Densification of thermodynamically unstable tin monoxide using cold sintering process

SnO is a thermodynamically unstable phase and undergoes thermal decomposition into SnO₂ and Sn at a relatively low temperature when heating under ambient conditions. With the cold sintering process (CSP), SnO can be densified up to 89% of theoretical density within 100 min by applying uniaxial pressure of 350 MPa and transient liquid phase. 15-fold BET specific surface area reduction is observed between the ball-milled powder and the cold-sintered pellet, indicating experimental evidence of sintering. The temperature profiles of 70–265 °C show densification while maintaining the phase purity. Water and 2 M acetic acid solution are studied as transient liquid phases which promotes dissolution-precipitation on the particle surface and induces crystalline texture. Electrical properties of the cold sintered bulk, notably electrical conductivity and Seebeck coefficient, are measured as a function of temperature.     

Preparation of Na0.5Bi0.5TiO3 ceramics by hydrothermal-assisted cold sintering

A recently proposed novel technique, termed “cold sintering process” (CSP), can provide dense ceramic solids at remarkably low temperatures around 180?°C. In a recent work, we successfully obtained dense Na_0.5Bi_0.5TiO₃ ceramics by this method. Bismuth titanate sodium nanoparticles were prepared as the raw material powder by the hydrothermal synthesis route. A hydrothermal precursor solution was used as the transient solvent for cold sintering. Under the combined action of pressure and temperature, the Na_0.5Bi_0.5TiO₃ green body was densified by dissolution-precipitation, and a preliminary densified ceramic sheet was obtained. The amorphous phase in the ceramic sheet was then transformed into a crystalline phase by annealing. Finally, densified Na_0.5Bi_0.5TiO₃ ceramic sheets were obtained, with density of up to 99%, relative permittivity of 681, and dielectric loss of 0.08 at 10?kHz and room temperature. The piezoelectric coefficient d₃₃ of the sample was 52.5?pC/N. The properties of the prepared ceramics were comparable to those of the conventional sintered ceramics.     

Rutile TiO2 microwave dielectric ceramics prepared via cold sintering assisted two step sintering

In this work, a sintering route named cold sintering assisted two step sintering process (CSP-TS) is presented to prepare rutile TiO₂ ceramics with submicron grain sizes. Cold sintering process at 300 °C with tetrabutyl titanate and water as the liquid phase yields a ‘green body’ with a relatively high density of ～80 %, and finally dense (98.5–99.8 %) rutile TiO₂ ceramics with grain sizes of ～600 nm can be obtained in the second sintering process at 950?1000 °C. The microstructural analysis with SEM and TEM indicates that the CSP-TS samples sintered at 950 °C have an obvious phenomenon of recrystallization, accompanying by a decrease of amorphous phases and a formation of clear grain boundaries. Besides, the rutile TiO₂ ceramics prepared by CSP-TS possess excellent microwave dielectric properties with relative permittivity of 92.0–98.4 and Q × f values of 27,800?31,900 GHz. Therefore, it is feasible to utilize CSP-TS to prepare ceramics with small grain sizes at low sintering temperatures.     

Sintering of a sodium-based NASICON electrolyte: A comparative study between cold,field assisted and conventional sintering methods

Scandium-substituted NASICON (Na_3.4Sc_0.4Zr_1.6Si₂PO₁₂) is a promising electrolyte material for sodium-ion solid state batteries, with the highest ionic conductivity reported to date for a NASICON material. Low-temperature densification and control of microstructure are important factors to enable the low-cost manufacturing of such new battery type. Non-conventional sintering techniques such as Field Assisted Sintering Technology / Spark Plasma Sintering (FAST/SPS) and Cold Sintering are therefore investigated and compared to conventional free sintering. FAST/SPS enables to get rapidly dense samples (99% TD) at lower temperatures than the ones required by conventional sintering routes and with similar electrical properties. Cold sintering experiments, involving the addition of aqueous solutions as sintering aids and high mechanical pressure, enable a moderate densification, but at temperatures as low as 250 °C. Further heat treatments still below the conventional sintering temperature increased the achieved density and ionic conductivity.     

Fabrication and properties of C/SiC porous ceramics by grinding-mould pressing-sintering process

High temperature resistant porous ceramics are considered to be prime candidates for applications in the transpiration cooling system of a hypersonic vehicle. This paper describes a new preparation process including grinding-mould pressing-sintering process, which is successfully used to fabricate C/SiC porous ceramics with high compressive strength and excellent permeability. The effects of carbon fiber content on the microstructure, mechanical property, pore size distribution and permeability of this porous ceramic are investigated in detail. The results indicate that this porous ceramic prepared in this study exhibits high compressive strength (～270.82 MPa) and excellent permeability (～3.937 × 10^?8 mm²). The C/SiC porous ceramics fabricated in this study will have potential application in active thermal protection systems.     

Gel-casting of transparent magnesium aluminate spinel ceramics fabricated by spark plasma sintering (SPS)

In this paper, a transparent magnesium aluminate spinel ceramic was fabricated through the newest colloidal gel casting method, using a synthetic powder with the average particle size of 90 nm and Isobutylene-Maleic Anhydride (ISOBAM) additive. ISOBAM served as both a dispersant and a gelation agent to achieve a dense body. Also, the suspension rheological behavior was optimized by the solid loading of 85 wt%, the additive content of 0.7 wt%, and the gelation time of 350 s. This led to a green body with a density equal to 65% of theoretical density and the green strength of 14.48 MPa. The results revealed that the reduction of porosity and the uniform distribution of pores in the green body (smaller than half of the initial powder particle size, 35 nm), as accompanied by spark plasma sintering (SPS), resulted in the final body density of 99.97%, as well as the high in-line transmittance of 86.7% at the wavelength of 1100 nm.     

Microwave sintering of cordierite precursor green bodies prepared by starch consolidation

This paper reports on a study of the microwave sintering behavior of green disks prepared by the starch consolidation forming method to produce cordierite-based porous materials. Green disks were formed by thermogelling the aqueous suspensions of talc, kaolin and alumina (29.6 vol.%) and potato starch (11.5 vol.%) at 75 and 85 °C for 4 h, drying and calcining. They were characterized by bulk density and apparent porosity measurements, and SEM. Microwave sintering was carried out at 1300 and 1330 °C for 15, 20 and 25 min, applying 50 °C/min. For purposes of comparison, an analysis of green disks prepared and calcined in the same conditions and conventionally sintered (1330 °C for 4 h) was also made. The materials were characterized by XRD, bulk density and apparent porosity measurements, and microstructurally analyzed SEM. The results were analyzed considering the behavior of starch in aqueous suspension at varying temperatures, and the experimental conditions of consolidation and sintering.     

Flash sintering of Mg-doped tricalcium phosphate (TCP) nanopowders

β-tricalcium phosphate is a bioceramic with unique osteoinductive and osteoconductive properties. Its processing is limited by the undesired β→α phase transition which occurs upon sintering at 1398 K. The reduction of sintering temperature or the stabilization of β phase (by doping) are therefore of particular interest.In this work, flash sintering was used to consolidate β-tricalcium phosphate nanopowders synthesized by wet chemical methods. Pure and Mg-doped powder was studied, pointing out a strong effect of the dopant on the flash behavior and on the phase transitions. The results point out that tricalcium phosphate can be consolidated in few seconds at relatively low furnace temperature. The sintered pellets contain only the desired β phase in the case of Mg-doped powder, whereas some retained α phase is present in the undoped ones. Starting from the electrical behavior of the material a first processing map to attain β-tricalcium phosphate by flash sintering is proposed.     

Porous SiC/SiCN composite ceramics fabricated by foaming and reaction sintering

Porous SiC/SiCN composite ceramics with heterogeneous pore structure and rod-like SiCN grains were fabricated by foaming and reaction sintering. The mixture slurry containing SiC and silicon as raw materials, cornstarch as binder, Y₂O₃ as sintering additive and an electrosteric dispersant was stirred with foams derived from pre-foaming using foaming agent. The casted green body was sintered at 1650 °C under nitrogen atmosphere. The results demonstrated that the porous SiC/SiCN ceramics exhibited hierarchical vias ranging from 1 μm to 1 mm and the rod-like crystalline SiCN grains generated in the SiC matrix.     

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 sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.     

Transparent yttria stabilized zirconia from glycine-nitrate process by spark plasma sintering

Nanosized cubic yttria-stabilized zirconia (ZrO₂-8 mol% Y₂O₃) powder was synthesized via a glycine-nitrate process combining with high-energy ball milling. Effect of the calcination temperature on the sintering activity of the powders was discussed. The present investigations demonstrated the most favorable calcination temperature was 900 °C for obtaining fine nanopowders with high sinterability. Consolidation of the nanopowder was carried out by spark plasma sintering at 1200-1350 °C for 5 min. Transparent ceramics fabricated could be achieved at 1300 °C. Optical transmittance calculation based on Mie theory fits well with the experimental results of the transparent specimen sintered at 1300 °C, while the inconsistence for the specimen sintered at 1350 °C above 600 nm might be attributed to the scattering by grain boundaries and higher oxygen vacancy content.     

多孔TiO2陶瓷光催化剂降解糠醛废水

以钛酸四丁酯等为原料,采用简单工艺制得多孔TiO2陶瓷光催化剂;并研究了利用多孔TiO2陶瓷光催化降解糠醛废水。结果表明,在紫外灯下照射60min,糖醛废水的COD去除率可达59．8％,而通入空气有利于污染物的去除。     

The effect of slip casting and spark plasma sintering (SPS) temperature on the transparency of MgAl2O4 spinel

MgAl₂O₄ bulk samples were fabricated by two different approaches to investigate the effect of slip casting and sintering temperature on their transparency. Three MgAl₂O₄ samples containing 1 wt% LiF, as the sintering aid, were prepared by the spark plasma sintering process (SPS) at 1400 °C and 1500 °C, under 100 MPa, for 15 min. Also, another MgAl₂O₄ sample was prepared by slip casting followed by SPS under similar conditions. It was observed that utilizing slip casting led to more transparency (10% in the visible region and 20% in the IR region) due to the more homogeneous structure. It was also observed that by reducing the SPS temperature from 1500 °C to 1400 °C, the transparency increased (20% in the IR region) because of the lower grain growth rate at the lower temperature.     

Pressure-assisted microwave sintering: A rapid process to sinter submicron sized grained MgAl2O4 transparent ceramics

This work focuses on the development of an original process based on a 2.45 GHz single-mode microwave cavity equipped with a uniaxial press, to sinter transparent spinel MgAl₂O₄ ceramic in air. The samples were conventionally pre-sintered to a density of 90% TD before microwave sintering to the final stage of densification. The influence of thermomechanical cycle on the material properties was investigated. Transmittance, grain size distribution, hardness and fracture toughness of the samples were measured and correlated to the microstructure. This new sintering process has allowed obtaining transparent samples with sub micrometric grain size and high mechanical properties, with relatively short times and low temperature. These first results can be compared to some obtained by SPS or HIP. The technical input of this method is that all the process is here conducted in air atmosphere.     

Fabrication of high performance silicon nitride ceramics with TiO2 additive by annealing process

The high performance Si₃N₄ ceramic was prepared firstly for TiO₂, Y₂O₃ and MgO as pressureless sintering additives. Si₃N₄ ceramic with relative density of 99.6% and flexural strength of 785 ± 23.3 MPa could be obtained with 3 mol% TiO₂ and sintered at 1800 °C for 2 h. After annealing at 1700 °C, the facture toughness of sample of 1 mol% TiO₂ increased from 8.31 ± 0.28 MPa m^1/2 to 9.84 ± 0.16 MPa m^1/2. The flexural strength of sample of 2 mol% TiO₂ increased from 707 ± 26 MPa to 981 ± 16 MPa, thermal conductivity increased from 57.8 W/(m·K) to 68.49 W/(m·K). The XRD results showed that the ratio of I₁₀₁/I₂₁₀ and grain height reached to 1.84 and 5 μm of the sample of 3 mol% TiO₂, respectively. The present investigation revealed that the three-dimensional array of highly oriented crystalline Si₃N₄ micro rods could be prepared which array on the homogeneous substrates by using TiO₂ as agent. This phenomenon may propose a method that the mechanical properties the Si₃N₄ ceramics added TiO₂ can be improved significantly after annealing process.     

Enhanced energy storage properties of Ba0.4Sr0.6TiO3 lead-free ceramics with Bi2O3-B2O3-SiO2 glass addition

In this study, we present an effective strategy to enhance the energy storage properties of Ba_0.4Sr_0.6TiO₃ (BST) lead-free ceramics by the addition of Bi₂O₃-B₂O₃-SiO₂ (BBS) glass, which were prepared by the conventional solid state sintering method. The phase structure, microstructure and energy storage properties were investigated in detail. It can be found that the Ba_0.4Sr_0.6TiO₃-x wt%(Bi₂O₃-B₂O₃-SiO₂) (BST- x wt%BBS, 0 ≤ x ≤ 12) ceramics possess large maximum polarization (P_max), low remanent polarization (P_r) and slim polarization electric field (P-E) hysteresis loops. The breakdown strength (BDS), recoverable energy storage density (W_rec) and energy storage efficiency (η) are enhanced obviously with the addition of BBS glass. The BST-9 wt%BBS ceramic is found to exhibit excellent energy storage properties with a W_rec of 1.98 J/cm³ and a η of 90.57% at 279 kV/cm. These results indicate that the BST-x wt%BBS ceramics might be good candidates for high energy storage applications.     

The effect of solidification direction with respect to gravity on ice-templated TiO2 microstructures

Unidirectional ice-templating produces materials with aligned, elongated pores via: (i) directional solidification of particle suspensions wherein suspended particles are rejected and incorporated between aligned dendrites, (ii) sublimation of the solidified fluid, and (iii) sintering of the particles into elongated walls which are templated by the ice dendrites. Most ice-templating studies utilize upward solidification techniques, where solid ice is located at the bottom of the solidification mold (closest to the cold source), the liquid suspension is above the ice, and the solidification front advances upward, against gravity. Liquid water reaches its maximum density at 4 °C; thus, liquid nearest the solid/liquid interface, at 0ºC, is less dense than warmer liquid above (up to 4 °C, above which, a density inversion occurs, and liquid density decreases with increasing temperature). The lower density liquid nearest the solidification front is thus expected to rise due to buoyancy, promoting convective fluid motion in the liquid during solidification. Here, we investigate the effect of solidification direction with respect to the direction of gravity on ice-templated microstructures to study the role of buoyancy-driven fluid motion during solidification. We hypothesize that, for upward solidification, the convective fluid motion that results from the liquid density gradient occurs near the solidification front. For downward solidification, we expect that this fluid motion occurs farther away from the solidification front. Aqueous suspensions of TiO₂ nanoparticles (10–30 nm in size, 10, 15, and 21 vol.%) are solidified upward (against gravity, with ice on bottom and water on top), downward (water on bottom, ice on top), and horizontally (perpendicular to gravity). Microstructural investigation of sintered samples shows evidence of buoyancy-driven, convective fluid flow during solidification for samples solidified upwards (against gravity), including (i) tilting of the wall (and pore) orientation with respect to the induced temperature gradient, (ii) ice lens defects (cracks oriented perpendicular to the freezing direction), and (iii) radial macrosegregation. These features are not observed for downward nor horizontal solidification configurations, consistent with the hypothesis that convective fluid motion does not interact directly with the solidification front for downward solidification.     

Porous fibrous ZrO2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting

Mullite fiber was used to fabricate ZrO₂-mullite based porous ceramic via tert-butyl alcohol (TBA)-based gel-casting process using zirconite and bauxite as raw materials. Phase compositions, microstructure, pore size distribution, linear shrinkage, bulk density, apparent porosity, thermal conductivity, and compressive strength were analyzed to investigate influences of mullite fiber content and added Y₂O₃ on prepared porous ceramics. Results show that bird nest-like three-dimensional fibrous reticular skeleton structure was constructed with mullite fibers that evenly enwrapped rod-like mullite and ZrO₂ grains. Prepared porous fibrous ZrO₂-mullite ceramics had narrow pore size distribution that consisted of mullite and m-ZrO₂. With an increase in mullite fiber content, linear shrinkage and bulk density decreased, apparent porosity increased, and relatively good thermal conductivity was obtained. In addition, added Y₂O₃ reacted with Al₂O₃ and SiO₂ to form Y-Al-Si-O glass phase, which promoted sintering and densification of the ceramic, thus improving its compressive strength.