TiB2-reinforced B4C composites produced by reaction sintering at high-pressure and high temperature

ABSTRACT

Obtaining both high hardness and toughness is a challenge in B₄C-nano-adhesive composites. Solving the inhomogeneous distribution of nano-adhesives in B₄C and forming the chemical bonding at grain boundary is an effective method. Here, we reported that the uniform distribution of titanium diboride (TiB₂)-reinforced B₄C composites synthesized by high-pressure and high temperature (HPHT). It is found that HPHT sintering can effectively inhibit the grain growth and increase the relative density. Moreover, HPHT sintering can cross high reaction energy barrier and effectively promote the formation of chemical bonding at grain boundary between B₄C and TiB₂. The optimal hardness and toughness value reach 30.0?±?0.9?GPa and 7.87?MPa·m^1/2, respectively. The improvement of hardness and toughness in the final products are ascribed to the strengthening of nanoTiB₂ connection of B₄C boundary and intergranular fracture mechanism. This work suggests a new way to achieve the uniform distribution of nanoTiB₂ in B₄C and form the chemical bonding at grain boundary, which is of great significance to the further development of TiB₂-reinforced B₄C composites with excellent mechanical properties.     

Constitutive modelling of the densification of micron-grain-sized alumina ceramics

In the present work, the densification behaviour of micron-grain-sized alumina during stage 1 sintering is studied by the constitutive models employed and the experimental results obtained. Grain-boundary diffusion is identified as the dominant densification mechanism of the materials system investigated, and the corresponding model is examined as an appropriate form. The activation energy for densification is evaluated as 384 kJ mol m1 , which provides strong support for the dominant densification mechanism investigation. The appropriate expressions for some important parameters, namely the sintering potential and dimensionless density functions, in the constitutive law are identified from those reported in the literature. The relationship between the sintering potential and the relative density is studied and the trend that the determined sintering potential increases with increase in the relative density is shown. The grain-boundary diffusion constitutive law is then used to study the process of sintering of a micron-grain-sized alumina tape on a rigid alumina substrate. As a result, the model provides a reasonably good prediction for the densification behaviour of the constrained-sintered tape. The tensile stress induced by the constraint is estimated and it is shown that the stress increases as the relative density increases.     

Electronic structures and mechanical properties of Al(111)/ZrB2(0001) heterojunctions from first-principles calculation

Employing first-principles density functional theory (DFT), the structures and electronic and mechanical properties of Al(111)/ZrB₂(0001) heterojunctions are investigated. It is found that both B-terminated ZrB₂(0001) and Zr-terminated ZrB₂(0001) can form heterojunction interfaces with Al(111) surface. The heterojunction with B-terminated ZrB₂(0001) is demonstrated to be most stable by comparing the surface adhesion energies of six different heterojunction models. In the stable configurations, the Al atom is found projecting to the hexagonal hollow site of neighbouring boron layer for the B-terminated ZrB₂(001), and locating at the top site of the boron atoms for Zr-terminated ZrB₂(001) interface. The mechanisms of interface interaction are investigated by density of states, charge density difference and band structure calculations. It is found that covalent bonds between surface Al atoms and B atoms are formed in the B-terminated heterojunction, whereas the Al atoms and Zr atoms are stabilised by interface metallic bonds for the Zr-terminated case. Mechanical properties of Al/ZrB₂ heterojunctions are also predicted in the current work. The values of moduli of Al/ZrB₂ heterojunctions are determined to be between those of single crystal Al and ZrB₂, which exhibit the transition of mechanical strength between two bulk phases. DFT calculations with the current models provide the mechanical properties for each heterojunction and the corresponding contributions by each type of interface in the composite materials. This work paves the way for industrial applications of Al(111)/ZrB₂(0001) heterojunctions.     

Hot-pressing of bimodally distributed magnesium fluoride powder

In the present study, nearly dense polycrystalline magnesium fluoride ceramics were fabricated by hot-press sintering, using bimodal size distribution MgF₂ powder. Densification was carried out at 710 °C under 300 MPa pressure for 60 min. The effect of weight percent of ultrafine powder on densification and also transparency, hardness and density of the samples produced are discussed. The results shown that sample with 15 wt.% ultrafine component, has the highest transparency, density, hardness, and a well developed microstructure, containing the lowest amount of porosities with small size.     

Dramatic reduction in the densification temperature of garnet-type solid electrolytes

Cubic garnet Li₇La₃Zr₂O₁₂ (LLZO) and similar compositions of fast ion-conducting solid-state electrolytes have shown great potential for the development of high-energy-density solid-state Li-ion batteries. Although these materials have shown unprecedented ionic conductivities and chemical stability, these materials require high processing temperatures for synthesis. For many of the common compositions of LLZO, temperatures above 1000 °C are required to form the cubic garnet phase and to achieve high conductivities. Therefore, lowering the processing temperatures of these materials is of great interest for the purposes of scalability and fabrication. It has been reported that a Bi co-dopant not only stabilizes the cubic garnet phase but also lowers the densification temperature. In this study, Li₆La₃ZrBiO₁₂ (LLZBO) was prepared by a rapid-induction hot-pressing technique and characterized using a variety of techniques, including X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. We demonstrate the ability to synthesize phase-pure LLZBO with higher relative densities (~?94%) than can be achieved by pressure-less sintering methods, at pressing temperatures of only 850 °C. The ionic conductivity was measured to be 0.1 mS cm^?1, which is comparable to the best reported conductivities of high-density LLZO. This demonstrates the ability to fabricate dense, phase-pure, and high-conductivity LLZBO at temperatures significantly lower than other garnet compositions, which will prove useful for scalability and reducing reactivity with cathodes during densification.     

Laser etching of glass substrates by 1064 nm laser irradiation

TiB₂-based cermets are prepared by combustion synthesis followed by a pressing stage in a granulate medium. Products obtained by combustion synthesis are characterized by a large remaining porosity (typically 50%). To produce dense cermets, a subsequent densification step is performed after the combustion process and when the reacted material is still hot. To design the process, numerical simulations are carried out and compared to experimental results. In addition, physical and electrical properties of the products related to electrical contact applications are evaluated. PACS 81.20.Ka; 81.05.Mh; 72.80.Tm     

Raman scattering in ZrB<Subscript>12</Subscript> cage glass

High-precision measurements of the Hall effect and Raman scattering have been performed for single crystals of ZrB₁₂ superconductor in the wide temperature range of 5–300 K. For ZrB₁₂, the boson peak with ω_max ~ 100 cm^–1 has been observed for the first time within the low-frequency range of the Raman spectrum I(ω). The sizes of vibrational clusters with the correlation length ranging from 25 to 35 Å are estimated. The relation between the renormalization of the low-frequency density of vibrational states accompanying the transition to the cage-glass phase (T* ~ 90 K) and the enhancement of superconductivity in ZrB₁₂ is discussed.     

Effect of Al-Mo codoping on the structure and ionic conductivity of sol-gel derived Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Zr<Subscript>2</Subscript>O<Subscript>12</Subscript> ceramics

Al-Mo codoped Li₇La₃Zr₂O₁₂ ceramics with fine grain were prepared by sol-gel method. The influences of Al-Mo codoping on the structure, microstructure, and conductivity of Li₇La₃Zr₂O₁₂ were investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and impedance spectroscopy. The cubic phase Li₇La₃Zr₂O₁₂ has been stabilized by partial substitution of Al for Li and Mo for Zr. Li_6.6-3yAl_yLa₃Zr_1.8Mo_0.2O₁₂ (0?≤?y?≤?0.1) has been sintered at 1040–1060 °C for 3 h. The liquid sintering facilitated its densification. The relative density of the composition with x?=?0.075 was approximately 96.4%. Results indicated that the Al-Mo codoped LLZO synthesized by sol-gel method effectively lowered its sintering temperature, accelerated densification, and improved the ionic conductivity.     

Effect of hot-consolidation method on the superconducting properties of B- and C-doped FeSe0.5Te0.5

The effect of hot consolidation method on the microstructures and superconducting properties of FeSe_0.5Te_0.5 (+B, C) was examined. FeSe_0.5Te_0.5 (+B, C) compounds were prepared by four different hot-consolidation methods: in-situ sintering, infra-red radiation sintering, hot-pressing and spark plasma sintering. The results showed that, among the employed consolidation methods, the spark plasma sintering is the most effective processing route for enhancing the critical current density due to enhanced densification. The doping of B or C also increased the critical density of FeSe_0.5Te_0.5, in particular, for spark plasma sintered specimens, but its beneficial effect was less pronounced for the specimens prepared by the other methods.     

The effect of titanium diboride addition on the thermoelectric properties of β-FeSi2 semiconductors

β-FeSi₂-TiB₂ composites with various amounts of TiB₂, from 0 up to 30 vol%, were prepared by hot pressing. The electrical and thermal conductivities, and the Seebeck coefficient were measured as a function of temperature. The results show that the thermal and electrical transport behavior of the composites is different as the volume fraction of TiB₂ is below and above about 0.255. A 5 vol% TiB₂ added sample has higher figure of merit than one without TiB₂ for temperatures above 650 K. The influence of an additional phase, ε-FeSi, formed during the hot pressing, on the thermoelectric properties of the β-FeSi₂-TiB₂ composites was also discussed.     

Preparation of dense La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>Ga<Subscript>0.8</Subscript>Mg<Subscript>0.2</Subscript>O<Subscript>3-δ</Subscript> with high ionic conductivity by solid-state synthesis

The results of a systematic investigation on the effects of processing steps, via solid-state reactions, on structural phase characteristics and ionic conductivity of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ solid electrolyte are reported. The main purpose of this work is to establish an optimized route for obtaining good densification and high ionic conductivity of this solid electrolyte. Processing routes with three successive calcinations at 1250 °C followed by attrition milling (R1), and with two sequences of calcination at 1350 °C with intermediary attrition milling (R2) give rise to near full density at 1450 °C sintering temperature. The rate of grain growth is fast when the relative density reaches 95%. Elemental mapping reveals uniform distribution of the constituents in the matrix along with La₄Ga₂O₉, LaSrGa₃O₇ and sub-micrometer MgO grains at grain boundaries. The ionic conductivity of grains remains unchanged with the processing route and sintering profile. The blocking effect of charge carriers at grain boundaries decreases with increasing the dwell temperature.     

Preparation of YSZ films by magnetron sputtering for anode-supported SOFC

YSZ films for anode-supported SOFCs were prepared by reactive sputtering method. It was found that the surface morphology of anode substrate has a very important effect on the quality of sputtered films. By applying an anode functional layer and making the anode surface smooth, dense and uniform YSZ films of 10 µm in thickness were successfully fabricated. The sintering behaviors of the sputtered YSZ films were also discussed. It is suggested that the optimized densification condition for the deposited YSZ films is sintering at 1250 °C for 4 h. Single cells with sputtered YSZ film as electrolyte and LSM-YSZ as active cathode materials were tested. 1.08 V open circuit voltage and a 700 mW/cm² maximum power density were achieved at 750 °C by using humidified H₂ as fuel and air as oxidant.     

Effect of low-temperature high-pressure sintering on BiFeO3 density,electrical magnetic and structural properties

Single-phase multiferroic BiFeO₃ (BFO) powders were prepared by hydrothermal microwave synthesis and dense BiFeO₃ ceramics were fabricated for the first time by the low-temperature high-pressure (LTHP) sintering technique. Effect of sintering temperature ranging from 400 to 800 °C (3 min and 10 min) and pressure of 3–8 GPa on structural, microstructural, electric and magnetic properties were investigated through X-ray diffraction, scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS), density and magnetic measurements. The results highlighted that LTHP sintering method, thanks to the high pressure involved, requires lower temperature and shorter time than other techniques, avoiding BiFeO₃ phase degradation. SEM images show that for short experimental time (t = 3 min) the average grain size of the sintered samples was approximately the same size of raw powder. Extending the sintering time up to 10 min the grain growth phenomena occurred. Moreover the results indicate that the best obtained density value was around 98% of theoretical density. The dielectric behavior of BiFeO₃ ceramics was not significantly influenced by the LTHP sintering conditions. Magnetic measurements showed that ceramic BiFeO₃ is weakly ferromagnetic at room temperature.     

On the use of NiO as sintering additive for BaCe0,9Y0,1O3 − α

The effect of nickel oxide addition on the densification behaviour and electrical conductivity of BaCe_0.9Y_0.1O_3 ? α (BCY10) is investigated. Small addition (4 mol%) of this sintering aid reduces the sintering temperature by 200 °C and promotes the densification. 95% of the theoretical density was reached after sintering at 1250 °C for 10 h in air. Addition of NiO has no detrimental effect on the total conductivity of BCY10 in wet hydrogen. Nevertheless, the activation energy of the recorded blocking effect is higher than that of the bulk contribution, indicating different electrical properties between grains and internal interfaces in pure and NiO doped BCY10.     

Synthesis and electrical properties of Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics

Ceramic compositions of a combination between lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃, and lead titanate, PbTiO₃, were fabricated by using Mg₄Nb₂O₉ precursor technique. Their electrical properties with respect to temperature and frequency were examined and the effect of sintering conditions on phase formation, densification, microstructure and electrical properties of the ceramics were examined. It has been found that optimisation of sintering condition can lead to a highly dense and pyrochlore-free PMN–PT ceramics. The gradual decreasing of the physical properties of the sintered ceramics was related to the gradual decrease of density and inhomogeneous microstructure. The results also revealed that for the lower concentration of lead titanate a relaxor behavior is noticed with a high electrostrictive effect, which was almost hysteretic free. However, higher amount of lead titanate led to a normal ferroelectric behavior.     

Ultrafast sintering of La–Ca–Mn–O bulk ceramics by thermal plasma assisted heating

This paper reports a novel and low-cost thermal plasma assisted heating method for ultrafast sintering of rare-earth-based perovskite manganites. An indigenously designed DC thermal plasma reactor has been used to sinter La_0.67Ca_0.33MnO₃ (LCMO) colossal magnetoresistance (CMR) material. Highly dense (96% of theoretical density) crystalline bulk LCMO ceramics were prepared within 2.5 min of sintering time. Plasma-sintered LCMO bulk ceramics showed enhanced T_C (272 K), which is close to T_IM (275 K) as compared to the conventional sintered LCMO [J.M. De Teresa, M.R. Ibarra, J. Blasco, J. Garcia, C. Marquina, P.A. Algarabel, Z. Arnold, K. Kamenev, C. Ritter, R. von Helmolt, Phys. Rev. B 54 (1996) 1187] sample. We observe that the thermal plasma heating process offers a unique advantage for quick physical densification and hence sintering within a few minutes.     

Effect of nanocrystalline particles on the magnetic properties of Z-type hexaferrites

In order to improve the magnetic properties of Z-type hexaferrites such as high initial permeability and high-quality factor, the nanocrystalline hexaferrite particles (NHPs) with the same chemical composition were introduced. The influence of NHPs on the densification, microstructures and magnetic properties of the ceramics prepared by a combined method was investigated. The results show that these NHPs, which spread around the micron-sized hexaferrite particles (MHPs), enhance the densification by increasing the inter-diffusion of the particles due to the increase of contact area; simultaneously, the grain growth in the direction of c-plane is dominant. Due to the special microstructure, high sintering density, and no addition sintering aids with different chemical composition, relatively high initial permeability and Q-factor than those of the samples with 1.0 wt% Bi₂O₃ were obtained in the samples with proper nanocrystalline particles sintered at 900 °C.     

氧离子导体La1.9Y0.1Mo2O9细晶粒陶瓷的制备和电学性质研究

采用溶胶凝胶法合成的La_1.9Y_0.1Mo₂O₉纳米晶粉体, 结合微波烧结技术制备出不同晶粒度的La_1.9Y_0.1Mo₂O₉块体样品. 利用X射线衍射仪(XRD)、高分辨透射显微镜(HRTEM)、场扫描显微镜(SEM)对粉体及陶瓷块体的物相、 形貌进行了表征, 利用交流阻抗谱仪测试了样品不同温度下的电导率. 实验结果表明, 掺Y的La_1.9Y_0.1Mo₂O₉能将高温立方β 相稳定到室温; 块体样品致密均匀, 平均晶粒度范围在60 nm–4 μm之间; 致密度高的样品表现出高的电导率, 其中900 ℃烧结样品的电导率600 ℃时高达0.026 S/cm, 比固相反应法制备的La_1.9Y_0.1Mo₂O₉样品高出约1倍. 总结认为样品的致密性对电导率影响较大, 是通过影响晶界电导率来影响总电导率的, 样品的晶粒度(在60 nm–4 μm范围内)对电导率的影响还不能确定. 关键词： 氧离子导体 1.9Y_0.1Mo₂O₉')" href="#">La_1.9Y_0.1Mo₂O₉ 细晶粒陶瓷 微波烧结     

Cathode-supported micro-tubular SOFCs based on Nd1.95NiO4 + δ: Fabrication and characterisation of dip-coated electrolyte layers

Fabrication conditions to obtain dense electrolyte double layers on a porous micro-tubular cathode substrate were investigated. Porous tubes of a nickelate material, Nd_1.95NiO_4 + δ, were fabricated by cold-isostatic pressing (CIP) and sintering. Thin films of electrolyte powders Ce_0.9Gd_0.1O_1.95 (GDC) and Zr_0.89Sc_0.1Ce_0.01O_1.95 (SSZ) were applied by dip-coating in aqueous suspensions. Electrolyte powders had been attrition-milled to decrease their particle size and densification temperature. The quantity of polyacrylic acid (PAA) as dispersant was optimised by ζ-potential measurements. The densification behaviour was studied by dilatometry and SEM-imaging. A sintering temperature of 1250 °C was found to densify GDC whereas SSZ-layers remained slightly porous.     

二硼化钛的高温高压制备及其物性

以化学计量配比的Ti,B元素为原料,在高温高压条件下成功制备出颗粒均匀、致密性大于99％的二硼化钛(TiB2)体材料.物性测试结果表明:TiB2的维氏硬度高达39.6 GPa(接近超硬材料的40 GPa);并呈现出金属导电特性,电阻率在10-8 Ω.m的数量级(接近TiB2单晶样品值).TiB2的高硬度与金属特性,可能与该方法制备的TiB2体材料中均匀的细小晶粒尺寸有关.该方法为制备功能陶瓷材料提供了新的思路.