Ceramics For High-temperature Applications Research Articles

Investigation of pyrochlore-type A2Sn2O7 (A = La, Nd, Sm, or Gd) ceramics as negative temperature coefficient thermistors for high-temperature application

This study puts forward a series of thermosensitive materials based on A2Sn2O7 (A = La, Nd, Sm, or Gd) ceramics for high-temperature applications. The A2Sn2O7 (A = La, Nd, Sm, or Gd) ceramics were synthesized following the solid-state route and characterized using techniques involving X-Ray diffraction, scanning electron microscopy, and Raman spectroscopy. It could be verified that the A2Sn2O7 (A = La, Nd, Sm, or Gd) ceramics had pyrochlore structures with relative densities ranging from 95.8 to 97.6%. The resistance-temperature measurements confirm that these ceramics have unique oxygen-insensitive negative temperature coefficient properties. The logarithm of resistivity depends on the reciprocal of temperature with excellent linearity over a wide temperature range of 400–1300 °C. A2Sn2O7 (A = La, Nd, Sm, or Gd) ceramics also exhibited well-ageing characteristics at their maximum upper applicable temperature of 1300 °C, in addition to the high resistivity and thermal constant B value that assured high accuracy. These properties outperformed those of the majority of high-temperature negative temperature coefficient ceramics based on scheelite or perovskite structures. The results suggested that the A2Sn2O7 (A = La, Nd, Sm, or Gd) ceramics could be potentially used for fabricating high-temperature NTC thermistors.

Templated grain growth of Bi(Zn0.5Zr0.5)O3 modified BiScO3−PbTiO3 piezoelectric ceramics for high temperature applications

ABSTRACT 2.5Bi(Zn0.5Zr0.5)O3−37.5BiScO3−60PbTiO3 (BZZ-BS-PT) ceramics were successfully textured in the [001] by templated grain growth (TGG) process using 5 vol% <001>-oriented BaTiO3 (BT) plate-like templates. The templates were aligned in the matrix powder via tape casting and textured BZZ-BS-PT ceramics sintered at 1100°C for 3 h in air had a relative density of 98% and a Lotgering factor of 0.91. Chemically stable BT templates in Pb and Bi-rich BZZ-BS-PT system gave rise to the formation of highly oriented polycrystalline structure with larger block-like grains. A very high unipolar strain of 0.3% and low-field (<5 kV/cm) piezoelectric charge coefficient (d33) of 930 pm/V when driven at 50 kV/cm were achieved in highly textured ceramics together with a Curie temperature (T C) of 403°C. The TGG approach significantly improved piezoelectric properties of the BZ-BS-PT ceramic system without deteriorating the dielectric properties, which is a very promising high-performance candidate for high-temperature sensor, transducer and actuator applications.

High-entropy carbide ceramics with refined microstructure and enhanced thermal conductivity by the addition of graphite

Aiming at the refined microstructure and enhanced thermal conductivity of high-entropy carbide (HEC) ceramics for high-temperature applications, the addition effect of graphite was comprehensively investigated in this study. HEC ceramics incorporated with different contents of graphite were solidified by spark plasma sintering (SPS) using self-synthesized high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C powder and graphite as starting materials. The results demonstrate that the incorporated graphite removed the oxygen impurity in the mixed powders, decreased the oxygen content and increased the lattice parameter of the HEC phase, and improved the densification behavior of HEC ceramics. On the other hand, the addition of graphite brings a refinement of HEC grains and improves the mechanical properties. More importantly, the thermal conductivity of the HEC ceramics was significantly increased owing to the removing effect of oxide impurity by the added graphite. It is considered that the lattice "purified" HEC grains with low oxygen content contribute to the improvement in thermal conductivity of the ceramics.

Enhanced ionic conductivity and thermal shock resistance of MgO stabilized ZrO2 doped with Y2O3

The present work focused on the effect of Y2O3 co-doping on the phase composition, microstructure, ionic conductivity and thermal shock resistance of 8 mol% MgO stabilized ZrO2 (Mg-PSZ) electrolyte ceramics for high temperature applications. The addition of Y2O3 could promote the process of monoclinic-to-cubic/tetragonal phase transformation and became the metastable phase at room temperature. Meanwhile, the grain size of Mg-PSZ decreased. It was demonstrated that an appreciable increase in the ionic conductivity and compressive strength occurred on substituting MgO with Y2O3 in the Mg-PSZ electrolyte ceramics across the measured temperature range. Moreover, the Y2O3 addition could restrain the adverse effect of the cyclic thermal shock on the ionic conductivity and compressive strength of Mg-PSZ. The main reason was that the increase of the amount of monoclinic phase caused by cubic/tetragonal-to-monoclinic phase transformation by the cyclic thermal shock was restrained after the Y2O3 addition.

Investigation of enhanced performance in BF-xPT-0.05BZ ternary ceramics for high temperature applications

Abstract (0.95-x)BiFeO3-xPbTiO3-0.05BaZrO3 (BF-xPT-0.05BZ) ternary ceramics were synthesized by the solid-state reaction method, which present a perovskite structure without detectable second phase. BF-0.32PT-0.05BZ ceramics display the dominant rhombohedral structure with a small amount of tetragonal phase, revealing the characteristics of morphotropic phase boundary (MPB). The well-densified ceramics possess the average grain size of 8–15 μm with different PT contents. The ferroelectric-paraelectric phase transition of BF-xPT-0.05BZ ceramics shows the characteristic of the first-order transition, which occurs at around 550 °C. It is noticeable that BF-0.32PT-0.05BZ ceramics exhibit the giant remnant polarization of 60 μC/cm2 under the field of 175 kV/cm. Moreover, BF-0.32PT-0.05BZ ceramics show the highest piezoelectric constant d33 of 105 pC/N and Qm of 501. The depolarization temperatures of BF-xPT-0.05BZ ceramics are much close to their Curie temperature TC, proving the extensive prospect for high temperature piezoelectric applications.

Mechanical properties, thermal expansion performance and intrinsic lattice thermal conductivity of AlMO4 (M=Ta, Nb) ceramics for high-temperature applications

The vital thermo-mechanical properties of thermal and environment barrier coatings (TBCs/EBCs) include high hardness, low Young's modulus, matching thermal expansion coefficients (TECs) with substrate and low thermal conductivity. The effect of distortion degree of crystal structure on thermo-mechanical properties of AlMO4 (M=Ta, Nb) ceramics are assessed in this work. AlMO4 ceramics display modest TECs and no phase transformation is detected from room temperature to 1200 ℃. The experiment thermal conductivity can be dropped further as the theoretical minimum thermal conductivity of AlTaO4 and AlNbO4 is 1.48 W m−1 K−1 and 1.05 W m−1 K−1, respectively. The temperature dependent phonon thermal diffusivity of AlMO4 ceramics has been confirmed; the intrinsic lattice thermal conductivity is determined. The extraordinary thermo-mechanical properties make it clear that AlMO4 ceramics are suitable for high-temperature applications.

Giant piezoelectric behavior in relaxor ferroelectric environment friendly Na0.52K0.44Li0.04Nb0.84Ta0.10Sb0.06O3 ceramics for high temperature applications

Lead free Na0.52K0.44Li0.04Nb0.84Ta0.10Sb0.06O3 (NKLNTS) ferroelectric ceramic was synthesized to replace lead based ceramics with high piezoelectric performance for application in sensors, transducers and other electronic devices. The synthesis of NKLNTS ceramic was carried out using solid state reaction technique and the sintering temperature was taken to be 1090 °C. The structural phase of synthesized ceramic was found to be pure perovskite as observed from powder X-ray diffraction. FE-SEM micrographs revealed the formation of homogeneous microstructure with well defined grains. The excellent dielectric response with a high value of Curie temperature (in the range 305–315 °C for all frequencies) and low dielectric loss (0.06 at high frequency of 2 MHz at room temperature) was observed. J–E curve was analyzed and conduction mechanism in NKLNTS ceramic was found to be Ohmic. A ‘Displacement–Voltage’ butterfly loop was traced and a very large value of piezoelectric charge coefficient (d33* ~ 754 pm/V) was obtained. P–E hysteresis loops were traced to study ferroelectric behavior of NKLNTS ceramic. The value of pyroelectric coefficient was obtained to be very high ∼ 1921.9 µCm− 2 °C−1. Fatigue test was performed over 107 switching cycles and fatigue free nature of NKLNTS ceramic was observed. Remanent hysteresis task was performed to determine the true-remanent polarization of NKLNTS ceramic. Time-dependent compensated hysteresis task was carried out which revealed resistive leakage free nature of the ceramic.

Cellular Ceramics Produced from Ceramic Shell: Processing and Characterization

In this work, ceramic shell (mullite source), an industrial solid waste from the lost-wax casting process, after crushing and milling steps, was used and evaluated as an alternative raw material source for the production of cellular ceramics for high temperature applications (> 1400 °C). The obtained ceramic shell powder, with particle size distribution (d50 < 2 µm) suitable for the production of ceramic foams, was characterized from the point of view of their physical, chemical, morphological and crystallographic properties. Samples were prepared and obtained by direct foaming and gelcasting routes, dried at room temperature and then fired in two steps (650 °C/2 h and 1550 °C/2 h), and cooled in the furnace to room temperature. The effects of solids loading (35 and 42 vol. %) and stirring velocity (500 and 2000 rpm) on the cellular structure (pore size distribution and porosity) and mechanical properties of the produced ceramic shell foam samples were evaluated. The results showed that it is possible to obtain ceramic foams based on mullite-zirconite, with pore sizes between 100 and 900 µm, porosities up to 77% and compressive strength varying from 3 to 20 MPa.

Transparent Mg–α/β-Sialon:Eu2+ ceramics as a yellow phosphor for pc-WLED

Thin transparent yellow phosphor Sialon ceramics were fabricated by hot pressed sintering method. The fabricated Sialon ceramics were co-doped with Eu2O3 and its yellow luminescence was observed around 570nm wavelength when excited by UV to blue light spectrum. Furthermore, the white light was observed when thin yellow phosphor Sialon plates were coupled with blue led (455nm). From XRD patterns, major α-Sialon phase and a minor β-Sialon phase were observed with the addition of Eu2O3. The microstructure revealed that the average grain size was below 500nm and other phases like AlN polytypoids exists as minor phase. Small amount of Y2O3 addition significantly increased the luminescence intensity as compared to only Eu2O3 addition. Sialon ceramics are generally known as advanced ceramics for high temperature applications; the fabricated transparent yellow Sialon phosphor also exhibits good mechanical properties and suitable for remote phosphor.

Investigation of (1−x)(Bi0.94La0.06)(Ga0.05Fe0.95)O3–xPbTiO3 ceramics for high temperature applications

BiFeO3–PbTiO3 binary system doped with small amount of La3+ and Ga3+ was investigated for the development of piezoelectric ceramics with high Curie temperatures. Crystalline solutions of (1−x)(Bi0.94La0.06)(Ga0.05Fe0.95)O3–xPbTiO3 (BLGF–xPT) for x=0.35, 0.37, 0.4 and 0.45 have been fabricated by the solid-state reaction method. X-ray diffraction reveals that BLGF–PT has a single perovskite structure with a mixed phase region at x=0.35–0.37. The dielectric constant (K), loss tangent (tanδ), Curie temperature (TC), remnant polarization (Pr) and piezoelectric constant (d33) of 700, 0.021, 532°C, 30μC/cm2 and 63pC/N respectively, have been achieved for x=0.35. The strain under the electric field of 90kV/cm was measured to be 0.107% for x=0.37 and the planar electro-mechanical coupling coefficient (Kp) of 19% was stable under 310°C, about 60% of its TC, indicating high thermal stability of the piezoelectric properties.

Synthesis and properties of high aspect ratio SrBi4Ti4O15 microplatelets

(001) oriented SrBi4Ti4O15 (SBT) microplatelets with Aurivillius-type crystal structure were fabricated by molten salt synthesis. The effects of flux type selection and salt-to-oxide ratio variation on crystallization habit and morphology of the product were investigated. The sodium salt fluxes facilitate the formation of Sr2Bi4Ti5O18 large size platelets (≥17µm in length), while potassium salt fluxes create favorable condition for the growth of small size SrBi4Ti4O15 platelets (≤5µm in length). When using KCl flux, a higher salt-to-oxide ratio is beneficial to produce smaller SBT platelets with relatively narrow size distribution. The ferroelectricity and piezoelectricity of an individual SBT platelet were examined by piezoelectric force microscopy. These microcrystals are suitable template candidate to achieve highly textured ceramics for high temperature applications.

Tailoring the structure and piezoelectric properties of BiFeO3-(K0.5Bi0.5)TiO3–PbTiO3 ceramics for high temperature applications

There is a growing requirement for piezoelectric materials and systems which can operate in extreme environments, for example, oil &amp; gas, and aerospace. Here, we present the high temperature BiFeO3-K0.5Bi0.5TiO3-PbTiO3 (BF-KBT-PT) polycrystalline perovskite system. X-ray diffraction, impedance analysis, and Berlincourt measurements reveal a large region of phase coexistence, which can be tailored to optimise performance; Tc and the tetragonal spontaneous strain correlate strongly with the PbTiO3 concentration. The highest temperature composition has a d33 of 140 pmV−1 with a Tc = 542 °C, occupying previously unchartered territory on the classical d33–TC plot.

Improved ferroelectric and pyroelectric properties of Pb-doped SrBi4Ti4O15 ceramics for high temperature applications

• Sr 1−x Pb x Bi 4 Ti 4 O 15 (SPBT, x = 0 − 0.4) ceramics were synthesized by soft chemical method. • X-ray diffraction analysis confirmed the formation of bismuth layered structure. • SEM images showed plate like grain morphology with random orientation of plate faces. • Pb-doping resulted in improved ferroelectricity of SrBi 4 Ti 4 O 15 ceramics. • Pb-doped SrBi 4 Ti 4 O 15 exhibited improved pyroelectric properties with high T C . Ferroelectric properties of Pb-modified strontium bismuth titanate ceramics with chemical formula Sr 1− x Pb x Bi 4 Ti 4 O 15 ( x = 0–0.4) were investigated. Polycrystalline ceramics were synthesized by soft chemical method to study the effect of Pb-doping on its physical properties. X-ray diffraction analysis revealed a bismuth layered structure for all the compounds. The doping resulted in an increased tetragonal strain and improved ferroelectric properties. Scanning electron microscope images showed plate like grain morphology with random orientation of platelets. The ferroelectric transition temperature of the ceramics increased systematically from 525 °C to 560 °C with the increase of doping concentration. The piezoelectric coefficient ( d 33 ) of the ceramics was enhanced significantly with Pb doping, exhibiting a maximum value of 21.8 pC/N for 40 mol.% Pb-doped SBT. Pyroelectric studies carried out using the Byer–Roundy method indicated that the modified SBT ceramics are promising candidates for high temperature pyroelectric applications.

Piezoelectric Ceramics with Compositions at the Morphotropic Phase Boundary in the BiFeO3–PbZrO3–PbTiO3 Ternary System

Since ceramics in the PbZrO3–PbTiO3 binary system display excellent piezoelectric properties and those in BiFeO3–PbTiO3 exhibit high Curie temperatures, morphotropic phase boundary (MPB) compositions in the BiFeO3–PbZrO3–PbTiO3 ternary solid solution system are investigated for the development of piezoelectric ceramics for high temperature applications. It is found that the MPB compositions in the ternary system deviate away from the mixture of two binary MPB compositions 0.70BiFeO3–0.30PbTiO3 and 0.52PbZrO3–0.48PbTiO3. With decreasing amount of BiFeO3 in the MPB compositions in the ternary system, the Curie temperature TC is observed to decrease while the piezoelectric coefficient d33 is found to increase. Accompanied with this trend are the decrease in the c/a ratio of the tetragonal phase, the increase in the dielectric constant and the decrease in the loss tangent of the ceramics at room temperature. It is further noticed that the compositions in the rhombohedral‐rich side of MPB exhibit slightly better piezoelectric properties. An example of such compositions is 0.511BiFeO3–0.326PbZrO3–0.163PbTiO3, with TC of 431°C, d33 of 101 pC/N, and the electromechanical coupling factor kp of 0.50.

Porous alumina-spinel ceramics for high temperature applications

In order to reduce energy costs, high-temperature insulation porous refractory ceramics have been subjected to increasing demands. Among the techniques used to produce these materials (such as the addition of foaming agents and organic compounds), the pore generation via phase transformation presents key aspects, such as easy processing and the absence of toxic volatiles. In this study, this technique was applied to produce porous ceramics by decomposing an aluminum–magnesium hydro-carbonate known as hydrotalcite (Mg6Al2(CO3)(OH)16·4H2O). It was found out that by using this complex compound, a large fraction of pores can be generated and kept at high temperatures (above 1300°C) due to the in situ formation of spinel-like phases (MgAl2O4).

Realization of High Packaging Density on Ceramics for High Temperature Applications

This study is focused on the high density packaging of devices working at high temperature gradient. At the first step isothermal solidification based interconnections are fabricated on the sample contact pads, printed on a ceramic substrate, using a low melting metal layer. At the second step the attachable device has been connected on the same pad connections, fabricated on the same area, using different interlayer metal. High melting intermetallic phases are formed during every process-step and the interconnect remains unaffected of the next bonding process-temperature. This paper reveals the possibility of high temperature stable Pb-free bonding in an overlapping area.

Laser-joined Al 2O 3 and ZrO 2 ceramics for high-temperature applications

A laser process is presented that has been specially developed for joining oxide ceramics such as zirconium oxide (ZrO 2) and aluminium oxide (Al 2O 3). It details, by way of example, the design of the laser process applied for to producing both Al 2O 3–Al 2O 3 and ZrO 2–ZrO 2 joints using siliceous glasses as fillers. The heat source used was a continuous wave diode laser with a wavelength range of 808–1010 nm. Glasses of the SiO 2–Al 2O 3–B 2O 3–MeO system were developed as high-temperature resistant brazing fillers whose expansion coefficients, in particular, were optimally adapted to those of the ceramics to be joined. Specially designed measuring devices help to determine both the temperature-dependent emission coefficients and the synchronously determined proportions of reflection and transmission. The glass–ceramic joints produced are free from gas inclusions and macroscopic defects and exhibit a homogenous structure. The average strength values achieved were 158 MPa for the Al 2O 3 system and 190 MPa for the ZrO 2 system, respectively.

Piezoelectric Ceramics for High Temperature Applications

Ceramic materials based on lead titanate, lead niobate and bismuth layer-structured ferroelectrics (BLSF) were studied to develop piezoelectric ceramics for high temperature sensor applications. Compositional modification enabled lead titanate and lead niobate type ceramics to exhibit good piezoelectric properties at 500°C . The Curie temperature for one BLSF, CaBi4Ti4O15 was close to 800°C, though the piezoelectric constant was smaller than those of lead titanate and lead niobate ceramics. These ceramics seem to be good candidates for use as high temperature sensor materials. In addition, textured SrBi2Nb2O9 (SBN), another BLSF, ceramics with various orientation factors were fabricated through the templated grain growth (TGG) method. The resonant frequency of 76% textured SBN varied linearly with temperature and exhibited stable temperature characteristics. The temperature coefficient of the resonant frequency was –0.85 ppm/°C from –50 to 250°C, and was smaller than that of a quartz oscillator. Therefore, textured SBN ceramics are suitable for use as a resonator material when stable resonant frequency is needed in a high temperature range.

Paper derived SiC–Si 3N 4 ceramics for high temperature applications

Biomorphic Si 3N 4–SiC ceramics have been produced by chemical vapour infiltration and reaction technique (CVI-R) using paper preforms as template. The paper consisting mainly of cellulose fibres was first carbonized by pyrolysis in inert atmosphere to obtain carbon bio-template, which was infiltrated with methyltrichlorosilane (MTS) in excess of hydrogen depositing a silicon rich silicon carbide (Si/SiC) layer onto the carbon fibres. Finally, after thermal treatment of this Si/SiC precursor ceramic in nitrogen-containing atmosphere (N 2 or N 2/H 2), in the temperature range of 1300–1450 °C SiC–Si 3N 4 ceramics were obtained by reaction bonding silicon nitride (RBSN) process. They were mainly composed of SiC containing α-Si 3N 4 and/or β-Si 3N 4 phases depending on the nitridation conditions. The SiC–Si 3N 4 ceramics have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Raman spectroscopy. Thermal gravimetric analysis (TGA) was applied for the determination of the residual carbon as well as for the evaluation of the oxidation behaviour of the ceramics under cyclic conditions. The bending strength of the biomorphic ceramics was related to their different microstructures depending on the nitridation conditions.

Potential of SiC multilayer ceramics for high temperature applications in oxidising environment

Multilayered ceramics seem very promising for applications at very high temperatures in an oxidising environment. Actually, they present lower cost and better oxidation resistance than many conventional ceramic composites. The multilayered SiC oxidation and shock resistance has been investigated on tubular specimens processed by tape casting and pressureless sintering. Microstructure, oxidation and mechanical behaviour were investigated by micro-XRD, SEM, TGA–DTA-MS, indentation and radial compressive tests. The mechanical characterization showed that weak interfacial bonds are present between the layers. Together with the residual stresses left after the preparation phase, they caused crack deflection and improved toughness with respect to traditional ceramics. These mechanisms persisted even after long-term oxidation at 1600 °C or repeated thermal shock tests. The strength was found to depend on the thickness of the single SiC layer, however it was only slightly affected by thermal treatments.