Microstructure and properties of CaO–ZrO2–SiO2 glass–ceramics prepared by sintering

For the development of a new wear resistant and chemically stable glass-ceramic glaze, the CaO–ZrO₂–SiO₂ system was studied. Compositions consisting of CaO, ZrO₂, and SiO₂ were used for frit, which formed a glass-ceramic under a single stage heat treatment in electric furnace. In the sintered glass-ceramic, wollastonite (CaSiO₃) and calcium zirconium silicate (Ca₂ZrSi₄O₁₂) were crystalline phases composed of surface and internal crystals in the microstructure. The internal crystal formed with nuclei having a composition of Ca_1.2Si_4.3Zr_0.2O₈. The CaO–ZrO₂–SiO₂ system showed good properties in wear and chemical resistance because the Ca₂ZrSi₄O₁₂ crystals positively affected physical and mechanical properties.     

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO₂/Y₂O₃ ceramics from mixture of Al₂O₃, SiO₂, ZrO₂ and Y₂O₃ by gradually adding of α–β Si₃N₄ nanopowder from 1 to 5 wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400 °C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si₃N₄ additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33 g/cm³, while for traditionally sintered samples, the value is 2.55 g/cm³, and the compressive strength for SPS grows with Si₃N₄ additives, reaching 600 N/mm². In the case of traditional sintering, it decreases to approximately 100 N/mm². The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO₂ grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si₃N₄ additive, are filled with mullite crystalline formations.     

Liquid-phase sintering of ZrO2-based nanocrystalline glass–ceramics achieved by multielement co-doping

Liquid-phase sintering (LPS) is an effective pathway to assist the densification of ceramics. However, it has seldom been used to densify glass–ceramics. In the present study, a multielement co-doping strategy has been utilized to achieve LPS of a ZrO₂–SiO₂ nanocrystalline glass–ceramic. Compared with undoped samples densified by solid-state sintering, doping of equimolar Al, Y, and Ca promoted the densification of the glass–ceramic at lower temperatures with a faster densification rate. Ternary doping enhanced coarsening of ZrO₂ nanocrystallites during sintering and annealing. The distribution of dopants was carefully observed with X-ray energy-dispersive spectrometry technique in scanning electron transmission microscopy mode. Results showed that the three dopants showed different distribution behaviors. After sintering, Y dopants were predominately distributed in ZrO₂ nanocrystallites, whereas parts of Al and Ca dopants were distributed in ZrO₂ nanocrystallites and part of them co-segregated at the ZrO₂/SiO₂ heterointerfaces. Meanwhile, the segregation of Ca dopant at some intergranular films among ZrO₂ nanocrystallites was observed. Redistribution of dopants did not occur during annealing.     

Fracture toughness of ZrO2–SiC/MoSi2 composite ceramics prepared by powder metallurgy

ZrO₂–SiC/MoSi₂ composite ceramics were prepared by powder metallurgy. The room temperature and high temperature fracture toughness of 10 vol%ZrO₂+MoSi₂, 10 vol%ZrO₂+10 vol%SiC + MoSi₂, 20 vol%ZrO₂+10 vol%SiC + MoSi₂, 10 vol%ZrO₂+20 vol%SiC + MoSi₂ composite ceramics were studied. The room temperature fracture toughness was calculated by Vickers indentation crack. High temperature fracture toughness is measured by wavefront interference principle of moire interferometry and non-contact measurement of in-plane displacement. The synergistic effects mechanism of nano-ZrO₂ and SiC particles on the fracture toughness of MoSi₂ nanocomposite ceramics was discussed.     

Influence of the sintering conditions on the properties of 0.57PSN–0.43PT ceramics prepared from mechanochemically activated powder

The present work is a systematic study of the influence of the sintering conditions on the structural and electrical properties of 0.57Pb(Sc_1/2Nb_1/2)O₃–0.43PbTiO₃ ceramics prepared from mechanochemically activated powder. The ceramics were sintered at various temperatures and for a range of times. Three or even more contributions competed for influence on the functional properties of the ceramics, i.e., the density, the grain size and the phase composition. However, all these contributions combined in such a way that the best functional properties were obtained for the ceramics sintered at 1000 °C for 8 h.     

Study of sintering behavior of PAN–PZT using dilatometry and co-relation with piezoelectric properties

The sintering behavior of Pb(Al_0.5Nb_0.5)O₃–Pb(Zr_0.52Ti_0.48)O₃ (PAN–PZT) ceramics was investigated. The sintering process of PAN–PZT was divided into four stages based on the amount of in-situ linear shrinkage: (Stage 1) initial stage at sintering temperature RT≤T_s≤ to 770 °C, (Stage 2) perovskite formation stage at 770≤T_s≤990 °C, (Stage 3) densification stage at 990≤T_s≤1265 °C and (Stage 4) Pb-loss stage at T_s>1265 °C. During stage 1, heating of PAN–PZT compact did not cause structural changes except the thermal expansion. During stage 2, the PZT-perovskite structure was formed by rearrangement of crystals. During stage 3, the specimen was rapidly densified with two peaks of strain rate. During stage 4, Pb was volatilized and this loss resulted in sudden increase of shrinkage. On analyzing the sintering stages, the optimized sintering conditions were considered as 1250 °C just before of Pb-loss stage. The effects of T_s on the crystal structure, microstructure and the piezoelectric performance were analyzed and documented.     

Enhancement of thermal shock and slag corrosion resistance of MgO–ZrO2 ceramics by doping CeO2

The purpose of this paper was to develop ceramics materials with high thermal shock resistance and corrosion resistance for preparing gas blowing components. In this paper, MgO-rich MgO–ZrO₂ ceramics were obtained by using MgO powder and ZrO₂ powder as starting materials and CeO₂ as an additive. Changes in the properties in terms of thermal shock resistance, mechanical properties, and slag corrosion-resistance with chemical compositions were examined correlated to microstructure and phase changes. Especially, the effect of doping CeO₂ on phase transition of zirconia in MgO-rich system was discussed. The results showed that doping amount of CeO₂ significantly improved properties of MgO–ZrO₂ ceramics. Especially when doping amount of CeO₂ was 2 wt%, residual strength ratio was enhanced over 100% after thermal shock testing. In samples doped with CeO₂, ZrO₂ was stable in cubic or tetragonal form due to complete solution of CeO₂, which was important reason for the improvement of various properties of MgO–ZrO₂ ceramics.     

Preparation of nano-AlN powder by sol–gel foaming and its sintering properties

Uniformly dispersed nano-sized aluminum nitride powders were prepared by the sol–gel foaming method using aluminum nitrate as the aluminum source, sucrose as the carbon source, and ammonium chloride as the foaming agent. The effects of ammonium chloride content on the particle size and the sintering properties of aluminum nitride were investigated. The results showed that when the molar ratio of ammonium chloride to aluminum nitrate was .5, the colloidal foams were uniform, large, and fluffy, and amorphous alumina precursors with uniform particles could be prepared. Aluminum nitride powder with a particle size of 22–27 nm can be obtained by calcining these precursors in nitrogen atmosphere at 1400°C for 2 h. At the same time, aluminum nitride bulk material with a relative density of 95% can be obtained by sintering the compact samples in nitrogen atmosphere at 1700°C for 2 h.     

Far from equilibrium ultrafast high-temperature sintering of ZrO2–SiO2 nanocrystalline glass–ceramics

Ultrafast high-temperature sintering (UHS) is a novel sintering technique with ultrashort firing cycles (e.g., a few tens of seconds). The feasibility of UHS has been validated on several ceramics and metals; however, its potential in consolidating glass–ceramics has not yet been demonstrated. In this work, an optimized carbon-free UHS was utilized to prepare ZrO₂–SiO₂ nanocrystalline glass–ceramics (NCGCs). The phase composition, grain size, densification behavior, and microstructures of NCGCs prepared by UHS were investigated and compared with those of samples sintered by pressureless sintering. Results showed that NCGCs with a high relative density (～95%) can be obtained within ～50 s discharge time by UHS. The UHS processing not only hindered the formation of ZrSiO₄ and cristobalite but also enhanced the stabilization of t-ZrO₂. Meanwhile, owing to the ultrashort firing cycles, the UHS technology allowed the NCGCs to be consolidated in a far from equilibrium state. The NCGCs showed a microstructure of spherical monocrystalline ZrO₂ nanocrystallites embedded in an amorphous SiO₂ matrix.     

Enhanced piezoelectric properties of barium titanate–potassium niobate nano-structured ceramics by MPB engineering

Barium titanate (BaTiO₃, BT)–potassium niobate (KNbO₃, KN) (BT–KN) nanocomplex ceramics with various KN/BT molar ratios were prepared by the solvothermal method. From a transmittance electron microscopy (TEM) observation, it was confirmed that KN layer thickness of the BT–KN nanocomplex ceramics was controlled from 0 to 44 nm by controlling KN/BT molar ratios. Their dielectric constants were measured at room temperature and 1 MHz, and a maximum dielectric constant of around 400 was measured for the BT–KN nanocomplex ceramics with a KN thickness of 22 nm. TEM observation revealed that below KN thickness of 22 nm, BT/KN heteroepitaxial interface was assigned to the strained interface while over 22 nm, the interface was assigned to the relaxed one. These results suggested that the strained heteroepitaxial interface could be responsible for the enhanced dielectric constants.     

Effects of CeO2 addition on the piezoelectric properties of PNW–PMN–PZT ceramics

Temperature stability and electrical properties of the piezoelectric material are very important in piezoelectric transformers applications. In this study, it was investigated the temperature stability of PNW–PMN–PZT with CeO₂ additives and the variation of Zr/Ti ratio. Meanwhile, effects of CeO₂ additives and the variation of Zr/Ti ratio on the microstructure and electrical properties of PNW–PMN–PZT were investigated in detail. The results revealed that the optimized temperature stability of Δf_r/f_r25 °C = 0.15%, ΔK_p/K_p25 °C = −0.86% and ΔQ_m/Q_m25 °C = −45.26% could be attained at x = 0.1 wt.% and Zr/Ti = 51/49. Moreover, optimized electrical properties were achieved: K_p = 0.60, Q_m = 1405, d₃₃ = 388 pC/N, ?_r = 2140 and tan δ = 0.0059. The obtained temperature stabilities and electrical properties make this composition a good candidate for high power piezoelectric transformer applications.     

Microstructure and properties of ZrB2–SiC composites prepared by spark plasma sintering using TaSi2 as sintering additive

ZrB₂–SiC composites were fabricated by spark plasma sintering (SPS) using TaSi₂ as sintering additive. The volume content of SiC was in a range of 10–30% and that of TaSi₂ was 10–20% in the initial compositions. The composites could be densified at 1600 °C and the core–shell structure with the core being ZrB₂ and the shell containing both Ta and Zr as (Zr,Ta)B₂ appeared in the samples. When the sintering temperature was increased up to 1800 °C, only (Zr,Ta)B₂ and SiC phases could be detected in the samples and the core–shell structure disappeared. Generally, the composites with core–shell structure and fine-grained microstructure showed the higher electrical conductivity and Vickers hardness. The completely solid soluted composites with coarse-grained microstructure had the higher thermal conductivity and Young's modulus.     

Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

Structural,piezoelectric and dielectric properties of PSLZT–PMnN ceramics

Pb_0.94Sr_0.05La_0.01(Zr_0.54Ti_0.46)_0.9975O₃–Pb(Mn_1/3Nb_2/3)O₃ (PSLZT–PMnN) ceramics with pure perovskite structure were prepared by conventional mixed oxide method. The influence of PMnN on the structural, dielectric and piezoelectric properties was investigated. The experimental results showed that the perovskite structure changed from tetragonal to rhombohedral symmetry and the Curie temperature decreased gradually with the increase of PMnN. The composition with 3 mol% PMnN exhibited favorable properties of Q_m (400), d₃₃(660 pC/N), k_p (0.60), K^T (1940), tan δ (0.90%) and T_C (247 °C), exhibiting potential usage for piezoelectric actuator and sensor applications.     

Low-temperature ultrasound-activated joining of ZrO2 ceramics using Sn–Al–Cu solder

In this study, the low-temperature ultrasound-activated joining of ZrO₂ ceramics using Sn–4Al–0.7Cu solder was achieved at 350°C. It was found that a nanoscale amorphous Al₂O₃ layer formed at the solder-ceramic interface during the ultrasonic soldering process. The occurrence of the interfacial oxidation of aluminum could be attributed to the sonochemical effects of acoustic cavitation and turbulent streaming induced by the propagation of ultrasonic waves in the liquid solder. The formed butt joints exhibited an average tensile strength of 47.3 MPa.     

Densification and mechanical properties of cBN–TiN–TiB2 composites prepared by spark plasma sintering of SiO2-coated cBN powder

cBN–TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–1973 K using cubic boron nitride (cBN) and SiO₂-coated cBN (cBN(SiO₂)) powders. The effect of SiO₂ coating, cBN content and sintering temperature on the phase composition, densification and mechanical properties of the composites was investigated. SiO₂ coating on cBN powder retarded the phase transformation of cBN in the composites up to 1873 K and facilitated viscous sintering that promoted the densification of the composites. Sintering at 1873 K, without the SiO₂ coating, caused the relative density and Vickers hardness of the composite to linearly decrease from 96.2% to 79.8% and from 25.3 to 4.4 GPa, respectively, whereas the cBN(SiO₂)–TiN–TiB₂ composites maintained high relative density (91.0–96.2%) and Vickers hardness (17.9–21.0 GPa) up to 50 vol% cBN. The cBN(SiO₂)–TiN–TiB₂ composites had high thermal conductivity (60 W m⁻¹ K⁻¹ at room temperature) comparable to the TiN–TiB₂ binary composite.     

Gelcasting and pressureless sintering of silicon carbide ceramics using Al2O3–Y2O3 as the sintering additives

In this paper, silicon carbide ceramics were prepared by aqueous gelcasting and pressureless sintering using Al₂O₃ and Y₂O₃ as the sintering additives. In order to develop well dispersed SiC slurries in the presence of sintering additives, the Al₂O₃ and Y₂O₃ powder was treated in the citric acid solution in advance. Zeta potential measurement showed that the isoelectric point (IEP) of Al₂O₃ and Y₂O₃ powder moved toward low pH region after treatment. Rheological measurement confirmed that the addition of as-treated powder showed very limited influence on the slurry properties as compared to that of untreated powder. SiC slurries with solid content of 54 vol% and enough fluidity can be developed. After gelcasting and pressureless sintering, SiC ceramics with nearly full density, fine grained and homogeneous microstructure can be obtained. Results showed that the surface treatment of Al₂O₃ and Y₂O₃ with citric acid is effective for the gelcasting process of SiC.     

Alumina–copper joining by the sintered metal powder process

Sintered metal powder process (SMPP) is one of the high technology methods in ceramic–metal joining domain. The present study examines the effect of temperature and time of metalized layer sintering on the thickness and homogeneity of the joining layer, the leakage rate in alumina–copper joining zone, and also identifies the different phases formed during sintering. The samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). Microstructure studies indicate that sintering the metalized layer with a holding time of 90 min at the temperature of 1530 °C, and with an applied layer thickness of 50 μm with proper plating and brazing stages lead to a completely homogeneous joining zone with an adequate thickness (about 33 μm). The results of leak tests on alumina–copper specimen in this condition was less than 10^?9 Pa l s^?1.     

Flash sintering of Al2O3–ZrO2 ceramics under alternating current electric field

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al₂O₃–ZrO₂) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al₂O₃–ZrO₂ composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al₂O₃–ZrO₂ composite ceramic was fabricated via AC flash sintering under 60 mA mm^?2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering.