The effects of cold sintering parameters on the densification of Na2WO4 ceramics using Na2WO4·2H2O dry powders

Cold sintering process (CSP) combines transient liquid phase and/or external pressure to assist the densification of ceramics, the sintering mechanism of which is typically dominated by the “dissolution–precipitation” process. In this work, Na₂WO₄·2H₂O dry powders without liquid water have been used as the “liquid phase” to realize the densification of Na₂WO₄ ceramics. The effects of sintering parameters on the CSP of Na₂WO₄ ceramics using Na₂WO₄·2H₂O dry powders are systematically studied, including sintering temperature, external pressure, sintering process time and the amount of Na₂WO₄·2H₂O phase. It is found that the “liquid phase,” that is, Na₂WO₄·2H₂O is the key factor for controlling the CSP of Na₂WO₄ ceramics, and Na₂WO₄·2H₂O hydrate shows an advantage of homogeneous dispersion of liquid water. The sintering temperature affects the densification mechanism involved with plastic deformation and the “dissolution–precipitation,” external pressure accelerates the densification process, and enough sintering process time ensures the completion of CSP. Finally, the optimum cold sintering parameters of Na₂WO₄ ceramics by Na₂WO₄·2H₂O powders are obtained at 150°C and 240 MPa for 60 min with a pretreatment at 150°C for 30 min.     

Facile synthesis of MgO–Mg2SiO4 composite ceramics with high strength and low thermal conductivity

The recycling of solid waste is a win-win solution for humans and nature. For this purpose, magnesite tailings and silicon kerf waste were employed to prepare MgO–Mg₂SiO₄ composite ceramics by solid-state reaction synthesis in the present work. Then, effects of sintering temperature and raw material ratio on as-prepared ceramics were systematically studied. As-prepared ceramics showed improvement in their relative density (from 47.55%–68.12% to 90.96%–95.25%) and cold compressive strength (from 7.34–118.66 MPa to 303.39–546.65 MPa) with the increase in sintering temperature from 1300 to 1600 °C. In addition, it was found that Si promoted synthesis process of Mg₂SiO₄ phase through transient liquid phase sintering and Fe₂O₃ accelerated sintering process through activation sintering. Consequently, the presence of Mg₂SiO₄ phase effectively improved the density and strength of MgO–Mg₂SiO₄ composite ceramic, while reducing its thermal conductivity. This work provides a potential reutilization strategy for magnesite tailings, and as-prepared products are expected to be applied in fields of construction, metallurgy, and chemical industry.     

Cold sintering and microwave dielectric properties of dense HBO2-II ceramics

HBO₂-II ceramics were prepared by cold sintering with 10wt% dehydrated ethanol as the transient liquid phase. When the processing temperature is 30°C, the relative density of the mechanically robust HBO₂-II ceramics increases from 77.5% to 84.5% with increasing the uniaxial pressure from 200 to 500 MPa. It changes less than 0.2% for higher pressure up to 700 MPa. Under a constant uniaxial pressure of 500 MPa, the relative density further increases to 94.7% for the processing temperature of 120°C. HBO₂-I is observed as the secondary phase when the processing temperature is 150°C. In comparison, the compacts prepared in the absence of ethanol are fragile, and the relative densities are 78.5%-84.5% for the processing temperatures of 30-120°C and uniaxial pressure of 500 MPa. It is indicated that ethanol promotes the densification significantly through the dissolution-precipitation mechanism. The permittivity increases with increasing the processing temperature, while the Qf value decreases. The optimal properties with the relative density of 94.7%, ε_r = 4.21, Qf = 47 500 GHz, and τ_f = −70.0 ppm/°C were obtained in the single-phase HBO₂-II ceramics cold sintered at 120°C under 500 MPa for 10 minutes. The relative density and Qf value are significantly higher than those of the HBO₂-II ceramic prepared by sintering the H₃BO₃ compact at 180°C for 2 hours (70.3% and 32 700 GHz, respectively). The results indicate that the nonaqueous solvent can also be used as the transient liquid phase for cold sintering, so that more materials that are unstable or insoluble in water can be densified by this method.     

Transient liquid phase diffusion process for porous mullite ceramics with excellent mechanical properties

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO₂-rich liquid phase during the sintering process, and the liquid phase is transient in the Al₂O₃-SiO₂ system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105?MPa) have been obtained after fried at 1700?°C.     

Composition,microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering

In this paper, cold sintering was served as a forming method to assist the conventional sintering, which is so-called cold sintering assisted sintering (CSAS) method. Lead-free K_0.5Na_0.5NbO₃ piezoelectric ceramics were prepared by the CSAS method, and the effects of the different procedures on the sintering behaviors and electrical properties of KNN ceramics were studied. Compared with conventional sintering (CS), cold sintering process can induce potassium-rich phase on the KNN particle surface, and remarkably increase both the green and sintering density of KNN ceramics. Meanwhile, the potassium-rich phase would transform to K₄Nb₆O₁₇ second phase on the grain surface, and subsequently suppress the volatilization of potassium element. The sinterability and electrical properties were greatly improved, and KNN piezoelectric ceramics with high performance can be manufactured in a wide sintering temperature range (1055 °C–1145 °C), which proves that CSAS has the potential to be an excellent sintering technique for producing KNN based ceramics.     

Low temperature pressureless sintering of α-SiC with Al2O3 and CeO2 as additives

The combination of Al₂O₃ and CeO₂ was testified as suitable sintering additive for liquid phase sintering of SiC ceramics, which has lower sintering temperature than that sintered with Al₂O₃ and Y₂O₃ as sintering aids. However, the mechanical properties including flexural strength, Vickers’ hardness and fracture toughness of this system were similar to those of the samples sintered with Al₂O₃ and Y₂O₃ as sintering aids. The good wettability of the eutectic liquid phase on SiC plate, the high solubility of SiC particles into the liquid phase and the penetration of the liquid phase along the SiC–SiC grain boundaries all confirmed the suitability of the combination of Al₂O₃ and CeO₂ as liquid phase sintering additive for SiC.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

0-3型钛酸锶钡与聚四氟乙烯复合材料的冷烧结制备与介电性能研究

0-3型钛酸锶钡(BST)与聚四氟乙烯(PTFE)复合材料是一种新型的陶瓷/高聚物功能复合材料,可以兼具BST材料与PTFE材料的优点,可表现出较高的介电常数和介电可调性。但是受聚合物相介电常数低的限制,常规方法(流延法)制备的以聚合物为基体,以陶瓷为填充相的复合材料的介电常数基本在100以下。为了进一步提高BST/PTFE复合材料的介电性能,本研究采用一种新型烧结工艺——冷烧结工艺实现BST陶瓷与PTFE高聚物的共烧。在试验中以BST为基体,引入体积比例为5%的PTFE,并引入固相八水合氢氧化钡(Ba(OH)₂·8H₂O)作为过渡液相以辅助烧结过程进行,制备0-3型BST/PTFE复合材料,并探究了不同冷烧结条件下复合材料的介电性能。结果表明,复合材料样品在冷烧结温度为275 ℃,压力为200 MPa,时间为2.5 h的条件下,介电常数可达到500以上(25 ℃,1 kHz)。相对于常规制备工艺,冷烧结工艺制备出的复合材料的介电常数有很大改进,这对陶瓷/高聚物功能复合材料的低温制备与研究有一定参考意义。     

Liquid‐phase sintering of highly Na+ ion conducting Na3Zr2Si2PO12 ceramics using Na3BO3 additive

Na₃Zr₂Si₂PO₁₂ (NASICON) is a promising material as a solid electrolyte for all‐solid‐state sodium batteries. Nevertheless, one challenge for the application of NASICON in batteries is their high sintering temperature above 1200°C, which can lead to volatilization of light elements and undesirable side reactions with electrode materials at such high temperatures. In this study, liquid‐phase sintering of NASICON with a Na₃BO₃ (NBO) additive was performed for the first time to lower the NASICON sintering temperature. A dense NASICON‐based ceramic was successfully obtained by sintering at 900°C with 4.8 wt% NBO. This liquid‐phase sintered NASICON ceramic exhibited high total conductivity of ~1 × 10^?3 S cm^?1 at room temperature and low conduction activation energy of 28 kJ mol^?1. Since the room‐temperature conductivity is identical to that of conventional high‐temperature‐sintered NASICON, NBO was demonstrated as a good liquid‐phase sintering additive for NASICON solid electrolyte. In the NASICON with 4.8 wt% NBO ceramic, most of the NASICON grains directly bonded with each other and some submicron sodium borates segregated in particulate form without full penetration to NASICON grain boundaries. This characteristic composite microstructure contributed to the high conductivity of the liquid‐phase sintered NASICON.     

La2O3掺杂SnO2陶瓷的烧结机理探讨

利用共沉淀法制备La2O3掺杂二氧化锡陶瓷。通过观察晶粒及晶界情况分析了烧结机理。结合相图指出在烧结初期以固体烧结为主,随着烧结的进行出现少量液相,进一步促进了烧结,在烧结后期从主晶相中分离出第二相。     

The effect of liquid phase chemistry on the densification and strength of cold sintered ZnO

Cold sintering is a chemo-mechanical densification process which allows densification of ceramics at low temperatures below 300 °C. This substantial reduction in the sintering temperature is enabled by an externally applied pressure and a compatible transient liquid phase. In this paper, ZnO is cold sintered using various commercial organic acids: formic, acetic and citric acid. The effect of these different transient phases on densification, microstructural evolution and mechanical response is investigated. Fourier transform infrared spectroscopy, thermogravimetric analyses and transmission electron microscopy were conducted to explain the chemical interactions in the cold sintering process. High relative densities (～ 96 %) were achieved by formic and acetic acid, whereas poor densification was obtained for citric acid (< 80 %), despite the higher expected solubility of zinc oxide. The higher biaxial strength found in samples sintered with formic acid compared to acetic acid (i.e. ～90 MPa vs. ～40 MPa) is discussed supported by fractographic analyses.     

Densification behaviour and properties of manganese oxide doped Y-TZP ceramics

The effect of employing a short sintering holding time of 12 min as compared to that of the commonly employed holding time of 120 min (2 h) on the properties of undoped and 1 wt% manganese oxide (MnO₂)-doped Y-TZP ceramics were studied. Sintering studies was conducted over the temperature range of 1150-1600 °C. Bulk density, Young's modulus, Vickers hardness and fracture toughness tests were carried out on the sintered samples. The results revealed that the 12 min sintering holding time was effective in promoting densification of the 1 wt% MnO₂-doped Y-TZP without sacrificing tetragonal phase stability or mechanical properties and incurring grain growth. Microstructure investigation by using the scanning electron microscope (SEM) of the fracture MnO₂-doped Y-TZP samples which was subjected to rapid quenching in liquid nitrogen revealed distinct microstructural features believed to be associated with the presences of a transient liquid phase during sintering. A sintering mechanism was subsequently proposed to explain the densification behaviour of MnO₂-doped Y-TZP ceramics.     

The effect of rare earth oxides on the pressureless liquid phase sintering of α-SiC

Silicon carbide (SiC) ceramics have been fabricated by pressureless liquid phase sintering with Al₂O₃ and rare-earth oxides (Lu₂O₃, Er₂O₃ and CeO₂) as sintering additives. The effect was investigated of the different types of rare earth oxides on the mechanical property, thermal conductivity and microstructure of pressureless liquid phase sintered SiC ceramics. The room temperature mechanical properties of the ceramics were affected by the type of rare earth oxides. The high temperature performances of the ceramics were influenced by the triple junction grain boundary phases. With well crystallized triple junction grain boundary phase, the SiC ceramic with Al₂O₃–Lu₂O₃ as sintering additive showed good high temperature (1300 °C) performance. With clean SiC grain boundary, the SiC ceramic with Al₂O₃–CeO₂ as sintering additive showed good room temperature thermal conductivity. By using appropriate rare earth oxide, targeted tailoring of the demanding properties of pressureless liquid phase sintered SiC ceramics can be achieved.     

Cold Sintering of Na2WO4 Ceramics using a Na2WO4-2H2O Chemistry

Crystal water is a kind of water molecules that are incorporated into the crystal structure of hydrated salts, which are generally decomposed at very low temperatures (usually 100～200 °C). Na₂WO₄ is a compound that could react with H₂O forming the Na₂WO₄·2H₂O hydrated crystal salt under certain conditions. In this work, the cold sintering of Na₂WO₄ ceramics is studied using a Na₂WO₄-2H₂O chemistry at a low sintering temperature of 120 °C. Their sintering mechanism has been investigated using SEM, TEM, DSC, FTIR, together with the shrinkage rates of the samples. During the sintering process, the intentionally added liquid water reacts with Na₂WO₄ powders to produce a Na₂WO₄·2H₂O and Na₂WO₄ composite, and the crystal water can be decomposed from the Na₂WO₄·2H₂O phase under a certain sintering condition. The water in both liquid and solid forms contributes to the densification of Na₂WO₄ ceramics. Finally, dense Na₂WO₄ ceramics with excellent properties (permittivity: 5.7; Q × f: 70 000 GHz; TCF: ?70 ppm/°C) have been obtained by removing the residual water after a heat treatment. Such sintering process can be generalized to the densification of other inorganic materials that easily react with liquid water to form hydrated salts, which provides a strategy for the cold sintering with crystal water.     

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.     

Effect of dual liquid phase sintering aids on the densification and microstructure of low temperature sintered alumina ceramics

Alumina ceramics was prepared by pressureless sintering technology in which a CuO–TiO₂–Bi₂O₃ mixture (0–4.0 wt% Bi₂O₃ and 4.0 wt% CuO and TiO₂) was added as dual liquid phase sintering aids. The phase compositions, microstructural feature, and sintering behaviour of the alumina ceramics were analyzed. The results showed that adding 2.5 wt% Bi₂O₃ to alumina ceramics can increase the contribution rate of initial stage of sintering to the sintering process. The relative density of the sample reached 97.63% after sintering at 1200 °C for 90 min. Measurements from differential scanning calorimetry, with the addition of CuO–TiO₂–Bi₂O_3, demonstrated the formation of two liquid phase points, 827.4 and 936.8 °C. Notably, the solid solution temperature of TiO₂ and Al₂O₃ ceramics diminished thanks to the dual liquid phase sintering aids, and at the same time the activation energy required also dropped from 368.96 to 137.31 kJ/mol. Research indicates that the combined action of dual liquid phase sintering and solid-state reaction sintering has promoted the densification of alumina ceramics during the sintering process while at the same time inhibiting the growth of abnormal grains so that a homogeneous microstructure can be formed.     

High density nano-grained Gd2Zr2O7 ceramic prepared by combined cold and microwave sintering

In this study, nano-grained Gd₂Zr₂O₇ (NGZO) ceramic with a high relative density was prepared by a novel cold sintering process (CSP) assisted by microwave sintering (CSP-MS). The CSP with water as the liquid phase at 280 °C yielded nano-grained ceramic with a relative density of 71.5%. NGZO with a high relative density of 97.1% and average grain size of 73 nm was obtained by subsequent microwave sintering of the cold-sintered sample at 1300 °C. Therefore, CSP-MS is feasible in preparing dense NGZO ceramics with small grain sizes at relatively low sintering temperatures.     

Structural, microstructural and electrical properties evolution of (K,Na,Li)(Nb,Ta,Sb)O3 lead-free piezoceramics through NiO doping

Excellent piezoelectric properties have been reported in the (K,Na)NbO₃-LiTaO₃-LiSbO₃ system and have been regarded as a new candidate of lead-free piezoelectric material. Nevertheless, there are still some structural and electrical aspects that remain controversial with respect to the role of dopants in this system. NiO doping modifies the (K,Na,Li)(Nb,Ta,Sb)O₃ structure, giving rise to the appearance of the TTB-like secondary phase and to changes on the orthorhombic to tetragonal phase transition temperature. The microstructural characterization reveals that sintering process is assisted by a transient liquid phase. The presence of Ni in the liquid phase indicates that Ni^II ions could act as new nucleus for the secondary phase crystallization. Thus, as higher the amounts of liquid phase, higher the secondary phase appearance. The modifications on the structure and microstructure of the system cause a reduction of the piezoelectric constant, which is accompanied by an increase on the mechanical quality factor.     

Modelling and optimizing the liquid phase sintering of alumina/CaO–SiO2–Al2O3 ceramics using response surface methodology

Alumina is an attractive material for engineering applications due to its unique properties. In this study, CaO–SiO₂–Al₂O₃ eutectic phase was used as an additive phase and liquid phase sintering of the alumina/CaO–SiO₂–Al₂O₃ samples were investigated. The liquid phase sintering was modelled and optimized by Response Surface Methodology (RSM) using Central Composite Design (CCD) to achieve maximum fracture strength and density as responses. Sintering temperature, alumina particles size distribution (PSD), lubricant and eutectic phase content were selected as independent variables. Two cubic models were developed in terms of these variables to describe the responses. The validity and accuracy of the models were checked using Analysis of Variance (ANOVA).Phase identification of the synthesized eutectic phase was evaluated by XRD and fracture strength of the sintered samples was determined by Ring-on-Ring test method. SEM was used to study the fracture surface of the samples. The obtained models for predicting fracture strength and density of the sintered samples showed high conformity with the experimental results. Sintering temperature and alumina PSD were found as the most effective parameters. Therefore, optimized condition based on the defined constraints was obtained for sintering temperature of 1533 °C, alumina PSD of 25%, and lubricant and eutectic phase content of 1.5 wt% and 7.5 wt%, respectively. Results showed that after ball milling of the eutectic phase, the fracture strength of the optimized ceramic sample was improved and it reached to maximum values at smaller amounts of the additive phase.     

Cold-sintered V2O5-PEDOT:PSS nanocomposites for negative temperature coefficient materials

A low temperature sintering method, namely cold sintering process, was used to prepare 97 vol%V₂O₅-3 vol% PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate ceramic-polymer nanocomposites. The density, phase purity, microstructure, elemental distribution and electrical properties of sintered tape-cast films were investigated. The composition with 97 vol%V₂O₅-3 vol% PEDOT:PSS ceramic-polymer nanocomposites can be densified (∼90%) after a cold sintering of 140 °C for 45 min under a uniaxial pressure of 300 MPa. The Transmission Electron Microscopy (TEM) microstructure shows that a ∼10 nm thick intergranular polymer of PEDOT:PSS has been distributed around the V₂O₅ grains after cold sintering. The resistivity decreases with temperature increasing, indicating a typical negative temperature coefficient (NTC) characteristic. The resistivity at 25 °C, temperature coefficient α at 25 °C, and B coefficient (material constant) are 6.34 Ωm, −2.4% K⁻¹ and 2153 K, respectively. The V₂O₅-PEDOT:PSS nanocomposite materials are suitable for new NTC devices, with properties that are comparable to traditional NTC materials that are sintered at much higher temperatures and with much more complexed process and compositions.