Cihangir Duran

Templated Grain Growth of Textured Piezoelectric Ceramics

ABSTRACT Crystallographic texturing of polycrystalline ferroelectric ceramics offers a means of achieving significant enhancements in the piezoelectric response. Templated grain growth (TGG) enables the fabrication of textured ceramics with single crystal-like properties, as well as single crystals. In TGG, nucleation and growth of the desired crystal on aligned single crystal template particles results in an increased fraction of oriented material with heating. To facilitate alignment during forming, template particles must be anisometric in shape. To serve as the preferred sites for epitaxy and subsequent oriented growth of the matrix, the template particles need to be single crystal and chemically stable up to the growth temperature. Besides templating the growth process, the template particles may also serve as seed sites for phase formation of a reactive matrix. This process, referred to as Reactive TGG (RTGG), has been used to obtain highly oriented Pb(Mg1/3Nb2/3)O3-PbTiO3, Sr0.53Ba0.47Nb2O6, and (Na1/2Bi1/2)TiO3-BaTiO3. Highly oriented Bi4Ti3O12, Sr2Nb2O7, CaBi4Ti4O15, Pb(Mg1/3Nb2/3)O3-PbTiO3, Sr0.53Ba0.47Nb2O6 and (Na1/2Bi1/2)TiO3-BaTiO3 ceramics have been produced by TGG. The resulting ceramics show texture levels up to 90%, and significant enhancements in the piezoelectric properties relative to randomly oriented ceramics with comparable densities. For example, piezoelectric coefficients of textured piezoelectrics are from 2 to 3 times higher than polycrystalline ceramics and as high as 90% of the single crystal values. In textured PMN-PT, a low field (< 5 kV/cm) piezoelectric coefficient (d 33) of ∼1600 pC/N was obtained with > 0.3% strain (at 50 kV/cm). The high field dielectric and electromechanical properties of textured perovskites are more hysteretic than those of single crystals, probably as a result of clamping by the residual template particles, residual random grains, the presence of non-ferroelectric second phases, and a wide orientation distribution. Lateral clamping of one grain by another may also be an important factor in fiber-textured samples. Means to further improve the quality of texture and thus properties of textured piezoelectric ceramics by TGG are presented.

Dielectric and piezoelectric properties of textured Sr0.53Ba0.47Nb2O6 ceramics prepared by templated grain growth

Fiber textured Sr 0 . 5 3 Ba 0 . 4 7 Nb 2 O 6 ceramics were reactively sintered to ≥95% of the theoretical density from a mixture of SrNb 2 O 6 and BaNb 2 O 6 powders. Texture in (001) was obtained by templated grain growth on(001)-oriented acicular KSr 2 Nb 5 O 1 5 template particles. The most highly textured ceramics had a peak dielectric constant of 23,600 (at T c = 128 °C), a remanent polarization (P r ) of 20.3 μC/cm 2 , a saturation polarization (P s a t ) of 24 μC/cm 2 (69-96% of single crystal), and a piezoelectric charge coefficient (d 3 3 ) of 84 pC/N (76-93% of single crystal). A model, correlating polarization with grain orientation, predicts that P r increases sharply when a percolating grain network forms to transfer charge between elongated grains.

Processing and electrical properties of 0.5Pb(Yb1/2Nb1/2)O3-0.5PbTiO3 ceramics

Pb(Yb1/2Nb1/2)O3-PbTiO3 ceramics at the morphotropic phase boundary (50:50) were sintered by conventional and reactive methods to ≥95% theoretical density and grain sizes <10 μm. Excess PbO, added to enhance the densification, resulted in PbO-based non-ferroelectric phases that degraded the electrical properties. Volatilization of excess PbO by annealing the samples after sintering resulted in dense, perovskite samples and excellent electrical properties. The best electrical properties, obtained via reactive sintering, were a remanent polarization, Pr, of 0.36 C/m2, a maximum dielectric constant of 31,000 (at the Tc = 371°C and 1 kHz), a piezoelectric charge coefficient, d33, of 508 pC/N, and an electromechanical coupling coefficient, k33, of 0.61.

Colloidal processing, surface characterization, and sintering of nano ZrO 2 powders

Colloidally stable suspensions are required to fabricate dense samples with uniform microstructure by colloidal processing methods, which necessitate dispersion of ceramic powders in a liquid medium. Aqueous nano ZrO 2 suspensions were prepared using polyethylenimine (PEI) as a dispersant. PEI adsorption on nano ZrO 2 surfaces was promoted with increasing initial PEI content and suspension pH. Isoelectric point was shifted from pH 7 at 0 wt% PEI to pH 10.4 at 3 wt% PEI. Stable suspensions had mean particle sizes in the range of 100 to 150 nm and sedimentation rates less than0.4 mm/h, as compared to 2–5.5 μm and 10–50 mm/h for unstable suspensions. Samples with 98% relative density were fabricated after sintering at 1300 °C for 4 h from colloidally stable suspensions.

Liquid-phase sintering and properties of Cr3C2/NiCr cermets

Abstract Liquid-phase sintering of Cr 3 C 2 /NiCr cermets was investigated as a function of Cr 3 C 2 content (55–95 wt%) at a constant Ni-to-Cr ratio of 4. Specimens cold-isostatically pressed at 230 MPa were sintered in a vacuum and in the presence of a liquid phase, at 1300°C for 15 and 30 min. Specimens with a carbide content of 98%) within 15 min. The density of the succesfully sintered specimens decreases slightly with inreasing carbide content and sintering time. Using X-ray diffraction analysis and scanning electron micrographs, it was revealed that the microstructures consisted of a prismatic Cr 3 C 2 phase and a metallic binder phase. The hardness of the samples increased with increasing carbide content, whereas the transverse rupture strength decreased. The decrease in strength is attributed to the reduction in the metallic binder content and to poor densification. It was shown that almost fully-densified Cr 3 C 2 /NiCr cermets with carbide contents of 75–95 wt% could be produced in a vacuum through the liquid-phase sintering process at a low temperature, such as 1300°C, for a sintering time as short as 15 min.

Texture Development in KSr2Nb5O15 Ceramics Fabricated by Reactive Templated Grain Growth

Highly textured KSr2Nb5O15 ceramics (~ 94%) were reactively sintered in a matrix of SrNb2O6 and KNbO3 powders. Acicular KSr2Nb5O15 template particles (10 wt%) were used to texture the samples in [001]. Excess Nb2O5, added as a liquid former, facilitated preferential growth of template particles at the expense of matrix grains, resulting in improved f with sintering conditions. Introduction Templated grain growth (TGG) is a low cost process for fabricating textured polycrystalline ceramics. TGG entails providing a dense, fine-grained matrix for the growth of anisotropically-shaped (i.e. whisker, or platelet) template particles dispersed in the matrix [1-3]. Template particles must be large and anisometric in shape so that they can be oriented during green body formation and grow preferentially during sintering. The relative size of the matrix and template particles during densification plays an important role in the texture development. In addition, the final texture in the microstructure depends strongly on the number of template particles [4]. Reactive sintering is a process in which chemical reaction and densification are both achieved in the same heat treatment. The two steps can occur in sequence or concurrently, depending on the material and processing variables [5]. Typically, reactive sintering is done by sintering either the constituent oxides or intermediate products, which eliminates the prereaction (or calcination) step(s) and imparts an additional driving force for sintering due to the free energy of the chemical reaction [6]. Reactive sintering has been applied successfully in (Sr,Ba)Nb2O6 ceramics [6, 7]. KSr2Nb5O15 (KSN) has a tungsten-bronze (TB) crystal structure and shows electrical properties only in [001] direction [8]. Randomly-oriented polycrystalline KSN ceramics show inferior electrical properties compared to KSN single crystals due to abnormal grain growth that inhibits densification [9], limited number of polarization directions [8] and averaging of electrical properties. Therefore, texturing KSN ceramics in [001] is needed to induce enhanced electrical properties. There are limited number of results on textured KSN ceramics. Kimura et al. [10] obtained 86% texture, as measured by the Lotgering factor, by hot pressing, without using any templates. Other textured systems with a TB crystal structure include hot-pressed SrxBa1-xNb2O6 (x=0.30-0.65) (SBN30-SBN65) [11], hot-pressed Pb0.60Ba0.40Nb2O6 [12], and TGG of SBN53 using KSN templates [3].In this paper, we report a reactive TGG process to fabricate [001]-textured KSN, using acicular KSN templates and a reactive matrix of KNbO3 and SrNb2O6. Texture development was investigated as a function of sintering temperature and time in undoped and Nb2O5-rich matrices. Experimental Procedures KSr2Nb5O15 (KSN) ceramics were prepared by reactive sintering of SrNb2O6 (SN) and KNbO3 (KN) powders. SN or KN were prepared separately by ball milling SrCO3 (Alfa) or K2CO3 (Merck) and Nb2O5 (Aldrich) for 24 h in acetone using zirconia balls. Acetone was evaporated during fast stirring to eliminate differential settling. After drying, the SrCO3-Nb2O5 and K2CO3-Nb2O5 powder mixtures were calcined at 1050°C for 5 h and 900°C for 1 h, respectively. Phase purity was checked by x-ray diffraction (XRD) Key Engineering Materials Vols. 264-268 (2004) pp. 1285-1288 online at http://www.scientific.net

Densification and phase formation in seeded, reactively sintered Sr0.53Ba0.47Nb2O6 ceramics

Sr0.53Ba0.47Nb2O6 (SBN53) ceramics were reactively sintered from SrNb2O6 and BaNb2O6 powders. The formation temperature decreased from 1260°C for unseeded samples to 1130°C for the samples with 15.4 wt% seeds plus 0.85 mol% V2O5 at a heating rate of 4°C/min. For the V2O5-free samples, the activation energy was lowered from 554 ± 15 kJ/mol for unseeded samples to 241 ± 17 kJ/mol for the samples with 15.4 wt% seeds. In the V2O5-containing samples, densification and phase formation occurred simultaneously. In the V2O5-free samples, however, phase formation was completed before densification. In both cases, ceramics with ≥95% relative density were obtained. In all cases, SBN53 formed directly, rather than via a variety of intermediate SBN solid solutions. The microstructure evolution was also studied.

Photocatalytic Efficiency of ZnO/TiO2 Composite Plates in Degradation of RR180 Dye Solutions

Tape casting method was used to prepare ZnO/TiO2 composite plates for photocatalytic degradation of Reactive Red 180 (RR 180) textile dyes in aqueous solutions. TiO2 content of the plates was selected as 20 mol%. The final sintering temperature of the plates is 700°C. The sintering temperature of 700°C was selected to obtain relatively high surface area of the plates which enhances the photocatalytic activity in the degradation processes. The plates were characterized by using TG-DTA, BET, XRD and SEM. The efficiencies of the composite plates during the degradation of the RR180 dye were determined using the laboratory-scale quartz photoreactor under UVA light irradiation. Over 97% color removal efficiency was obtained in 150min process time for the dye solution at 50 mg/L concentration. Efficient reuse of plates indicated that the successful photocatalytic degradation was maintained after three consecutive runs. The addition of TiO2 is found to increase the photocatalytic activity of the ZnO plates in degradation of the selected dye from water, however, the pure ZnO plates sintered at the same temperature yielded less color removal efficiency for the same dye solution.

Hydrothermal Synthesis of Nano ZrO2 Powders

Hydrothermal synthesis of nano ZrO2 powders were carried out at a temperature range of 100 to 200°C, using zirconium solution. Formation of ZrO2 formation was accelerated with temperature and precipitation of ZrO2 took place directly from the solution. XRD analysis revealed that monoclinic ZrO2 was formed at all conditions. Crystallite size was found to range from 2.6 to 4 nm. Mean particle size increased from 93 nm at 135°C to 125 nm at 200°C after reaction time of 2 h.

Processing and properties of a 85%Cr3C2-10.5%Ni-4.5%Fe cermet

Sintering of the 85%Cr{sub 3}C{sub 2}-10.5%Ni-4.5%Fe cermet was carried out in both the solid-state (T 1,275 C) regimes. Samples sintered at temperatures above 1,275 C exhibit better density and hardness, owing to the liquid phase formed during sintering. The best mechanical properties and fine microstructures were obtained from the samples sintered at 1,300 C. Properties of solid-state sintered specimens are poorer, but improve with sintering time. Abnormal and elongated growths of carbide grains are favored by high sintering temperature and sintering time. The binder phase which is a solid solution of Ni-Fe-Cr{sub 3}C{sub 2} is distributed as separate islands in the skeleton of almost pure Cr{sub 3}C{sub 2} phase.