Fabrication of zero-thermal-expansion ZrSiO4/Y2W3O12 sintered body

We focused on the linear negative thermal expansion of Y₂W₃O₁₂ in a wide-temperature range and on the chemical stability of ZrSiO₄ in the fabrication of the composite material ZrSiO₄/Y₂W₃O₁₂ with a zero-thermal-expansion. The compact composed of Y₂W₃O₁₂ and ZrSiO₄ had a thermal shrinkage rate smaller than that of Y₂W₃O₁₂ and higher than that of ZrSiO₄. SEM–EDX observation clarified that the ZrSiO₄/Y₂W₃O₁₂ sintered body fabricated at 1400 °C for 10 h had a microstructure composed of ZrSiO₄ and Y₂W₃O₁₂ grains, and XRD indicated that only ZrSiO₄ and Y₂W₃O₁₂ phases existed in the sintered body. The relative density of the ZrSiO₄/Y₂W₃O₁₂ sintered body reached 92%, which was larger than that of the ZrSiO₄ sintered body because Y₂W₃O₁₂ grains could be sintered at lower temperatures. The average linear thermal expansion coefficients of the ZrSiO₄/Y₂W₃O₁₂ sintered body were −0.4 × 10⁻⁶ and −0.08 × 10⁻⁶ °C⁻¹ in the temperature ranges from 25 to 500 °C and from 25 to 1000 °C, respectively, which showed an almost zero-thermal-expansion.     

Luminescent properties of Pr doped ZrSiO4 phosphors

Mixtures of ZrO₂, SiO₂, Pr₂O₃, and H₃BO₃ were fired at the temperature of 1150–1350 °C under H₂ atmosphere. A single phase ZrSiO₄:Pr phosphors could be achieved at the firing temperature above 1150 °C. Crystallinity and PL properties strongly depended on the flux amounts, firing temperature, and dopant concentrations. ZrSiO₄:Pr phosphors showed a strong red emission at 615 nm and a weak red emission at 622 nm with an excitation wavelength of 295 and 450 nm, respectively. Two sets of the emission bands and excitation spectra originated from Pr³⁺ ions at Zr and Si sites, respectively.     

ZrSiO4对Si3N4结合SiC制品强度和抗氧化性的影响

研究了硅酸锆(ZrSiO     

Developments on the mechanical properties of MgO-MgAl2O4 composite refractories by ZrSiO4-3 mol% Y2O3 addition

Improvements and the effects of additions of ZrSiO₄-3 mol% Y₂O₃ into MgO-MgAl₂O₄ composite refractories on mechanical properties and thermal stress resistance parameters were investigated. Significant improvements were achieved on mechanical properties and R-R_st parameters up to ∼2 and ∼3-fold ratios. The major parameters improving mechanical properties and thermal behaviour of refractories were determined as follows: (i) the increase in resistance to crack initiation and propagation due to formation of Mg₂SiO₄ phase after decomposition of zircon; (ii) propagation of the microcracks formed in the structure for a short distance by interlinking each other; (iii) arresting or deviation of microcracks when reaching pores or ZrO₂ grains released after dissociation of zircon, located together with Y₂O₃ particles, and furthermore; (iv) co-presence of both intergranular and transgranular types of cracks, and with incorporation of zircon-Y₂O₃; (v) increase in density; and (vi) a significant reduction in MgO grain size.     

Porous (SiCw-Si3N4w)/(Si3N4-SiC) composite with enhanced mechanical performance fabricated by 3D printing

SiC whisker and Si₃N₄ whisker-reinforced Si₃N₄-SiC ((SiC_w-Si₃N_4w)/(Si₃N₄-SiC)) composite was synthesized by 3D printing for the first time, by the combination of printed-Si-body nitridation and chemical vapor infiltration-SiC methods. The mechanical properties of the composite could be optimized through the adjustment of SiC_w content and load direction. A SiC_w content of 3?wt% was found to be the optimal scheme, and accordingly, the average bulk density increased by 22.4%, the bending strength increased by 63.6%, the compressive strength parallel to the printing layer increased by 404.8%, and the compressive strength perpendicular to the printing layer increased by 157.1%, compared with the bulks without whiskers. The enhanced mechanical performance was mainly attributed to the process of densification by CVI, and the effect of the homogeneous whiskers bridging, pull-out and deflecting crack to expend energy. The achieved indices meet the requirements for 3D-printed porous ceramic matrix composite targeted for commercial and military field applications.     

Synthesis and characterization of praseodymium-containing ZrSiO4 solid solutions from gels

The preparation and characterization of a series of praseodymium-zircon solid solution (Pr_x-ZrSiO₄) materials with increasing nominal amounts of Pr is reported. Pr-doped zircon gels were prepared by gelling mixtures of zirconium n-propoxide, praseodymium acetylacetonate and tetraethylorthosilicate, and annealed over the range of temperature up to the formation of Pr-zircon solid solutions. The reaction sequence was followed by X-ray powder diffraction (XRD), ultraviolet-visible diffuse reflectance (DR) and infrared spectroscopy (IR). The first crystalline phase detected on annealing gels was a tetragonal praseodymium-containing ZrO₂ phase (t-Pr-ZrO₂). On further annealing, the subsequent transformation to the monoclinic form (m-Pr-ZrO₂) took place. The formation of final Pr-ZrSiO₄ solid solutions occurred by the reaction between m-Pr-ZrO₂ and amorphous silica phase. The mechanism of solid solution formation inferred from energy dispersive X-ray microanalysis (SEM/EDX) data, variation of lattice parameters and DR of final Pr-ZrSiO₄ solid solutions involved the replacement of Zr⁴⁺ by Pr⁴⁺ in dodecahedral sites of the zircon structure. DR revealed that a relatively small amount of Pr³⁺ was still present in final Pr-containing ZrSiO₄ products. The estimated solubility of praseodymium in the zircon was around 0.067 mol of praseodymium per mol of zircon (∼11.5 wt% as Pr₂O₃). This study opens new perspectives to the development of more ecological zircon-based ceramic pigmenting systems by using mineralizer-free sol-gel synthetic techniques.     

Zircon-zirconia (ZrSiO4-ZrO2) dense ceramic composites by spark plasma sintering

Dense reinforced zircon (ZrSiO₄)-20 vol.% zirconia (ZrO₂) ceramic composites were obtained by spark plasma sintering (SPS) starting from high energy milled commercially available powders. The sintering temperature and holding time resulted in two different microstructural configurations.In the first configuration, the nano sized zirconia nanoparticles (100 nm) act as a bonding phase continuously dispersed along the zircon micronic (1-4 μm) grains. In the second one, a continuous zircon phase with well dispersed zirconia grains was achieved.Both configurations led to improvement in fracture toughness and Vickers hardness if compared to pure zircon material processed under the same sintering condition. By comparing the developed microstructure configurations, the second one exhibits higher fracture toughness (almost 4.0 MPa m^1/2) due to the more effective zirconia reinforcement effects.     

The preparation and properties of SiCw/B4C composites infiltrated with molten silicon

Silicon carbide whisker (SiC_w) toughened B₄C composites have been prepared by pressureless infiltration of B₄C–SiC_w–C preforms with molten silicon under vacuum at 1500 °C. The effect of SiC_w addition on bulk density, hardness, bending strength, fracture toughness and microstructure of SiC_w/B₄C composites is discussed. It is revealed that the addition of SiC_w improves the fracture toughness of B₄C ceramic, but reduces its bending strength at the same time. The maximum fracture toughness for SiC_w/B₄C composite with 24 wt% SiC_w addition is 4.88 MPa m^1/2, which is about 9% higher than that of the one without SiC_w, but at the same time, the bending strength reduces to the minimum value 243 MPa, reduced by 25%. XRD analysis shows that the phase composition of reaction bonded SiC_w/B₄C composites is B₄C, SiC, Si, and B₁₂ (C, Si, B)₃, with no residual C. And the main toughening mechanism of SiC_w is whisker pulling up.     

Improvements on corrosion behaviours of MgO-spinel composite refractories by addition of ZrSiO4

Corrosion behaviours of MgO-spinel-ZrSiO₄ compositions were investigated. The influence of corrosion resistance based on the microstructural changes occurred due to the solubilities of constituents in corroded regions was examined using SEM/EDX analysis. The following observations were determined by microstructural characterisation performed at the interface of clinker-refractory: (i) the formation of ZrO₂ and Mg₂SiO₄ phases among MgO grains after sintering, (ii) the formation of CaZrO₃ phase during penetration, (iii) prevention of penetration by new phases formed making a barrier effect against clinker with an improvement in densification, and (iv) the decrease in the amount of CaO and the increase in the quantity of MgO using EDX analysis made moving from clinker towards refractory. The addition of ZrSiO₄ reduced the values of penetration and spreading areas of the corroded regions of composite refractories and improved the corrosion resistance significantly, leading to a long service life of MgO-spinel-zircon based refractories for industrial applications.     

Shrinkage-free ZrSiO4-ceramics: Characterisation and Applications

Dense, shrinkage-free ZrSiO₄-ceramics can be produced by a reaction-bonding process using ZrSi₂, ZrO₂, and a polysiloxane as starting materials. Sinter shrinkage is compensated by the volume increase during oxidation of ZrSi₂. In addition, the use of a Si-containing so called low-loss-binder [polymethylsilsesquioxane (PMSS)] reduces shrinkage further. Near net-shape ceramic compacts can be produced by an embossing process. As the green bodies are extremely dense but not brittle, they can be mechanically machined. The reaction-bonding process has to be controlled in a way that first the pyrolysis of PMSS, followed by the oxidation of ZrSi₂, and finally the formation of ZrSiO₄ and the sintering to dense compacts take place. Due to the absence of shrinkage and the good mechanical properties obtained, these ceramics open up new applications in fields requiring high dimensional accuracy such as microsystem engineering or dentistry. ©     

3Y—TZP／Al2O3复合粉末的相变研究

采用化学共沉淀法制备了3Y－TZP／Al2O3纳米级复合粉末,研究了Al2O3含量和煅烧温度对粉末的相结构和ZrO2晶格常数的影响。研究表明：800℃,1h煅烧的复合粉末只出现t-ZrO2相,不出现Ａl2O3的任何晶相,当Al2O3质量分数不大于10％时,ZrO2晶格常数随Al2O3含量增加而减小,但当Al2O3质量分数为20％时,ZrO2晶格常数反而略有增大;当温度升至1200℃时,开始形成α-Al2O3,Al2O3对ZrO2晶格常数的影响减小;当温度达到1450℃时,完成了向α－Al2O3的转变,故Al2O3不再对ZrO2晶格常数产生影响。     

Hydrothermal synthesis of precursors for Y-TZP/α-Al2O3 composite

A coprecipitated xerogel having the composition of 90 vol.% zirconia (3 mol% Y₂O₃) and remaining 10 vol.% alumina was crystallized under hydrothermal conditions by adopting conventional (110 °C for 7 days) or microwave (250 °C for 2 h) routes. The hydrothermal treatments of crystallization were performed in the presence of (KOH+K₂CO₃) (K), (CH₃)₄NOH (TMA), or (C₂H₅)₄NOH (TEA) mineralizer solutions at different concentrations.The role of the two different hydrothermal routes, of the nature and the concentration of the mineralizer solution on the thermal behaviour of the resulting powders are discussed.     

Laser surface nanostructuring for reliable Si3N4/Si3N4 and Si3N4/Invar joined components

IR pulsed laser radiation in air was applied to Si₃N₄ and Invar to obtain reliable Si₃N₄/Si₃N₄ and Si₃N₄/Invar adhesive bonded components. The laser pre-treatment produced a homogeneous nanostructured oxide layer on the surfaces, which effectively increased the adhesion at the adhesive/adherends interface and led to cohesive failure in the joining material. The mechanical strength of Si₃N₄/ Si₃N₄ and Si₃N₄/Invar joined components was measured, with and without laser nanostructuring, before and after thermal cycling from room temperature to 50?K, and it resulted that the exposure to extremely low temperatures did not affect the mechanical integrity of the joints. It was also demonstrated that this laser pre-treatment did not alter the mechanical properties of the ceramic substrate.     

Subcritical Crack Growth in Y-TZP and Al2O3-Toughened Y-TZP

Crack velocity curves for Y-TZP and Al₂O₃-toughened Y-TZP were determined for long cracks in compact tension specimens with an in situ fracture device on the stage of an optical microscope. Indications for a crack velocity threshold were found for both materials. Above this threshold, at 2.6 MPa·m^1/2 for Y-TZP and 3.6 MPa·m^1/2 for Al₂O₃-toughened Y-TZP, chemically assisted subcritical crack growth occurs over an extended regime of applied stress intensity factors of width 2.1–2.8 MPa·m^1/2. It is recognized that the dependence of the shielding term on the crack-tip stress field renders transformation-toughened materials particularly susceptible to stress-corrosion cracking. This interrelation leads to the definition of a steady-state velocity at constant applied stress intensity factor. This velocity is obtained in the situation where the shielding term is fully defined by the present crack-tip stress field, not depending on prior loading history.     

High-Pressure Synthesis of Perovskites Pb(Li1/4Nb3/4)O3 and Pb(Li1/4Ta3/4)O3

Complex perovskite-type compounds with the general formula Pb(B⁺_1/4B⁵⁺_3/4)O₃, where B⁺= Li⁺ and B⁵⁺= Nb⁵⁺ or Ta⁵⁺, were synthesized using a high-pressure technique and studied by X-ray powder diffraction. The X-ray patterns were indexed on the basis of a cubic cell with a ₀= 4.071 Å for Pb(Li_1/4 Nb_3/4)O₃ and a ₀= 4.052 Å for Pb(Li_1/4Ta_3/4)O₃. Electrical properties of the new perovskites were also studied.     

稀土固体超强酸SO2-4/TiO2/La3+催化合成醋酸丁酯

研究了以固体超强酸SO24-/TiO2/La3+催化剂,醋酸和正丁醇为原料合成醋酸丁酯,并考察了影响反应的因素.结果表明醇酸物质的摩尔比为1.61、催化剂用量0.6 g、带水剂甲苯7 mL、反应时间2.5 h是最适宜的反应条件,其酯化率可达96.2%.     

Al_2(SO_4)_3/FeCl_3催化合成乙酸异戊酯

以冰乙酸和异戊醇为原料,Al_2(SO_4)_3/FeCl_3为催化剂,对催化合成乙酸异戊酯的条件进行研究。考察催化剂用量、乙酸与异戊醇物质的量比以及反应时间对乙酸酯化率的影响。结果表明,Al_2(SO_4)_3/FeCl_3具有良好的催化活性,在乙酸物质的量为0.1 mol、乙酸与异戊醇物质的量比为1∶4、催化剂用量1.0 g、反应时间2.0 h和带水剂环己烷用量10 m L反应条件下,重复实验3次,平均乙酸酯化率为93.50%。     

Effect of Fe2O3 doping on colour and mechanical properties of Y-TZP ceramics

Yttria-stabilized zirconia (Y-TZP) samples with different Fe concentrations were prepared aiming to study the effects of Fe₂O₃ doping on colour and mechanical properties. Since colour is an important optical property for jewellery and watchmaking, the investigation of colour in zirconia ceramics has a great scientific and technological interest. An investigation of the mechanical and optical properties, specifically the colour, was developed starting from commercial partially yttria-stabilized zirconia (Y-TZP) powders produced by Emulsion Detonation Synthesis (EDS). Within the strategies to get colours, the use of colouring oxides such as iron oxide (Fe₂O₃) was the chosen approach. The addition of specific ions into the ZrO₂ matrix can be used to tune zirconia colour without compromising its outstanding mechanical properties. Doping with iron oxide has proved to be a suitable, reproducible and irreversible colouring mechanism, allowing the development of a chromatically beige stable material with respect to its use in different processing conditions such as different atmospheres and temperature ranges. XRD results suggested that iron ions dissolved into tetragonal zirconia phase are at interstitial positions since the unit-cell volume of the tetragonal zirconia increases with increasing iron content. The effect of dopant addition on the mechanical properties of Y-TZP ceramics was also assessed. Compared to the undoped samples, doped ones exhibit a similar Vickers hardness (>1200?MPa) and biaxial flexural strength (>1000?MPa). However, it was observed that Fe₂O₃ doping slightly decreased the fracture toughness of Y-TZP ceramics.     

可见光增强g-C_3N_4/Co_3O_4胶体催化剂的催化产氢性能

通过水热法制备了具有可见光增产氢高性能的g-C_3N_4/Co_3O_4胶体催化剂,采用XRD、TEM、SEM和EDS等分析样品的组成和形貌结构。催化产氢结果表明,光照条件下g-C_3N_4/Co_3O_4胶体催化剂具有极高的催化产氢活性,TOF值高达58.2 min~(-1),通过拟合温度动力学曲线,得到了催化反应的活化能为15.73 kJ·mol~(-1)。对样品进行UV-vis和PL测试发现,g-C_3N_4/Co_3O_4胶体催化剂具有极高的光能利用率和电子-空穴分离率,并进一步阐述了光能促进催化产氢的作用机理。     

Joining of Si3N4/Si3N4 with in-situ formed Si-Al-Yb oxynitride glasses interlayer

Si₃N₄ ceramic was successfully joined to itself with in-situ formed Yb-Si-Al oxynitride glass interlayer. The joints were composed of three parts: (I) Si₃N₄ matrix, (II) oxynitride glass interlayer in which hexagonal or fine elongated β-sialon grains and a few ball-like β-Si₃N₄ grains exist, and (III) diffusion zone in Si₃N₄ matrix containing a thin dark layer and a ~ 25?µm thick bright layer. The seam owned similar microstructure to matrix and was inosculated with the matrix as a whole. The strength of the joint tended to increase with the increase of bonding temperature and reached the value of 225?MPa, when the joints were prepared at 1600?°C for 30?min under a pressure of 1.5?MPa. The high-temperature strength remained 94.7% and 75.2% of R.T. strength when the joints were tested at 1000?°C and 1200?°C, respectively. It may be contributed to the high softening temperature of the Yb-Si-Al oxynitride glass phase formed in the seam. Even suffered to the air exposure for 10?h at 1200?°C, the residual strength of the joints was still 143?MPa, attributed to the existence of YbAG phase.