Additional Solid Solution Research Articles

Effect of Ba(Y1/2Nb1/2)O3 and BaZrO3 on the cation order and properties of Ba(Co1/3Nb2/3)O3 microwave ceramics

Abstract The effect of different solid solution additives and heat treatments on the cation order and dielectric properties of Ba(Co1/3Nb2/3)O3 (BCoN) were investigated. Pure Ba(Co1/3Nb2/3)O3 undergoes an order–disorder transition at ∼1400 °C, the Q value of the ordered phase (Q·f=64,400) is significantly higher than the disordered variant (Q·f=10,800). The substitution of BaZrO3 destabilizes the 1:2 order and a 1:1 ordered phase are formed for ∼10–20 mol% BZ. The order–disorder temperatures for the 1:1 BCoN–BZ phases are quite low (

Thermally Induced Coarsening of the Chemically Ordered Domains in Pb(Mg1/3Nb2/3)O3 (PMN)‐Based Relaxor Ferroelectrics

The effect of different tetravalent octahedral‐site substitutions (M4+= Zr, Tb, and Ce) on the stability of the 1:1 chemically ordered phase in Pb(Mg1/3Nb2/3)O3 (PMN) relaxor ferroelectrics were investigated using X‐ray diffractometry and transmission electron microscopy. In their as‐sintered form, all the samples adopted a phase‐separated microstructure that consisted of nanosized 1:1 ordered domains dispersed in a disordered matrix. However, after annealing at 1250°C for 24 h, a fully 1:1 ordered microstructure with large domains (>100 nm) was stabilized in 10 mol%‐terbium‐doped PMN. No ordered‐domain coarsening was observed in pure or zirconium‐doped PMN, which indicated that the size of the solid‐solution additives was critical to stimulating the growth and stability of the 1:1 order. The observation of complete 1:1 order in the terbium‐doped system supports the “random‐site” model description of the Pb(β′1/2β″1/2)O3 structure. This model also can be used to explain the insensitivity of the diffuse frequency‐dependent dielectric properties to the size of the chemically ordered domains.

Chemical order in PMN-related relaxors: structure, stability, modification, and impact on properties

High temperature thermal treatments were used to modify the cation order in several tantalate and niobate members of the Pb(Mg 1/3Nb 2/3)O 3 (PMN) family of relaxors. The observation of complete 1:1 structural order in several compositions, and the refined cation occupancies of well-ordered samples conflict with the predictions of the “space charge” model, and support the “random site” description for the B-site order. In this charge balanced model one of the positions in the ordered structure is solely occupied by Ta (Nb), while the other contains a random distribution of Mg and the remaining Ta (Nb) cations. The stability of the order and magnitude of the domain growth is strongly influenced by solid solution additives. For Pb(Mg 1/3Ta 2/3)O 3 (PMT), ordering-enhancing Zr, Sc, or La substituents increase the cation order–disorder transition temperature (∼1375°C in pure PMT) and promote extensive domain coarsening. For pure PMN a low temperature (<1000°C) order–disorder transition prevents any structure modification, and domain growth could only be realized with additives (Tb, Sc, or La) that stabilize the order to temperatures where the samples are “kinetically active”. The retention of relaxor behavior in all the fully 1:1 ordered, large-domain PMT and PMN-based ceramics suggests that the disorder on the random site is critical in frustrating ferroelectric order. By systematically controlling the concentration of ferroelectrically active cations on this position in fully ordered (1− x)Pb(Mg 1/3Ta 2/3)O 3–( x)Pb(Sc 1/2Ta 1/2)O 3 solid solutions, a crossover from relaxor to normal ferroelectric behavior was induced at x=0.5.

Strengthening of iron aluminides by vacancies and/or nickel

At room temperature, iron aluminides may be strengthened by quenched-in thermal vacancies as well as by solid solution alloying additions. Since the separation of these two effects requires precise vacancy concentration measurements, a novel extrapolation procedure was developed to determine vacancy volume fractions with an accuracy approaching 1 part in 10 000. The rate of removal of quenched-in vacancies during annealing, as well as the hardening by vacancies, were found to be influenced by solid solution additions of Ni to Fe-45 at.% Al. First, yield strength and density measurements showed that nickel slows down the removal of vacancies, making it more difficult to obtain iron aluminides with low vacancy concentrations. Second, in the absence of vacancies, additions of nickel result in substantial strengthening. Third, for high vacancy concentrations (e.g. 0.2 vol.%), nickel additions can cause softening. These results are rationalized in terms of the interaction between the vacancies and the nickel atoms.

Controlling the Chemical Order in PMN-Type Relaxors

AbstractInvestigations of pure and chemically substituted tantalate and niobate members of the Pb(Mg1/3 Nb2/3)O3 (PMN) family of perovskite relaxors demonstrate that the degree of cation ordering and size of the chemically ordered domains can be extensively modified by thermal treatment. These observations and refinements of the cation occupancies in well ordered samples conflict with the predictions of the widely accepted “space charge model”, and instead support a “random site” model for the 1:1 B-site ordering. In this model one of the positions in the ordered structure is occupied by Ta (or Nb) and the other contains a random distribution of Mg and the remaining Ta cations. The stability of the order and magnitude of the domain growth can be enhanced by relatively small concentrations of selected solid solution additives (e.g. Zr in Pb(Mg1/3Ta2/3)O3 (PMT); Th or Sc in PMN). Correlations between the stability of the 1:1 order and the size of the solid solution additive can be used to understand the different responses of the cation order in PMN and PMT to thermal treatment. Results are also presented for the dielectric response of fully ordered relaxors in the (l–x) Pb(Mg1/3Ta2/3)O3 – (x) Pb(Sc1/2 Ta1/2)O3 system. While large domain samples of PMT-rich compositions exhibit a relaxor response, a crossover to normal ferroelectric behavior is observed for x = 0.5. The change in the dielectric response can be rationalized in terms of the effect of the chemistry of the random site sub-lattice on the correlation length of the ferroelectric coupling.

EFFECT OF SOLID SOLUTION ADDITIVES ON THE DENSIFICATION AND CREEP OF GRANULAR CERAMICS

The effect of solid solution additives on the densification and creep of granular ceramics was investigated for a model system consisting of CeO2 solid solutions with Y2O3 as the additive. In the sintering of powder compacts at 1150°C, the densification rate of CeO2 at a given density decreased significantly with increasing Y3+ concentration. The reduction in the densification rate reached a factor of ≈ 100 for a Y3+ concentration of 6 at.%. Creep of dense specimens was investigated at constant strain rates of 10−5 and 10−4 s−1 in air at 1200°C. After compensation for differences in grain size, the creep rate was also found to decrease significantly with increasing Y3+ concentration. If the creep rate is assumed to be controlled by a mechanism of grain boundary diffusion, then the magnitude of the decrease is in good agreement with that observed in the sintering experiments. The results strongly indicate that it may be possible to predict changes in the steady-state creep behavior from observed changes in the sintering behavior provided that matter transport occurs by the same mechanism. They also indicate that the solid solution approach may have considerable merit for controlling the creep resistance of rare earth oxides that commonly have a high solubility for many cations. © 1997 Acta Metallurgica Inc.

ChemInform Abstract: Effect of Solid Solution Additives on the Sintering of Ultra‐fine CeO2 Powders.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Effect of solid solution additives on the sintering of ultra-fine CeO 2 powders

Ultra-fine CeO 2 powders (particle size ≈ 10–15 nm) containing up to 20 at% of various divalent and trivalent cations (Mg 2+, Ca 2+, Sc 3+, Y 3+ and Nd 3+) were prepared by chemical precipitation under hydrothermal conditions. The effects of the cation concentration, size and valency on the densification and grain growth of the compacted powders were examined during sintering at a constant heating rate of 10 °C/min. Compared to undoped CeO 2, all of the additive cations caused a shift in the densification curve to higher temperatures. However, the density and grain size achieved after sintering depended significantly on the elemental composition of the additive. When the radii of the additive cations were larger than that of the host Ce 4+ cation, (i.e. Ca 2+, Y 3+ and Nd 3), nearly full density and ultrafine grain size were achieved. Under identical sintering conditions, lower density (≈ 93–95% of the theoretical) and larger grain size were achieved when the radii of the additive cations were smaller than that of the host, (i.e. Mg 2+ and Sc 3+). Powders doped simultaneously with two cations, (e.g. Ca 2+ and Mg 2+) showed sintering and grain growth characteristics which were intermediate between those for the powders doped with single cations at the equivalent concentration.

Microstructure and chemistry of second phases in MgO- and NiO-codoped alumina by analytical transmission electron microscopy

Small amounts of additives can greatly affect the sintering of ceramic powders. Since the addition of small amounts of MgO (~ 0.25 wt%) to A1203 enables it to sinter close to theoretical density [1], the influence of small amounts of additives on the sintering of A1203 has been widely studied [2-4]. The additives form second phases which profoundly affect the sintering behaviour, i.e. reduce grain boundary mobility and inhibit exaggerated grain growth. MgO inhibits grain growth in fully dense alumina, and the degree of inhibition depends on the purity of the starting powder [5]. Studies have also been conducted concerning the influence of NiO on grain growth in alumina [4]. NiO has been reported to behave similarly to MgO [4, 6]. Sintering in MgOand FeO-codoped alumina has been studied by Zhao and Harmer [7] in order to investigate the role of multiple solid-solution additives in sintering. They observed that MgO inhibits grain growth strongly in very pure powders and FeO promotes grain growth more than densification in alumina. Therefore, FeO was not a favourable additive. In the study reported here, MgO and NiO were used as multiple solid-solution additives. The MgO-NiO system consists of a complete solid solution extending from the higher melting point of MgO to the lower melting point of NiO [8]. The purpose of this study was to investigate the microstructure and chemistry of second phases, segregated particles and crystalline defects in alumina codoped with MgO and NiO using analytical transmission electron microscopy (TEM). As a result, it became possible to infer the location of MgOand NiO-codopants and impurities during the sintering process. Alumina codoped with 0 .15wt% MgO and 0.10 wt % NiO was fabricated by hot pressing at 1480 °C and 27.58 MPa in vacuum. TEM samples were sectioned from the alumina using a low-speed diamond saw and mechanically ground to -300 ~m. Circular discs of 3 mm in diameter were core drilled from the ground sections, mechanically ground to -120/~m, then dimpled to -30/ ira . The samples were ion milled with 5 kV Ar ÷ ions at an incident angle of 12 ° until perforation was achieved. A light carbon film was evaporated on the samples to prevent charging in the electron microscope. The microstructure and chemistry of second phases, segregated particles and crystalline defects in the alumina were investigated by bright field image, convergent beam electron diffraction (CBED),

SOLID SOLUTION APPROACH FOR THE FABRICATION OF CERAMICS WITH HIGH DENSITY AND CONTROLLED MICROSTRUCTURE

Abstract The use of solid solution additives is a very effective approach for the formation, by conventional sintering, of ceramics with high density and controlled grain size. However, the number of systems for which solid solution additives have been found remains quite smalt and, with the possible exception of the MgO-doped Al2O3 system, the role of the additives in sintering is very ambiguous. The reasons for the gap in understanding the additive role are outlined. Current work for advancing the understanding in this important area of ceramic processing is described for the doped CeO2 system.

Grain growth and Ostwald ripening in chromia-doped uranium dioxide

The use of solid solution additives such as «corundum» types oxides is a practical mean for enhancing grain growth of stoichiometric uranium dioxide. The present paper deals with chromia-doped UO 2 samples. Large-grain sized fuels can be produce by addition of less than 1000 ppm of chromia to UO 2 powder prior to sintering. Above the solubility limit, excess of insoluble Cr 2 O 3 precipitates from solid solution to create second phase inclusions in UO 2 matrix. These inclusions coalesce by an Ostwald ripening phenomenon

Solid Solution Additives and the Sintering of Ceramics

ABSTRACTThe use of solid solution additives has been shown to be very effective for the formation, by conventional sintering, of ceramic materials with high density and with controlled grain size. However, the number of systems for which such additives have been successfully found remains quite small, and the role of the additives is fairly well understood in only two or three of these. This paper describes the initial part of a systematic study into the effects of solid solution additives on the sintering of ceramics. Cerium oxide was chosen as a model host powder for this work because it has appreciable solubility for many additives. A combination of kinetic data and microstructural observations indicate that the sintering and grain growth are influenced significantly by the additive size but less significantly by the additive charge. The density versus grain size relationship is almost independent of the additive below relative densities of = 0.90 but depends strongly on the additive above this density. The data are interpreted in terms of the effect of the additives on the densification to coarsening ratio.

The effect of boron on the microstructure and physical properties of chemically vapour deposited nickel films

The solid solution addition of boron greatly enhances the strength and hardness of chemically vapour deposited (CVD) nickel while dramatically changing the microstructure. The solid solubility of boron in nickel is limited, and single-phase alloys containing in excess of 0.3 at% B are supersaturated with respect to the formation of one or more intermetallic boride phases. Single-phase Ni-B alloys containing 0 to 13.0 at% B were produced by CVD on polycrystalline copper substrates at 155° C in an atmospheric pressure process. The microstructure, mechanical and physical properties were characterized for the alloys both as-deposited and after various thermal treatments by using optical microscopy, transmission electron microscopy, X-ray diffraction and micro-indentation hardness testing with a diamond pyramid indentor. The grain size of the alloy was found to decrease sharply with rising boron content. Concomitantly, the defect density of the material rose significantly, the microhardness increased and the ductility decreased. With annealing at a temperature of 300° C or greater, precipitation of the Ni3B intermetallic phase, recovery and grain growth occurred.

Solid Solution Additives and The Sintering of Ceramics

Trace additives have long been known to be capable of greatly enhancing the sinterability of ceramic powder systems. Explanations for this effect have generally been sought either in the ability of the additive to prevent abnormal grain growth or in its ability to enhance the extent of densification in comparison to that of coarsening in response to the surface energy driving forces. Recent studies of sample microstructures and of sintering dilatometry lend weight to the first explanation, particularly in systems where a degree of microstructural inhomogeneity is to be found. PM/0356

Developments in The Production of Oxide Dispersion Strengthened Superalloys

AbstractDevelopments in the production of prealloyed oxide dispersion strengthened superalloy powders have resulted in the commercial scaling-up of the mechanical alloying process. This method incorporates a uniform dispersion of yttrium oxide particles of 20–40 nm dia. into superalloy base compositions and effectively extends the maximum temperature capabilities of such alloys by several hundred degrees kelvin. Powder metallurgical consolidation techniques and controlled thermo-mechanical working processes are required to achieve optimum properties in a range of commercial alloys and product forms. One of the alloys, Inconel alloy MA 6000, is an advanced turbine blade material strengthened by solid solution additions, γ′, and yttrium oxide compounds. A zone annealing technique is used to achieve a highly directional coarse grain structure which further enhances creep resistance. In addition to components in the hot section of gas turbines, Inconel alloy MA 754 and Incoloy alloy MA 956 now are found in a ...

The interaction between dislocations and precipitates in Al-Li alloys studied by tem

The superalloy Ma 6000 E is a Ni-base alloy developed for high creep resistance at intermediate as well as at high temperatures. The alloy has a complex composition being strengthened with several solid solution additions, y' precipitates for intermediate temperature strength and inert yttria for extreme elevated temperature creep resistance. The volume fraction of the y' (Ni3Al) precipitates is approximately 50% and their size about 0.2 pm in diameter. The yttriumoxide (Y2O3) volume fraction is 2.9% with an average particle size of 28 nm. Precipitates in an as received sample, deformed in tension to rupture at 760°C, are identified with an energydispersive X-ray system (EDS). Dislocation structures are studied using a 200 kV transmission electron microscope (JEM 200 cx). It is expected that dislocations bypass the yttriumoxide particles by looping, whereas y' precipitates can be bypassed either by loopinq or shearing. With increasing temperature dislocations will tend to climb rather than form coplanar Orowan loops around the particles. The experimental results are compared with predictions based on a model calculation for the critical particle diameter where cutting is taken over by looping.

Effects of heat treatment and doping with Zr on the superconducting critical current densities of multifilamentary Nb3Sn wires

Various wire samples consisting of 19-Nb3Sn filaments in a normal metal matrix were made by heat treating composite wires of 19-Nb, Nb-0.5 wt% Zr, Nb-0.75 wt% Zr, and Nb-2.5 wt% Zr filaments embedded in a Cu-10 wt% Sn matrix. The Nb-0.75 wt% Zr rod contained the precipitate ZrO2, while the Zr in the remaining Nb rods was in solid solution. The over-all size of the composite was 2.5×10−2 cm and each filament was approximately 2.5×10−3 cm in diameter. The critical current density Jc at 4.2 °K was measured at 40 and 100 kG for a variety of heat treatments. It was found that the critical current density was a strong function of the thickness of the Nb3Sn layers formed during heat treatment. This dependence of Jc on the thickness d could be approximately represented by the expression Jc(d)≃J0 exp (−nd), where J0 is the extrapolated zero thickness critical current density. J0 was found to be approximately 2.4×106 A/cm2 at 40 kG for the Nb3Sn formed from pure Nb, 1.2×106 A/cm2 for Nb doped with solid-solution additions of Zr, and 9.5×105 A/cm2 for that formed from Nb doped with ZrO2. The exponent n falls in the range 0.52–0.67 (μm)−1 for pure Nb3Sn depending on heat treatment temperature, 0.154–0.287 (μm)−1 for Nb doped with solid-solution additions of Zr, and only 0.11 (μm)−1 for the Nb3Sn composite wire containing ZrO2. The magnetic field dependence of Jc for some of the composites was also measured to 150 kG.

Some effects of solid solution additions of copper on grain boundary sliding in aluminium

Grain boundary shear behaviour at constant shear stresses, and at temperatures in the range 350–430°C, has been examined in bicrystal specimens of aluminium and aluminium containing various additions of copper in solid solution. At grain boundary shear stresses between 50 and 90 g/mm 2 the sliding displacement was linearly related to the extent of the accompanying transcrystalline deformation. Copper additions increased the ratio of sliding to crystal deformation (γ) by increasing the rate of sliding and decreasing the rate of crystal deformation. The ratio γ also increased with decreasing stress but was little affected by temperature changes. This was in accordance with differential tests showing sliding to have a lower stress sensitivity than crystal deformation, but a temperature sensitivity only slightly higher than that of crystal deformation, and led to the conclusion that sliding was not uniquely controlled by bulk crystal deformation. At a lower grain boundary shear stress of 10 g/mm 2, when macroscopic crystal deformation was absent, copper no longer increased the sliding rate. The observed effects of copper led to the suggestion that the sliding was regulated by a slide hardening process which was intensified by macroscopic crystal deformation. Mechanisms of sliding and slide hardening are discussed.