Effects of pore shape and porosity on the properties of porous PZT 95/5 ceramics

Porous lead zirconate titanate (PZT 95/5) ferroelectric ceramics were prepared by sintering compacts consisting of PZT and pore formers. The piezoelectric, dielectric and ferroelectric properties of porous PZT ceramics were investigated as a function of pore shape and porosity. Piezoelectric coefficient (d₃₃), dielectric constant (ɛ₃₃) and remnant polarization (P_r) decreased with an increase in porosity, and the porous PZT ceramics with spherical pores exhibited better properties than that with irregular pores. Furthermore, the electrical conductivities of PZT ceramics were investigated to explain the phenomena that porous PZT ceramics exhibited lower dielectric loss (tan δ) than dense PZT ceramics in the temperature range from 250 to 500 °C.     

Effects of poling state and pores on fracture toughness of Pb(Zr0·95Ti0·05)O3 ferroelectric ceramics

Abstract

Abstract

The effects of poling state and pores on the fracture toughness of Pb(Zr_0·95Ti_0·05)O₃ (PZT 95/5) ferroelectric ceramics were investigated. X-ray diffraction analysis and piezoelectric constant measurements reveal that the phase structures of PZT 95/5 ceramics change with the poling state, which significantly affects the fracture toughness. The poled PZT 95/5 ceramics demonstrate higher fracture toughness than the unpoled ceramics, and their fracture toughness significantly increases after the pressure depoling. As the porosity of ceramics increases with addition of poreformer during preparation, their fracture toughnesses all decrease accordingly either in poled state or unpoled state. The effect of pore size on the fracture toughness is subtle for the poled ceramics, but for the hydrostatic pressure depoled porous PZT 95/5 ceramics, their fracture toughness increases with the increase in pore size. A new stress model is proposed to explain the pore size effect on the fracture toughness of hydrostatic pressure depoled PZT 95/5 ceramics.     

Densification and microstructure evolution of reactively sintered transparent spinel ceramics

Reactive sintering is an effective and simple method to prepare transparent spinel ceramics. In this research, transparent MgO·nAl₂O₃ (0.98?≤ n?≤?2) spinel ceramics were prepared via reactive sintering in air followed by hot isostatic press (HIP), using MgO and γ-Al₂O₃ powders as raw materials. The influence of composition on densification and microstructure evolution was systemically investigated. More importantly, the relationship between microstructure of presintered samples and final properties of transparent ceramics was singled out. Thermodynamically stable large pores were easily generated in magnesia-rich and stoichiometric samples after presintering in air, causing severe abnormal grain growth during the HIP treatment and poor optical quality of the resulting samples. The presintering temperature of alumina-rich samples widely varied with composition. No large pores were observed in the presintered sample, which was beneficial for the elimination of residual pores in the following HIP process. Highly transparent spinel ceramics with n?=?1.1 and 1.3 were successfully fabricated with the transmittance above 84% even at the short wavelength of 400?nm, close to the theoretical value.     

Influence of heating rate on optical properties of Nd:YAG laser ceramic

Using commercial α-Al₂O₃, Y₂O₃ and Nd₂O₃ as raw materials, 0.8 at% Nd:YAG ceramics were fabricated by solid-state reaction and vacuum sintering technology, with tetraethoxysilane (TEOS) as sintering aid. The Nd:YAG ceramics were obtained by sintering at 1750 °C for 20 h under vacuum. The sintering process with different heating rate of the Nd:YAG ceramics have been studied during the present work. The grain sizes, pores and secondary phase amounts increased versus increasing the heating rate. The optical properties of the Nd:YAG ceramics were closely related to the microstructures of the specimens. The lasing performance of the Nd:YAG ceramics changed drastically with change in pores and secondary phase amounts.     

Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores

Porous alumina ceramics having unidirectionally aligned cylindrical pores were prepared by extrusion method and compared with porous ceramics having randomly distributed pores prepared by conventional method, and their gas permeability and mechanical properties were investigated. SEM micrographs of the porous alumina ceramics prepared by the extrusion method using nylon fibers as the pore former showed excellent orientation of cylindrical pores. The bending strength and Weibull modulus of the extruded porous alumina ceramics with 39% porosity were 156 MPa and 17, respectively. These mechanical properties of extruded samples were higher than those of the conventional porous alumina ceramics. The strength decreased from 156 to 106 MPa with increasing pore size from 8.5 to 38 μm. The gas permeability of the extrusion samples is higher than that of the conventional samples and increased with increasing of porosity and pore size.     

Effect of pore size and porosity on piezoelectric and acoustic properties of Pb(Zr0.53Ti0.47)O3 ceramics

We studied the effect of porosity and pore morphology on the functional properties of Pb(Zr_0.53Ti_0.47)O₃ (PZT) ceramics for application in high frequency ultrasound transducers. By sintering a powder mixture of PZT and polymethylmetacrylate spherical particles (1.5 and 10?μm) at 1080°C, we prepared ceramics with ～30% porosity with interconnected micrometer sized pores and with predominantly ～8?μm spherical pores. The acoustic impedance was ～15?MRa for both samples, which was lower than for the dense PZT. The attenuation coefficient α (at 2.25?MHz) was higher for ceramics with ～8?μm pores (0.96?dB?mm^??1?MHz^??1), in comparison to the ceramic with smaller pores (0.56?dB?mm^??1?MHz^??1). The high α value enables the miniaturisation of the transducer, which is crucial for medical imaging probes. The dielectric and piezoelectric coefficients, polarisation, and strain response decreased with increased porosity and decreased pore/grain size. We suggest a possible role of pore/grain size on the switching behaviour.     

Facile processing of oriented macro-porous ceramics with high strength and low thermal conductivity

Unidirectionally oriented architectures demonstrate a notable efficiency in enhancing the properties of macro-porous materials, yet are difficult to construct in a time- and cost-effective fashion. Here a facile approach was exploited for fabricating oriented macro-porous ceramic materials by employing natural graphite flakes as a fugitive material and preferentially aligning the flakes within ceramic matrices using accumulative rolling technique. Flaky to near-ellipsoid shaped pores with a homogeneous distribution were created in macro-porous zirconia ceramics with their porosity and microstructural characteristics adjustable by controlling the additive amounts of graphite flakes. The resulting materials exhibited a good combination of properties with high compressive strength up to over 1.5 GPa, which exceeds those of most other porous zirconia ceramics with similar porosities, along with low thermal conductivity of 0.92–1.85 Wm^?1·K^?1. This study offers a simple means for developing new oriented macro-porous materials with enhanced properties, and may promote their application by allowing for easy mass production.     

Fast sinter crystallisation of waste glasses

Abstract

Two glasses, belonging to the CaO–Al₂O₃–SiO₂ system and corresponding to the melting of mixtures of industrial wastes (recycled glasses, mining residues, ashes, asbestos containing cements, etc.), have been successfully converted into dense glass ceramics by sintering with concurrent crystallisation. The usage of fine glass powders (<37 μm) allowed very short sintering treatments, due to the enhanced nucleating activity of glass surfaces. In particular, dense glass ceramics could be produced by direct insertion of pressed glass powders in the furnace at the sintering temperature, followed by rapid cooling at room temperature after a 30 min holding time. The proposed approach evidences the feasibility of sintered glass ceramics by the fast and economic processes employed for traditional ceramics, with the advantage of superior mechanical properties (bending strength exceeding 100 MPa, Vickers' microhardness exceeding 6 GPa). Like in traditional ceramics, clay and water could be used for the shaping of pressed tiles, thus posing the conditions for massive industrial production.     

A novel method for the fabrication of porous calcium hexaluminate (CA6) ceramics using pre-fired CaO/Al2O3 pellets as calcia source

Herein, porous calcium hexaluminate ceramics that contain pores exhibiting multiple morphologies were fabricated via in situ reaction sintering using α-Al₂O₃ powders and pre-fired CaO/Al₂O₃ pellets. The results indicated that the composition of the pre-fired CaO/Al₂O₃ pellets significantly affected the pore morphology, reaction-diffusion mechanisms, sintering behaviour and properties of the porous CA₆ ceramics. For the specimens containing low CaO/Al₂O₃-ratio (0.37) pellets, the main reaction occurred by solid state diffusion, i.e. ion diffusion through the solid reactant phase, which resulted in a slow process and low CA₆ formation rate at an elevated sintering temperature. With higher CaO/Al₂O₃-ratio (0.57) pellets, large-sized pores were observed because of transient liquid phase diffusion during the sintering process. The transient liquid phase diffusion effect increased the porosity of the porous ceramics and promoted the formation of a large number of plate-like CA₆ grains in the walls of the pores, enhancing their mechanical properties and high-temperature performance. The porous CA₆ ceramics containing high CaO/Al₂O₃-ratio (0.57) pellets sintered at 1700 °C exhibited high open porosity (55.88%), low thermal conductivity and excellent high-temperature performance.     

Enhanced mechanical properties of 3D printed alumina ceramics by using sintering aids

Stereolithography based 3D printing provides an efficient pathway to fabricate alumina ceramics, and the exploration on the mechanical properties of 3D printed alumina ceramics is crucial to the development of 3D printing ceramic technology. However, alumina ceramics are difficult to sinter due to their high melting point. In this work, alumina ceramics were prepared via stereolithography based 3D printing technology, and the improvement in the mechanical properties was investigated based on the content, the type and the particle size of sintering aids (TiO₂, CaCO₃, and MgO). The flexural strength of the sintered ceramics increased greatly (from 139.2 MPa to 216.7 MPa) with the increase in TiO₂ content (from 0.5 wt% to 1.5 wt%), while significant anisotropy in mechanical properties (216.7 MPa in X-Z plane and 121.0 MPa in X–Y plane) was observed for the ceramics with the addition of 1.5 wt TiO₂. The shrinkage and flexural strength of the ceramics decreased with the increase in CaCO₃ content due to the formation of elongated grains, which led to the formation of large-sized residual pores in the ceramics. The addition of MgO help decrease the anisotropic differences in shrinkage and flexural strength of the sintered ceramics due to the formation of regularly shaped grains. This work provides guidance on the adjustment in flexural strength, shrinkage, and anisotropic behavior of 3D printed alumina ceramics, and provides new methods for the fabrication of 3D printed alumina ceramics with superior mechanical properties.     

Effects of BaCO3 addition on the microstructure and electrical properties of La-doped barium titanate ceramics prepared by reduction-reoxidation method

Effects of trace BaCO₃ addition were investigated on microstructure and electrical properties of La-doped BaTiO₃ ceramics prepared via reduction-reoxidation method. The addition of BaCO₃ could effectively facilitate the densification and grain growth of ceramics. SEM images suggested that numerous pores presented in ceramics due to the decomposition of BaCO₃ to expel generated CO₂ gas. The presence of pores promoted reoxidation of the reduced ceramics. As a result, lowering room-temperature (RT) resistivity and improving PTCR jump were simultaneously achieved as suitable amount of BaCO₃ was added. An optimal addition of 0.3 mol% BaCO₃ led to a low RT resistivity of about 28 Ω cm and a PTCR jump up to 10^3.7 after ceramics were fired at 1100 °C in high-purity N₂ and reoxidized at 800 °C in air.     

Evaluation of the pore morphologies for piezoelectric energy harvesting application

Piezoelectric energy harvesting has attracted significant attention in recent years due to their high-power density and potential applications for self-powered sensor networks. In comparison to dense piezoelectric ceramics, porous piezoelectric ceramics exhibit superiority due to an enhancement of piezoelectric energy harvesting figure of merit. This paper provides a detailed examination of the effect of pore morphology on the piezoelectric energy harvesting performance of porous barium calcium zirconate titanate 0.5Ba(Zr_0.2 Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ (BCZT) ceramics. Three different pore morphologies of spherical, elliptical, and aligned lamellar pores were created via the burnt-out polymer spheres method and freeze casting. The relative permittivity decreased with increasing porosity volume fraction for all porous BCZT ceramics. Both experimental and simulation results demonstrate that porous BCZT ceramics with aligned lamellar pores exhibit a higher remanent polarization. The longitudinal d₃₃ piezoelectric charge coefficient decreased with increasing porosity volume fraction for the porous ceramics with three different pore morphologies; however, the rate of decrease in d₃₃ with porosity is slower for aligned lamellar pores, leading to the highest piezoelectric energy harvesting figure of merit. Moreover, the peak power density of porous BCZT ceramics with aligned lamellar pores is shown to reach up to 38 μW cm^-2 when used as an energy harvester, which is significantly higher than that of porous BCZT ceramics with spherical or elliptical pores. This work is beneficial for the design and manufacture of porous ferroelectric materials in devices for piezoelectric energy harvesting applications.     

Microstructure and properties of pore-created SiC ceramics with calcium chloride as pore former

Pore-created silicon carbide (SiC) ceramics were liquid phase sintered (LPS) by using Al₂O₃–Y₂O₃ as sintering additive and calcium chloride (CaCl₂) as pore former. The CaCl₂ did not react with other compositions, and accumulatively formed CaCl₂ crystals on the grain boundary of SiC ceramics. The addition of CaCl₂ decreased the sintering and mechanical properties of SiC ceramics, but obviously reduced the dry friction coefficient. The pores on the surface and inside of SiC ceramics could be continuously created by the solubility and non-volatility of calcium chloride.     

Effect of closed pores on dielectric properties of 0.8Na0.5Bi0.5TiO3-0.2K0.5Bi0.5TiO3 porous ceramics

Porous 0.8Na_0.5Bi_0.5TiO₃-0.2K_0.5Bi_0.5TiO₃ ceramics are fabricated via the pore-forming agent method with polymethyl methacrylate (PMMA) and stearic acid (SA) as pore forming agents, and microstructure observations demonstrate that the porosity, pore shape, and pore sizes can be controlled by the synthesis technology. The dielectric properties of porous ceramics are found not only correlated to the pore-matrix composite model, but also have a significant grain-size effect. Based on the Zener Theory, pining forces exerted by pores on the grain boundary are calculated, to explain the shape effect of pores on grain boundary migration. A phase-field simulation is carried out to investigate pore shape effect on the grain size regulation in porous polycrystalline, and simulation results are in good agreements with experiential results as well as theoretical calculations. Thus, a modified equation is proposed to predict the effective permittivity of the porous piezoelectric ceramics by considering effects of porosity, pore shape and grain size.     

Er3+-doped CaF2 polycrystalline ceramic with perfect transparency for mid-infrared laser

Er³⁺-doped CaF₂ transparent ceramics are promising mid-infrared gain materials because of their utra-low phonon energy as well as excellent physical, chemical, and optical properties. However, existing hot-pressed and hot-formed CaF₂ ceramics are very difficult to be used in practical applications due to residual pores and weak polycrystallization, respectively. Here, we developed the high quality Er³⁺-doped CaF₂ transparent polycrystalline ceramic by single crystal ceramization. The sample exhibits obvious polycrystalline structure, good mechanical properties, perfectly transmittance, and excellent mid-infrared performance, which provides significant and wide-ranging opportunities for advanced mid-infrared gain materials.     

Properties of porous zirconia ceramics fabricated by using various pore-forming agents

The present work proposes a new method for fabrication partially stabilized porous zirconia ceramics using monoclinic zirconia as raw material, yttrium nitrate and magnesium compounds as pore-forming agents and stabilizers. Effects of different pore-forming agents, firing temperatures and firing time on properties of samples were investigated. Thermal decomposition of yttrium nitrate and magnesium compounds creates a large number of pores, and thus porous zirconia ceramics were fabricated. ZrO₂ can be partially stabilized by Y₂O₃ and MgO derived from the precursors. The porous ZrO₂ ceramics obtained by using Y(NO₃)₃·6H₂O and 4MgCO₃·Mg(OH)₂·6H₂O as pore-forming agents had relatively high stabilization ratio, uniform pores and high strength. The optimum firing temperature and firing time are 1400 °C and 3 h, and the samples have the stabilization ratio of 45.7%, high cold crushing strength (26 MPa), uniform pores, and apparent porosity is about 40%.     

锆英石陶瓷生产工艺对力学性能的影响

对锆英石陶瓷的生产工艺对材料的显微结构、力学性能的影响进行了研究.结果表明:造粒粉颗粒的硬壳与空洞在坯体中形成大颗粒,如果在成型过程中末被完全破坏遗留在烧结体中而引起材料内部气孔的存在,使材料的力学性能明显下降.制定合理的锆英石材料生产工艺、控制气孔的产生,是提高材料强度和可靠性的有效措施.     

Effects of HfC addition on microstructures and mechanical properties of TiC0.7N0.3-based and TiC0.5N0.5-based ceramic tool materials

Effects of HfC addition on microstructures and mechanical properties of TiC_0.7N_0.3-based and TiC_0.5N_0.5-based ceramic tool materials were investigated. The results showed that the same by-products such as HfN_0.4, HfNi, Ti_0.76Hf_0.24Ni, TiNi_0.85Co_0.15 and HfCo₆ were discovered in these ceramics. To some extent, HfC additive can inhibit their denitrification in the sintering process. The HfC particles can produce the crack flection and a suitable HfC content can improve mechanical properties of these ceramics effectively. Besides, the TiC_0.5N_0.5-HfC ceramics exhibited a poor sinterability compared to TiC_0.7N_0.3-HfC ceramics, showing much more pores in fracture and polished surfaces micrographs; grains in TiC_0.7N_0.3-HfC ceramics are finer than that in TiC_0.5N_0.5-HfC ceramics; the HfC particles dispersion in TiC_0.7N_0.3-HfC ceramics is more uniform than that in TiC_0.5N_0.5-HfC ceramics; thus, the mechanical properties of TiC_0.7N_0.3-HfC ceramics are superior to that of TiC_0.5N_0.5-HfC ceramics. In addition, their fracture mechanism is a combination of transgranular fracture and intergranular fracture. As a consequence, the enhancement of their mechanical properties is conducive to the presence of intermetallic compounds, the fine microstructure and the suitable HfC content.     

The preparation and properties of high-strength porous mullite ceramics by a novel non-toxic gelcasting process

A novel approach to fabricate porous mullite ceramics with homogeneous pore size and high-strength using green non-toxic and cost-effective poly-γ-glutamic acid (γ-PGA) gelling system was reported for the first time. Effect of γ-PGA addition, additive amount and solid loading on rheological behavior of the slurries, and microstructure and properties of samples were investigated systematically. By optimizing the solid loading of mullite samples, we are able to get the sample with small pores (< 200 µm) dominating (93.3% of the total pores), and compressive strength of the sample reaches up to 26.62 MPa. In addition, the mullite ceramics exhibited high porosity of 75.7% with low thermal conductivity of 0.279 W/(m·K) at room temperature. This study not only provides a green and non-toxic gelling system but also offers porous mullite ceramics with low thermal conductivity and excellent mechanical strength as an energy-saving thermal insulation material.     

A novel and green preparation of porous forsterite ceramics with excellent thermal isolation properties

This work provides a novel and green approach to preparing porous forsterite ceramics by a transient liquid phase diffusion process based on fused magnesia and quartz powders without detrimental additives. The size of quartz particles markedly affected the sintering behaviors, phase composition, microstructure and properties of the porous forsterite ceramics. Fine quartz particles (D₅₀, 3.87?µm) accelerated the rate of the forsterite formation at elevated temperatures and promoted solid-state sintering behavior of the porous ceramics. Conversely, coarse quartz particles (D₅₀, 25.38?µm) reduced the rate of the solid state reaction and a large amount of unreacted SiO₂ and enstatite (MgSiO₃) phases transformed into a transient liquid-phase during the firing process. This effect resulted in a high porosity (approximately 58.89%) and formation of many large pores (mean pore size of 42.36?µm). These features contributed to the excellent thermal isolation properties of the prepared porous forsterite ceramics. The strength of the obtained porous ceramics (about 23.6?MPa) is relatively high compared with those of conventional ceramics.