Thermal expansion and thermal conductivity of SmxZr1−xO2−x/2 (0.1 ≤ x ≤ 0.5) ceramics

A study was conducted of the effect of additions of samarium oxide on the thermal expansion and thermal conductivity of zirconium oxide for thermal barrier coatings. Sm_xZr_1?xO_2?x/2 (0.1 ≤ x ≤ 0.5) ceramic powders synthesized with a chemical-coprecipitation and calcination method were sintered at 1873 K for 15 h. Structures of the synthesized powders and sintered ceramics were identified by X-ray diffractometer. The morphologies of ceramic powders were observed by transmission electron microscope. The thermal expansion coefficients and thermal diffusion coefficients of Sm_xZr_1?xO_2?x/2 ceramics were studied with a high-temperature dilatometer and a laser flash diffusivity technique from room temperature to 1673 K. The thermal conductivity was calculated from thermal diffusivity, density and specific heat of bulk ceramics. Sm_0.1Zr_0.9O_1.95 ceramics consists of both monoclinic and tetragonal structures. However, Sm_0.2Zr_0.8O_1.9 and Sm_0.3Zr_0.7O_1.85 ceramics only exhibit a defect fluorite structure. Sm_0.4Zr_0.6O_1.8 and Sm_0.5Zr_0.5O_1.75 ceramics have a pyrochlore-type lattice. With the increase of Sm₂O₃ content, the linear thermal expansion of Sm_xZr_1?xO_2?x/2 ceramics increases except for Sm_0.1Zr_0.9O_1.95. The thermal conductivities of Sm_xZr_1?xO_2?x/2 ceramics ranged from 1.41 at 873 K to 1.86 W m^?1 K^?1 at room temperature in a test temperature range of room temperature to 1673 K, and the results can be explained by phonon scattering mechanism.     

Influence of ytterbium- and samarium-oxides codoping on structure and thermal conductivity of zirconate ceramics

To increase operating temperature and improve performance of gas-turbine engines, it is urgently needed to develop new thermal barrier oxides with a lower thermal conductivity than 6–8 wt.% yttria-stabilized zirconia. (Yb_xSm_1?x)₂Zr₂O₇ (0 ≤ x ≤ 1.0) ceramics were synthesized by pressureless-sintered at 1700 °C for 10 h in air. The relative density, phase structure, morphology and thermal diffusivity coefficients of (Yb_xSm_1?x)₂Zr₂O₇ ceramics were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and laser-flash method. Sm₂Zr₂O₇ and (Yb_0.1Sm_0.9)₂Zr₂O₇ ceramics exhibit a pyrochlore structure, while (Yb_xSm_1?x)₂Zr₂O₇ (0.3 ≤ x ≤1.0) ceramics have a defect fluorite-type structure. The thermal conductivities of (Yb_xSm_1?x)₂Zr₂O₇ ceramics first gradually decrease with increasing temperature, and then increase slightly above 800 °C due to the increased radiation contribution. YbSmZr₂O₇ ceramics have the lowest thermal conductivity over the entire temperature range, which is caused by the reduction of cation mean free path in ytterbium–samarium zirconate system.     

Influence of TiO2 doping on thermo-optical properties of pyrochlore Sm2(Zr1–xTix)2O7 (0≤x≤0.15) ceramics

Sm₂(Zr_1–xTi_x)₂O₇ (0≤x≤0.15) ceramics have been fabricated by pressureless-sintering method at 1973 K for 10 h in air. The influence of TiO₂ doping on microstructure and thermo-optical properties of Sm₂(Zr_1–xTi_x)₂O₇ ceramics is investigated by X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy measurements. The partial substitution of Ti⁴⁺ for Zr⁴⁺ results in a significant increase in emissivity at low wavelengths contrasted with undoped Sm₂Zr₂O₇. Sm₂(Zr_0.85Ti_0.15)₂O₇ ceramic exhibits a high emissivity of above 0.70 at 1073 K in a wavelength range of 3–16 µm, where the highest value at this temperature is more than 0.90 especially in the wavelength range of 9–14 µm. FT-IR spectra and optical absorption spectra unveil the mechanisms of enhanced emissivity in Sm₂(Zr_1–xTi_x)₂O₇ (0.05≤x≤0.15) ceramics in the intermediate infrared range, especially at the wavelengths of 3–8 µm, due to Ti⁴⁺ ion substitution for Zr⁴⁺ ion.     

Influence of Ca substitution on microstructure and electrical properties of Ba(Zr,Ti)O3 ceramics

In the present work, lead-free (Ba_1?xCa_x)(Zr_0.04Ti_0.96)O₃ (x=0.00–0.09) ceramics were fabricated via a solid-state reaction method. The microstructure and electrical properties of the ceramics were investigated. The microstructure of the BCZT ceramics showed a core shell structure at compositions of x=0.03 and 0.06. The substitution of small amount of Ba²⁺ by Ca²⁺ resulted in an improvement of the piezoelectric, dielectric and ferroelectric properties of the ceramics. The orthorhombic–tetragonal phase transition was found in the composition of x≤0.03. Piezoelectric coefficient of d₃₃～392 pC/N and lowest E_c～3.3 kV/cm with highest P_r～14.1 μC/cm² were obtained for the composition of x=0.03 while its Curie temperature (T_C) was as high as 125 °C. However, the ferroelectric to paraelectric transition temperature had slightly shifted towards room temperature with increasing Ca²⁺ concentration.     

The PZT system (PbTixZr1−xO3, 0≤x≤1.0): Dielectric response of solid solutions in broad temperature (10≤T≤1000 K) and frequency (10−2≤f≤107 Hz) ranges (Part 4)

Solid solution (SS) ceramics of the PZT (PbTi_xZr_1?xO₃, 0≤x≤1.0) system were studied in broad temperature (10≤T≤1000 K) and electric field frequency (10^?2≤f≤10⁷ Hz) ranges. Several groups of SS were distinguished, which differ by nonmonotonic behavior of dielectric parameters in the cryogenic temperature range and at Т>300 K, which results both from the defective state and from the polymorphism of SS. A conclusion is made on the expediency of use of the obtained data during the application of materials based on the PZT system in the broad range of external actions.     

Phase evolution and chemical durability of Zr1-xNd xO2-x/2 (0 ≤ x ≤ 1) ceramics

A series of Zr_1-xNd _xO_2-x/2 (0 ≤ x ≤ 1) ceramics was prepared by solid-state reaction method. The effects of Nd content on the phase evolution were investigated. The chemical durability of resulting waste forms was also examined. The results show that the ceramics with x < 0.1 show monoclinic and cubic zirconia phase, with 0.2 ≤ x < 0.4 exhibit a single cubic phase, with 0.4 ≤ x ≤ 0.6 exhibit a single pyrochlore phase, with 0.6 < x < 0.8 exhibit a single cubic phase and remain cubic phases and hexagonal Nd₂O₃ when 0.8 ≤ x ≤ 1. The unit cell parameters of the Nd-doped zirconia samples increase as the Nd content increases. Moreover, the normalized element release rates of Nd element in Nd-doped zirconia ceramics firstly decrease with leaching time and almost no change after 21 days (∼0⁻⁶ g m⁻² d⁻¹), demonstrating its good chemical durability.     

Rapid synthesis of low thermal expansion materials of Ca1−xSrxZr4P6O24

A rapid synthesis method is introduced for the synthesis of low thermal expansion materials of Ca_1?xSr_xZr₄P₆O₂₄ (x = 0, 0.5 and 1) and optimum synthesizing conditions are obtained. It is shown that these materials can be synthesized by one-step sintering by putting the preheated mixture of CaCO₃, SrCO₃ and NH₄H₂PO₄ directly into a pipe furnace at the sintering temperature (1673–1873 K). With this method, the sintering procedure is much simplified and sintering time and energy exhausts are considerably reduced with respect to the conventional solid state reactions which usually require multiple-step and longer time sintering at different temperatures with intermediate grindings for the synthesis of these materials. By putting the samples directly at the sintering temperature, the formation of the secondary phase ZrP₂O₇ can be largely avoided. This insures the rapid synthesis of Ca_1?xSr_xZr₄P₆O₂₄. MgO is introduced to increase the density of Ca_0.5Sr_0.5Zr₄P₆O₂₄ ceramics. A sintered density of 3.10 g cm^?3, relative density of 96.2% for Ca_0.5Sr_0.5Zr₄P₆O₂₄ is obtained with 1.0 wt.% MgO. The coefficients of thermal expansion are about 0.27 × 10^?6 K^?1. Raman spectroscopic and differential scanning calorimetry (DSC) analyses reveal that there are no phase transitions of all the samples from 113 K to 1423 K.     

Effect of B2O3 addition on the sintering temperature and microwave dielectric properties of Zn2SiO4 ceramics

B₂O₃ (25.0 mol%) was added to Zn_2?xSiO_4?x ceramics (0.0 ≤ x ≤ 0.5) to decrease the sintering temperature. Specimens with 0.0 ≤ x ≤ 0.3 sintered at 900 °C were well sintered with a high density due to the formation of a B₂O₃ or B₂O₃–SiO₂ liquid phase. The Q × f value of the Zn₂SiO₄ ceramic was relatively low, 32,000 GHz, most likely due to the presence of a ZnO second phase. A maximum Q × f value of 70,000 GHz was obtained for the specimens with x = 0.2–0.3, and their ?_r and τ_f values were approximately 6.0 and ?21.9 ppm/°C, respectively. Ag metal did not interact with the 25.0 mol% B₂O₃-added Zn_1.8SiO_3.8 ceramic, indicating that Zn_2?xSiO_4?x ceramics containing B₂O₃ are a good candidate materials for low temperature co-fired ceramic devices.     

The effects of sintering temperature and content of x on phase formation,microstructure and dielectric properties of (1−x)(Bi0.4871Na0.4871La0.0172TiO3)−x(BaZr0.05Ti0.95O3) ceramics prepared via the combustion technique

(1?x)(Bi_0.4871Na_0.4871La_0.0172TiO₃)?x(BaZr_0.05Ti_0.95O₃) ceramics (abbreviated (1?x)BNLT?xBZT) where 0.1≤x≤0.3 were fabricated by the combustion technique using glycine as fuel. BNLT and BZT powders were calcined at temperatures of 825 °C for 4 h and 925 °C for 6 h, respectively. After that they were mixed with the different compositions. It was found that the optimum sintering temperature of (1?x)BNLT?xBZT ceramic was obtained at 1125 °C for 2 h. This ceramic had the highest density. The structure of the (1?x)BNLT?xBZT ceramics exhibited the co-existence of tetragonal and rhombohedral phases with x≤0.1. The tetragonality increases with the increase of x content. The average grain size, the density and the Curie temperatures decrease with increasing x content. The maximum dielectric constant and the highest P_r were at about 4850 and 12.7 μC/cm², respectively, and were obtained by the 0.85BNLT?0.15BZT sample.     

Effect of Ba0.85Ca0.15Ti0.90Zr0.10O3 content on the microstructure and electrical properties of Bi0.51(Na0.82K0.18) 0.50TiO3 ceramics

(1?x)Bi_0.51(Na_0.82K_0.18)_0.50TiO₃–xBa_0.85Ca_0.15Ti_0.90Zr_0.10O₃ [(1?x)BNKT–xBCTZ] ceramics were prepared by the conventional solid-state method, and the effect of BCTZ content on their microstructure and electrical properties was investigated. A stable solid solution with a pure perovskite phase is formed between BNKT and BCTZ, and these ceramics have a coexistence of rhombohedral and tetragonal phases in the range of 0 ≤ x < 0.15. Their T_m and T_d values are strongly independent on the BCTZ content. Moreover, the sintering temperature strongly affects the ferroelectric and piezoelectric properties of these ceramics with x = 0.02. These ceramics with x = 0.02 exhibit an optimum electrical behavior of d₃₃ ～ 205, k_p ～ 0.25, P_r ～ 31.8 μC/cm², and E_c ～ 19.1 kV/cm together with a high T_d value of ～91 °C when sintered at 1180 °C and poled at an optimum condition. As a result, the (1?x)BNKT–xBCTZ ceramic is a promising candidate material for lead-free piezoelectric ceramics.     

The structure and properties of 0.95MgTiO3–0.05CaTiO3 ceramics co-doped with ZnO–ZrO2

The (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were prepared by conventional solid-state route. The dielectric properties and structure of (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were investigated. It has been found that MgTiO₃ and CaTiO₃ are the main phases and a second phase CaZrTi₂O₇ appeared in 95MCT ceramics co-doped with Zn–Zr. With Zn–Zr additive, the sintering temperature of 95MCT ceramics can be reduced to 1300 °C, and adjust the temperature coefficient of dielectric constant. With the increasing of Zr content, dielectric constant ?_r decrease from 22.6 to 19.91 and the temperature coefficient of dielectric constant α_c from 5.93 to 2.52 ppm/°C when x = 0.01, 0.02, 0.03 and 0.04 mol respectively. The 95MCT ceramics with x = 0.02 has a dielectric constant ?_r of 22.02, a dielectric loss of 2.78 × 10^?4 and a temperature coefficient of dielectric constant α_c value of 2.98 ppm/°C.     

Influence of magnesia doping on structure and electrical conductivity of pyrochlore type GdSmZr2O7

Abstract

Polycrystalline ceramic samples of magnesia doped GdSm_1–xMg_xZr₂O_7–x/2 have been prepared by conventional solid state reaction method using high purity oxides. The influence of magnesia dopant content on densification, microstructure and electrical properties of GdSm_1–xMg_xZr₂O_7–x/2 ceramics are investigated. Magnesia doping promotes the sintering densification behaviour of GdSm_1–xMg_xZr₂O_7–x/2 ceramics. GdSm_1–xMg_xZr₂O_7–x/2 (x?=?0, 0·05, 0·10) ceramics have a single phase of the pyrochlore type structure, while GdSm_1–xMg_xZr₂O_7–x/2 (x?=?0·15, 0·20) ceramics consist of the pyrochlore type structure and a small amount of magnesia as the second phase. The total conductivity of GdSm_1–xMg_xZr₂O_7–x/2 ceramics obeys the Arrhenius relation, and gradually increases with increasing temperature from 723 to 1173 K. GdSm_1–xMg_xZr₂O_7–x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1·0×10^–4 to 1·0 atm at each test temperature. The maximum value of the total conductivity is 1·29×10^–2 S cm^–1 at 1173 K for the GdSm_0·85Mg_0·15Zr₂O_6·925 ceramic.     

Creating adjoining polymorphic phase transition (PPT) regions in ferroelectric (Ba,Sr)(Zr,Ti)O3 ceramics

In this work, we report the polymorphic phase transitions(PPT) in ferroelectric Ba_0.95Sr_0.05Zr_xTi_(1-x)O₃ (BSZT, x = 0.01–0.10) ceramics synthesized by using a solid-state reaction method. The doping elements and composition ratios were selected to create adjoining PPT phase boundaries near room temperature, hence to achieve a broadened peak of piezoelectric performance with respect to composition. The temperature-composition phase diagram was constructed and the effects of PPT on the electromechanical and ferroelectric properties of the ceramics were investigated. It was revealed that the two adjacent PPT regions at room temperature showed different characteristics in property enhancement. However, due to the proximity of the phase boundaries, Ba_0.95Sr_0.05Zr_xTi_(1-x)O₃ ceramics in a fairly broad range of compositions (0.02 ≤ x ≤ 0.07) showed excellent piezoelectric properties, including a large piezoelectric constant (312 pC/N ≤ d₃₃ ≤ 365 pC/N) and a high electromechanical coupling coefficient k_p (0.42 ≤ k_p ≤ 0.49).     

Relation between piezoelectric properties of ceramics and output power density of energy harvester

(1?x)Pb(Zr_0.47Ti_0.53)O₃–xPb[(Zn_0.4Ni_0.6)_1/3Nb_2/3]O₃ [(1?x)PZT–xP(ZN)N] ceramics with 0.26 ≤ x ≤ 0.31 were sintered at 1100 °C, and their energy harvesters were fabricated. All specimens exhibit a similar energy convergence efficiency. However, the transduction coefficient (d₃₃ × g₃₃) increased with x, reaching 21.5 × 10^?15 m²/N for the x = 0.31specimen; the figure-of-merit of the specimens shows a similar variation. The output energy density of the energy harvester also increased with x, and a high output energy density of 231 mW/cm³ was obtained for the harvester fabricated using the x = 0.31 specimen, indicating that the d₃₃ × g₃₃ value significantly affects the output energy density of the energy harvester.     

Synthesis and negative thermal expansion properties of solid solutions Yb2−xLaxW3O12 (0 ≤ x ≤ 2)

A new series of rare earth solid solutions Yb_2?xLa_xW₃O₁₂ were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb_2?xLa_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb_2?xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb_2?xLa_xW₃O₁₂. Negative thermal coefficients of the Yb_2?xLa_xW₃O₁₂ (0 ≤ x ≤ 2) also vary from ?7.78 × 10^?6 K^?1 to 2.06 × 10^?6 K^?1 with increasing substituted content of lanthanum.     

Synthesis and characterization of Bi1.5Zn0.92Nb1.5−xSnxO6.92−x/2 pyrochlore ceramics

The morphological, compositional, structural, dielectric and electrical properties of Bi_1.5Zn_0.92Nb_1.5?xSn_xO_6.92?x/2 ceramics have been investigated by means of scanning electron microscopy (SEM), X-ray energy dispersion spectroscopy (EDS), X-ray diffraction (XRD), temperature and frequency dependent dielectric constant and temperature dependent conductivity measurements for Sn-contents in the range of 0.00 ≤ x ≤ 0.60. It was shown that single phase of the pyrochlore ceramics can only be obtained for x ≤ 0.25. Above this value a ZnO phase appeared in the XRD patterns and SEM micrographs as well. An increase in the lattice constant and in the temperature coefficient of dielectric constant and a decrease in the dielectric constant values with increasing Sn content was observed for the ceramics which exhibited a single phase formation. A temperature dependent but frequency invariant dielectric constant was observed for this type of ceramics. The lowest electrical conductivity and highest dielectric constant was observed for the sample which contains 0.06 Sn. The Bi_1.5Zn_0.92Nb_1.5?xSn_xO_6.92?x/2 pyrochlore ceramic conductivities are thermally active above 395 K. For temperatures greater than 395 K, the conductivity activation energy which was found to be 0.415 eV for the pure sample increased to 1.371 eV when sample was doped with 0.06 Sn.     

High-temperature conductivity,stability and redox properties of Fe3−xAlxO4 spinel-type materials

Iron-based oxides are considered as promising consumable anode materials for high temperature pyroelectrolysis. Phase relationships, redox stability and electrical conductivity of Fe_3?xAl_xO₄ spinels were studied at 300–1773 K and p(O₂) from 10^?5 to 0.21 atm. Thermogravimetry/XRD analysis revealed metastability of the sintered ceramics at 300–1300 K. Low tolerance against oxidation leads to dimensional changes of ceramics upon thermal cycling. Activation energies of the total conductivity corresponded to the range of 16–26 kJ/mol at 1450–1773 K in Ar atmosphere. At 1573–1773 K and p(O₂) ranging from 10^?5 to 0.03 atm, the total conductivity of Fe_3?xAl_xO₄ is nearly independent of the oxygen partial pressure. The conductivity values of Fe_3?xAl_xO₄ (0.1 ≤ x ≤ 0.4) at 1773 K and p(O₂) ～10^?5 to 10^?4 atm were found to be only 1.1–1.5 times lower than for Fe₃O₄, showing high potential of moderate aluminium additions as a strategy for improvement of refractoriness for magnetite without significant deterioration of electronic transport.     

Phase evolution and microstructural studies in CaZrTi2O7 (zirconolite)–Sm2Ti2O7 (pyrochlore) system

From the characterization of a series compositions with general stoichiometry as Ca_1−xZr_1−xSm_2xTi₂O₇ (0.00 ≤ x ≤ 1.00), the phase evolution between zirconolite (CaZrTi₂O₇) and pyrochlore type Sm₂Ti₂O₇ has been elucidated. All the compositions were prepared by high temperature solid state reaction and characterized by powder X-ray diffraction (XRD) and electron probe for microanalyses (EPMA). Three major phase fields, namely two layer (2-M) or four layer (4-M) monoclinic zirconolite and cubic pyrochlore structure types were observed in this system. In addition, a feeble amount of perovskite type phase is found to coexist with zirconolite phase. 4-M zirconolite phase is observed as single phase field at the composition with x = 0.30 and 0.35, while cubic pyrochlore phase is observed as single phase at the compositions with x ≥ 0.60. Further, the composition and microstructure of coexisting phases are verified by back scattered electron image and EPMA studies.     

Enhanced electrical properties of lead-free BNLT–BZT ceramics by thermal treatment technique

Electrical properties of lead-free solid solution ceramics from the Bi_0.4871Na_0.4871La_0.0172TiO₃ (BNLT) and BaZr_0.05Ti_0.95O₃ (BZT) system have been improved by a thermal treatment technique. A modified two step mixed-oxide method was employed for the preparation of the (1?x)BNLT–xBZT ceramics, where x=0.06, 0.09, 0.12 and 0.20. After sintering at 1125 °C for 4 h, the BNLT–BZT ceramics were annealed at 825, 925 and 1025 °C. The annealing treatment caused an increase in dielectric constant of BNLT–BZT ceramics with x≤0.09 mol% and with x higher than 0.09 mol% the dielectric value dropped considerably. The ferroelectric properties of all annealed ceramic samples tend to decrease with increasing annealing temperature as confirmed by the slimmer P–E loops. The piezoelectric coefficient (d₃₃) increased with annealing temperatures and a maximum value of ～170 pC/N was obtained from the ceramic samples annealed at 1025 °C with x=0.02.     

Effects of structural characteristics on microwave dielectric properties of (1 − x)Ca0.85Nd0.1TiO3–xLnAlO3 (Ln = Sm,Er and Dy) ceramics

Dependence of microwave dielectric properties on the structural characteristics of (1 − x)Ca_0.85Nd_0.1TiO₃–xLnAlO₃ (Ln = Sm, Dy and Er) ceramics were investigated as a function of LnAlO₃ content (0.05 ≤ x ≤ 0.25). For the specimens with SmAlO₃, a single phase with orthorhombic perovskite was obtained through the entire composition, however, Dy₂Ti₂O₇ and Er₂Ti₂O₇ were detected as a secondary phase along with the orthorhombic perovskite phase for the specimens with DyAlO₃ (x = 0.25) and ErAlO₃ (0.10 ≤ x ≤ 0.25), respectively. With an increase of LnAlO₃ content, the dielectric constant decreased due to the smaller ionic polarizability of LnAlO₃ than Ca_0.85Nd_0.1TiO₃. The temperature coefficient of the resonant frequency (TCF) decreased with LnAlO₃ content resulted from an increase of oxygen octahedral distortion.