gb at the specimen surface was observed compared with that of the specimen interior. The result was mainly due to the difference in the dihedral angle of the intergranular phase, which was ∼20° at the surface and ∼100° at the interior.     

Formation of Potential Barrier Related to Grain-Boundary Character in Semiconducting Barium Titanate

Resistance–temperature ( R – T ) characteristics were measured directly at single-grain boundaries in 0.1-mol%-niobium-doped barium titanate bicrystals that had been fabricated from polycrystalline sinters, to determine a geometrical grain-boundary character dependence of the positive temperature coefficient of resistivity (PTCR) effect. Both random boundaries and low-Σ boundaries exhibit a similar grain-boundary character dependence of the PTCR effect through a simple geometrical analysis, using the coincidence of reciprocal lattice points. Differences of the R – T characteristics in individual boundaries have been explained in terms of the formation of a potential barrier that is associated with the oxidation of grain boundaries during cooling, after sintering or annealing. The grain-boundary character is likely to affect the diffusivity of O²⁻ ions and, hence, is crucial to the formation of the potential barrier.     

Space-Charge Contribution to Grain-Boundary Diffusion

The contributions of the space-charge cloud to impurity and self-diffusion in the grain-boundary region of ionic materials are shown by calculations for doped KC1. The magnitude of these effects is modest (< 10³ D_l ) and limited in dimension (10 to 200 Å). Both enhancement and reduction in the charge cloud contribution to boundary diffusivities are predicted. In the charge cloud, appreciable diffusivity enhancement is indicated only for the ion on the sublattice opposite that of the principal impurity. The large boundary-diffusion enhancements and boundary widths often reported in "pure" and doped samples cannot be explained by the space-charge contribution.     

Oxygen Grain-Boundary Diffusion in MgO

Proton activation and autoradiography were combined to measure ¹⁸O grain-boundary diffusion in undoped and Fe-doped MgO bicrystals. Additions of 7000 ppm Fe enhance O grain-boundary diffusion at 1700°C by at most a factor of 5 over that in samples that contain 1000 atomic ppm total Al and Ca. From these results, the relative importance of grain-boundary and volume diffusion to O transport in dense polycrystalline MgO at 1700°C was estimated.     

Low-Frequency Grain-Boundary Relaxation in Alumina

The internal friction spectrum of polycrystalline, sintered alumina was determined at 15 to 25 cps and 25° to 1550°C. Two relaxation mechanisms appeared to be operative. A high-temperature relaxation interpreted as stemming from diffusion-controlled grain-boundary motion was dominant above 1150°C. Assuming a mean diffusion path of 10⁻⁴ cm gave a diffusivity of D = 5.2 × 10⁻² exp (−52,000/RT) which was independent of impurity content. A lower-temperature relaxation was observed in specimens of higher impurity content at 900° C. This relaxation reached a maximum but did not behave as a Debye-type relaxation. Single-crystal alumina exhibited neither of these relaxations.     

Grain-Boundary Viscosity of BaO-Doped SiC

Internal friction characterization of the viscosity of a residual SiO₂/BaO glass, segregated to grain boundaries of polycrystalline SiC, is presented. The anelastic relaxation peak of internal friction, arising from viscous slip along grain boundaries wetted by a glass phase, is analyzed. Two SiC polycrystals, containing SiO₂/BaO glasses with different compositions, are studied and compared with a SiC polycrystal containing only pure SiO₂. The internal friction peak is first analyzed with respect to its shift upon frequency change. This analysis allows quantitative assessment of both the intrinsic viscosity and the activation energy for viscous flow of the grain-boundary phase. Both parameters markedly decrease with increasing amounts of BaO dopant, which is consistent with data reported in the literature on SiO₂ and SiO₂/BaO bulk glasses with the same nominal composition. Analysis of the peak morphology is also attempted, considering the evolution of peak width while varying the grain-boundary glass composition. Moreover, the role of microstructural parameters, such as the distributions of grain size and grain-boundary angles, on the broadening of the internal friction peak is addressed, and a procedure is proposed that allows quantitative evaluation of the activation energy for viscous flow of intergranular glass merely from the width of the internal friction peak.     

Grain-Boundary Viscosity of Polycrystalline Silicon Carbides

Internal friction experiments were conducted on three SiC polycrystalline materials with different microstructural characteristics. Characterizations of grain-boundary structures were performed by high-resolution electron microscopy (HREM). Observations revealed a common glass-film structure at grain boundaries of two SiC materials, which contained different amounts of SiO₂ glass. Additional segregation of residual graphite and SiO₂ glass was found at triple pockets, whose size was strongly dependent on the amount of SiO₂ in the material. The grain boundaries of a third material, processed with B and C addition, were typically directly bonded without any residual glass phase. Internal friction data of the three SiC materials were collected up to similar/congruent2200°C. The damping curves as a function of temperature of the SiO₂-bonded materials revealed the presence of a relaxation peak, arising from grain-boundary sliding, superimposed on an exponential-like background. In the directly bonded SiC material, only the exponential background could be detected. The absence of a relaxation peak was related to the glass-free grain-boundary structure of this polycrystal, which inhibited sliding. Frequency-shift analysis of the internal friction peak in the SiO₂-containing materials enabled the determination of the intergranular film viscosity as a function of temperature.     

Grain-Boundary Diffusion in MgO

Grain-boundary diffusion of Ni²⁺ was investigated in polycrystalline MgO and also in isolated grain boundaries in natural and prepared bi-crystals of MgO. Concentration distributions were determined with the aid of both electron microbeam probe spectroscopy and X-ray absorption analysis. Results from diffusion couples in which a surface was maintained at constant concentration during the diffusion annealing indicate that grain-boundary diffusion is the predominant transport mechanism in polycrystalline MgO at temperatures below 1700°C. Lattice diffusion becomes increasingly important at higher temperatures and eventually becomes the dominant mechanism. Concentration distributions for diffusion couples in which a fixed amount of NiO was supplied to the surface of the couple as a thin initial plating resemble those for lattice diffusion but yield anomalously high values for apparent lattice diffusion coefficients. Grain-boundary diffusion in MgO is not confined to a layer of atomic dimensions but extends over a zone of the order of microns. The activation energy for grain-boundary diffusion is less than that for lattice diffusion and is between 1 and 2 ev. Grain-boundary diffusion was observed even in tilt boundaries with a mismatch as low as 6°.     

Grain-Boundary Migration in Nonstoichiometric Solid Solutions of Magnesium Aluminate Spinel: II, Effects of Grain-Boundary Nonstoichiometry

The grain-boundary chemistry of magnesium aluminate spinel solid solutions MgO· n Al₂O₃ in which grain growth measurements were reported in part I has been investigated in order to understand the mechanism of grain-boundary migration. It is found that although segregation of impurity Ca and Si is common, much larger deviations in grain-boundary stoichiometry are present. There is an excess of Al and O relative to Mg at grain boundaries in all compositions. Grain-boundary migration appears to be rate-limited by solute drag from intrinsic defects accommodating lattice nonstoichiometry, rather than by extrinsic solutes, consistent with the observed impurity tolerance of grain-boundary mobility. Different rate-limiting defects are proposed for magnesia-rich and alumina-rich spinels.     

Relating Grain-Boundary Complexion to Grain-Boundary Kinetics I: Calcia-Doped Alumina

Because of the complex nature of internal interfaces it has been a continual challenge to link the grain growth behavior of alumina (especially the onset of abnormal grain growth) to the internal interface structure and chemistry, and the associated atomic transport rate. The present work considers the problem of normal and abnormal grain growth development in calcia-doped alumina, a system noted for its complex abnormal grain growth behavior, in terms of the new concept of interface complexions. Calcia-doped alumina was shown to exhibit four distinct grain-boundary complexions in the temperature range of 1325°–1870°C. All four complexions may coexist at a single temperature. Each complexion is associated with a characteristic grain-boundary mobility, all of which enhances the grain growth kinetics relative to undoped alumina. It was found that the activation energy for the different complexions (normal and abnormal grain growth) was approximately the same in each case (∼450 kJ/mol). This is discussed in the context of interface- versus diffusion-controlled grain growth, and it is concluded that normal and abnormal grain growth in this system is diffusion controlled.     

Grain-Boundary Characterization in a Nonstoichiometric Fine-Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space-Charge Layers

The grain-boundary chemistry of fine-grained spinel MgO· n Al₂O₃ (mean grain size below micron) has been investigated by STEM microanalysis. We have quantified the concentration of each element across the grain boundaries. Stoichiometry variations are observed from the grain-boundary region to the bulk. The Al/Mg ratio increases from 2.1 in the bulk to 2.35 at the grain-boundary regions. X-ray quantification allows us to reveal and to characterize the space-charge layer in the subgrain boundary. The grain-boundary cores are negatively charged due to     vacancies in excess, and in the subgrain-boundary region, an opposite, positive space-charge layer is obtained. The point defect composition and the characteristic (sign, space-charge potential Φ_∞) of the space-charge layer are discussed.     

Equivalent Circuit Model in Grain-Boundary Barrier Layer Capacitors

Electrical properties of BaTiO₃-based capacitors are investigated. A new model is developed to explain the frequency response of the impedance of grain-boundary barrier layer (GBBL) capacitors. This model takes into consideration the dipole polarization effect and provides a simple and effective approach to evaluate the performance of GBBL capacitors with various dopants and sintering in different atmospheres. When sintered in a reducing atmosphere, doped BaTiO₃ exhibits a higher dielectric constant and a relatively stable dieletric constant with respect to the frequency response and temperature dependence. Also, smaller grain resistivity is obtained with addition of both Dy₂O₃ and Nb₂O₅.     

Liquid-Channel Grain-Boundary Structures

A combination of independent methods was used for studying the formation, structure, and conductivity of liquid-channel grain-boundary structures (LGBS) with Bi₂CuO₄ and CuO as examples. It was shown that LGBS can be formed by partial thermal decomposition or liquid oxide permeation along boundaries of ceramic materials. It was found that the high ionic conductivity of ceramic LGBS originates from the fact that they are matrix-type distributed structures.     

Grain-Boundary Wetting-Dewetting in z= 1 SiAlON Ceramic

The grain-boundary structure of a model SiAlON polycrystal with nominal composition Si₅AlON₇ was characterized by transmission electron microscopy (TEM) both in an equilibrium (as-processed) state at room temperature and after quenching from elevated temperature. In addition, low-frequency (1–13 Hz) internal friction data were recorded as a function of temperature, showing a pronounced grain-boundary sliding peak positioned at 1030°C. High-resolution transmission electron microscopy (HRTEM) of the equilibrated low-temperature microstructure revealed residual glass only at multigrain junctions, but no amorphous intergranular films were observed. The detection of clean interfaces in the as-processed sample contradicts the internal friction data, which instead suggests the presence of a low-viscosity grain boundary phase, sliding at elevated temperatures. Therefore, a thin section of the as-sintered material was heated to 1380°C and rapidly quenched. HRTEM analysis of this sample showed, apart from residual glass pockets, wetted grain boundaries, which is in line with the internal friction experiment. This wetting-dewetting phenomenon observed in z = 1 SiAlON is expected to have a strong impact not only on high-temperature engineering ceramics but also on geological, temperature-activated processes such as volcanic eruptions.     

A Novel Method for the Uniform Incorporation of Grain-Boundary Layer Modifiers in Positive Temperature Coefficient of Resistivity Barium Titanate

The presently developed two-stage process involves diping the prefired porous disks of n -BaTiO₃ in nonaqueous solutions containing Al-buty rate, Ti-isopropoxide, and tetraethyl silicate and subsequent sintering. This leads to uniform distribution of the grain-boundary layer (GBL) modifiers (Al₂O₃+ TiO₂+ SiO₂) and better control of the grain size as well as the positive temperature coefficient of resistivity characteristics. The technique is particularly suited for GBL modifiers in low concentrations (< 1%).     

Effects of Yttrium Doping on the Grain and Grain-Boundary Resistivities of BaTiO3 for Positive Temperature Coefficient Thermistors

Several positive temperature coefficient (PTC) thermistors made of barium titanate with an excess of titania and containing additives such as yttria, and eventually silica, have been prepared following two different routes. The electrical properties of the ceramic samples have been studied at room temperature, i.e., below the transition temperature, using complex impedance spectroscopy. The latter proved to be very useful to measure separately the grain and grain-boundary resistivities which have been followed as a function of the yttrium concentration. They behave very similarly and go through a minimum for the same composition. From both electrical resistivity measurements and local chemical analysis, it is inferred that the average dopant concentration in the grains is lower than the nominal content in the starting powders. An overall interpretation is given, emphasizing the importance of liquid-phase sintering.     

Thermal Etching and Grain-Boundary Grooving Silicon Ceramics

Polished polycrystalline specimens of Si, Sic, and Si₃N₄ were heated to high temperatures and the rate of thermal etching was measured. Grain-boundary grooving occurred on silicon by surface diffusion, with a surface-diffusion coefficient given by Silicon carbide surfaces became extremely rough and very little grain-boundary grooving occurred. Silicon nitride decomposed in an N₂-H₂, atmosphere with an activation energy of 757 kJ/mol, which was very near the activation energy calculated from thermochemical data. The surfaces became fairly rough but grain-boundary grooves formed by an evaporation processsimilar to that for decomposition.     

Grain-Boundary Diffusion of U in Pure and Doped Uranium Carbides with Different C/U Ratios

"The grain-boundary diffusivity of U in UC was measured between 1200" and 2200°C in pure carbides with a wide range of C/U ratios (0.93 to 2.00) and in material doped with up to 2.4 wt% W, V, or Ta. Grain-boundary diffusion is ∼ 10³ to 10⁵ times faster than volume diffusion, and, for UC_1,0, has an activation enthalpy of 75.9 ± 9.1 kcal/mol, ∼ 55% of that for volume diffusion. In the monocarbide region, grain-boundary mobility increases as the C/U ratio decreases and is impeded by the addition of impurities.