Avoiding the intermediate phase Zr2WP2O12 to develop a larger-negative-thermal-expansion-coefficient material Zr2W2P2O15

Zr₂W₂P₂O₁₅ with larger-negative-thermal-expansion-coefficient (NTEC, −4.01×10⁻⁶ K⁻¹, 143–673 K) is developed by controlling reaction process to avoid the intermediate phase of Zr₂WP₂O₁₂, whose smaller NTEC limits its application. From 1473 K to1573 K, Zr₂WP₂O₁₂ forms easily but the reaction between Zr₂WP₂O₁₂ and WO₃ to generate Zr₂W₂P₂O₁₅ is difficult even followed by heating up to 1673 K. However, putting raw materials of ZrO₂, WO₃ and NH₄H₂PO₄ into a pipe furnace at sintering temperature 1673 K directly, the Zr₂W₂P₂O₁₅ is formed avoiding the intermediate phase of Zr₂WP₂O₁₂.     

Electrical and thermal properties of SiC-Zr2CN composites sintered with Y2O3-Sc2O3 additives

SiC-Zr₂CN composites were fabricated from β-SiC and ZrN powders with 2 vol% equimolar Y₂O₃-Sc₂O₃ additives via conventional hot pressing at 2000 °C for 3 h in a nitrogen atmosphere. The electrical and thermal properties of the SiC-Zr₂CN composites were investigated as a function of initial ZrN content. Relative densities above 98% were obtained for all samples. The electrical conductivity of Zr₂CN composites increased continuously from 3.8 × 10³ (Ωm)⁻¹ to 2.3 × 10⁵ (Ωm)⁻¹ with increasing ZrN content from 0 to 35 vol%. In contrast, the thermal conductivity of the composites decreased from 200 W/mK to 81 W/mK with increasing ZrN content from 0 to 35 vol%. Typical electrical and thermal conductivity values of the SiC-Zr₂CN composites fabricated from a SiC-10 vol% ZrN mixture were 2.6 × 10⁴ (Ωm)⁻¹ and 168 W/m K, respectively.     

The influence of nano alumina additions on the coefficient of thermal expansion of a LZS glass–ceramic composition

In this work a 19.58Li₂O·11.10ZrO₂·69.32SiO₂ (mol%) glass–ceramic matrix was prepared and milled in order to determine its coefficient of thermal expansion (CTE) and to study how it is influenced by the addition of nanosized Al₂O₃ particles (1–5 vol%) and submicrometric Al₂O₃ particles (5 vol%). Comminution studies from the LZS parent glass frit showed that a powder with an adequate particle size (3.5 µm) is achieved after 120 min of dry milling followed by a second step of 60 h wet milling. The obtained LZS glass–ceramic samples (fired at 900 °C/30 min) showed an average relative density of ∼98% with zirconium silicate and lithium disilicate as main crystalline phases. Prepared composites with 1, 2.5 and 5 vol% of nanosized Al₂O₃ and 5 vol% submicrometric Al₂O₃ showed average relative densities varying from 97% to 94% as the alumina content increased. The formation of β-spodumene in the obtained composites leads to reduce the CTEs, whose values ranged from 9.5 to 4.4×10⁻⁶ °C⁻¹. Composites with 5% nanosized alumina showed a CTE lower than that of the equivalent formulation with submicrometric alumina.     

Preparation of zirconium tungstate (ZrW2O8) by the amorphous citrate process

Citrate gels were prepared from aqueous solutions of zirconium dinitrate oxide, ammonium metatungstate and citric acid in the range of pH 1.1–10. The citric acid to total metal molar ratio was varied from 0.66 to 1.33. The thermal decomposition of these precursor gels and the oxide phase evolution were studied by thermal analysis (DTA, TG, EGA-MS) and X-ray powder diffraction (XRD), respectively. Complex stability appeared to be largely insensitive to pH variations, whereas consistency and thermal behaviour of the gels were found to be influenced by pH and molar ratio of the starting solutions. Under optimized thermolysis conditions, phase-pure trigonal ZrW₂O₈ was obtained which was converted to cubic ZrW₂O₈ by up-quenching to 1125 °C for a very short time (<1 min). The cubic ZrW₂O₈ has particle sizes in the lower micron to submicron range and shows a negative linear coefficient of thermal expansion CTE = ?10.2 × 10^?6 K^?1 (30–120 °C). The polymorphism of ZrW₂O₈ is discussed with respect to its preparation by the amorphous citrate process and other soft chemistry routes.     

Effect of sintering temperature on the thermal expansion behavior of ZrMgMo3O12p/2024Al composite

A 2024Al metal matrix composite with 10?vol% negative expansion ceramic ZrMgMo₃O₁₂ was fabricated by vacuum hot pressing, and the influence of sintering temperature on the microstructure and thermal expansion coefficient (CTE) of alloys was investigated. Experimental results showed that all ZrMgMo₃O_12p/2024Al composites sintered at 500–530?°C had a similar reticular structure and exhibited different linear expansion coefficients at 40–150?°C and 150–300?°C. The addition of 10?vol% ZrMgMo₃O₁₂ decreased the CTEs of 2024Al by ~ 16% at 40–150?°C and by ~ 7% at 150–300?°C. This addition also increased the hardness of 2024Al by ~ 23%. The density of the composites and the content of Al₂Cu in ZrMgMo₃O_12p/2024Al increased as the sintering temperature increased. The CTEs of the composites decreased, whereas hardness increased. Thermal cycling from 40?°C to 300?°C caused the CTEs of the composites to decrease gradually and reach a stable value after seven cycles. The lowest CTEs of 15.4?×?10^?6 °C^?1 at 40–150?°C and 20.1?×?10^?6 °C^?1 at 150–300?°C were obtained after 10 thermal cycles and were reduced by ~ 32% and ~ 17%, respectively, compared with the CTE of the 2024Al. Among the current reinforcements, ZrMgMo₃O₁₂ negative expansion ceramics showed the highest efficiency to decrease the CTE of Al matrix composites.     

Effect of CaO addition on the structure and electrical conductivity of the pyrochlore-type GdSmZr2O7

Polycrystalline GdSm_1?xCa_xZr₂O_7?x/2 (0 ≤ x ≤ 0.20) ceramics have been prepared by the solid-state reaction method. The effects of CaO addition on the microstructure and electrical properties of the pyrochlore-type GdSmZr₂O₇ ceramic were investigated. GdSm_1?xCa_xZr₂O_7?x/2 (x ≤ 0.05) ceramics exhibit a pyrochlore-type structure; however, GdSm_1?xCa_xZr₂O_7?x/2 (0.10 ≤ x ≤ 0.20) ceramics consist of the pyrochlore-type structure and a small amount of CaZrO₃. The total conductivity of GdSm_1?xCa_xZr₂O_7?x/2 ceramics follows the Arrhenius relation, and gradually increases with increasing temperature from 723 to 1173 K. GdSm_1?xCa_xZr₂O_7?x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10^?4–1.0 atm at each test temperature. The highest total conductivity is about 1.20 × 10^?2 S cm^?1 at 1173 K for the GdSm_0.9Ca_0.1Zr₂O_6.95 ceramic.     

Preparation and mechanical performance of ductile Csf/Al2O3–BN composites—Part 2: Effects of fiber contents and ablation properties

In this paper, ductile C_sf/Al₂O₃–BN composites were prepared by hot-press sintering. Effects of fiber contents on the mechanical performance and ablation behavior of the composites were investigated systematically. The results showed that all the composites fractured in non-brittle failure mode, exhibiting elastic region and non-linear region as shown in load–displacement curves. With the increase in fiber contents from 15 to 30 vol%, mechanical properties of the obtained composites first increased, reached the peak values at fiber content of 25 vol% and then decreased. When being exposed to high-speed oxyacetylene combustion flow for 60 s, the composites with fiber contents of 15, 20 and 25 vol% showed comparable ablation property and mass loss of 5.3, 7.2 and 8.4×10⁻⁴ g/s, respectively. The ablation mechanisms include fiber and ceramic matrix oxidation, decomposition of mullite, evaporation of both B₂O₃ and SiO₂, and mechanical exfoliation.     

Thermal expansion and mechanical properties of phenolic resin/ZrW2O8 composites

Phenolic resin/ZrW₂O₈ composites were successfully fabricated and their coefficient of thermal expansion (CTE) as well as mechanical properties was investigated. The CTE of the composites decreases from 46 × 10^–6 to 14 × 10^–6 K^?1 when the ZrW₂O₈ volume fraction increases from 0 to 52 vol %. The CTE of the composites is analyzed by some theoretical models; Schapery's upper bound provides the best estimate of the reduction in CTE. The Barcol hardness of the composites increases with an increase in the ZrW₂O₈ volume fraction. The bending strength of the composites with 19–25 vol % of ZrW₂O₈ fillers shows a maximum value of 130 MPa, which is 45% larger than that of phenolic resin without fillers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering

Electroconductive ZrO₂–Al₂O₃–25 vol% TiN ceramic nanocomposites were prepared by spark plasma sintering at 1200 °C for 3 min. The electrical resistivity of the composites decreased from 4.5 × 10^?4 Ω m to 3 × 10^?5 Ω m as the Al₂O₃ content in the ZrO₂–Al₂O₃ matrix increased from 0 to 100 vol%. SEM images graphically presented the microstructural evolution of the composites and a geometrical percolation model was applied to investigate the relationship between the electrical property and the microstructure. The results indicated that the addition of Al₂O₃ to ZrO₂–TiN improved the electrical conductivity of the material by tailoring the structure from “nano–nano” type for ZrO₂–TiN to “micro–nano” type for ZrO₂–Al₂O₃–TiN.     

Novel synthesis and thermal property analysis of MgO–Nd2Zr2O7 composite

MgO-Nd₂Zr₂O₇composites with ratios of 50–70 vol% MgO were produced via a one-pot combustion synthesis. A suite of characterization techniques, including X-ray diffraction, scanning and transmission electron microscopy were employed to investigate the structural properties while dilatometry, simultaneous thermal analysis and laser flash analysis were used to characterize the thermal properties of the composites. Dense pellets were produced after sintering at 1400 °C with grain sizes between 200 and 500 nm for both phases. The thermal properties of the composites are similar to those produced using standard methods. The composite with 70 vol% MgO was found to have the highest thermal conductivity below 1000 °C, while above this temperature the thermal conductivity was found to be similar and independent of MgO content. This novel synthesis route produces materials which show significant improvements in homogeneity with smaller particle sizes when compared to current standard synthesis techniques without significantly reducing thermal conductivity.     

Properties of polyphenylene sulphide/glass fibre composites with negative thermal expansion ceramic fillers

The high coefficient of thermal expansion (CTE) of polymeric composites can cause large deformation under temperature changes, affecting coupling with devices made of other materials in radio frequency (RF) communication systems and limiting their application in RF systems. In order to obtain polyphenylene sulphide (PPS)-based composites with low CTE, a series of PPS-based composites containing different loadings of ceramic powders (including Zr₂WP₂O₁₂, BN, AlN, Al₂O₃) were fabricated by melt extrusion method using PPS with 40 wt% glass fibre (GF) as matrix material. The experimental results showed that the PPS composites with Zr₂WP₂O₁₂ (ZWP) as a filler had a lower CTE compared to the samples with other fillers at the same filler loading. The CTE of PPS/GF/ZWP steadily decreased with increasing ZWP addition. At 20 vol% ZWP loading, a 67% (about 18 ppm/°C) reduction of CTE compared to the PPS/GF was achieved. The addition of ZWP powder to PPS/GF also led to an improvement in the dielectric loss of the composite. When the ZWP content is 20 vol%, the dielectric loss of the composites is about 0.0035, which is 24.4% lower than PPS/GF. Hence, the PPS/GF/ZWP composites have great potential for applications in RF communication systems.     

Preparation and properties of B6O/TiB2-composites

B₆O/TiB₂ composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900 °C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B₆O material showed that the addition of TiB₂ in a non-reactive sintering route promotes the B₆O densification. Further improvement was obtained by sintering reactive B–TiO₂ mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0 MPa m^1/2 (SEVNB) and 2.6–5.0 MPa m^1/2 (IF method) while simultaneously a high hardness of up to 36 GPa (HV_0.4) and 28 GPa (HV₅) could be preserved. The high temperature properties at 1000 °C of hardness, thermal conductivity and CTE were up to 20 GPa, 18 W/mK and 6.63 × 10^?6/K, respectively.     

Investigation of thermal expansion of 3D-stitched C–SiC composites

Carbon fiber reinforced silicon carbide (C–SiC) composites are promising materials for a severe thermo-erosive environment. 3D-stitched C–SiC composites were fabricated using liquid silicon infiltration. The infiltration was carried out at 1450–1650 °C for 10–120 min in vacuum. Coefficient of thermal expansion (CTE) of the composites was determined in in-plane and through-thickness directions in the temperature range from room temperature to 1050 °C. The in-plane CTE varies in the range (0.5–2) × 10^?6/°C, while that in the through-thickness direction, it varies in the range (1.5–4) × 10^?6/°C. The effect of siliconization conditions is higher in the through-thickness direction than in the in-plane direction. The CTE values are lower than the values reported for chemical vapor impregnation based 3D C–SiC composites. An extensive microstructure study was also carried out to understand the thermal expansion behavior of the composites. It was found out that CTE behavior is closely related to the composition of the composite which in turn depends upon siliconization conditions. The best conditions were 1650 °C and 120 min.     

Tailored thermal expansion and electrical properties of α-Cu2V2O7/Al

α-Cu₂V₂O₇/Al composites (with 5–80 wt% of Al) were prepared by a solid state method. Their structural stability, thermal expansion, hardness and electrical properties were studied in detail. The coefficient of thermal expansion (CTE) and hardness of α-Cu₂V₂O₇/Al composites sample can be tailored with the content of Al. The CTE is only 0.49×10⁻⁶ K⁻¹ (RT–780 K) when the Al content is 10 wt%, which is near-zero thermal expansion. The electrical conductivity of α-Cu₂V₂O₇/Al composites increases with increasing the content of Al. When the content of Al is larger than 40 wt%, the α-Cu₂V₂O₇/Al composites exhibit excellent electrical conductivity, which can be mainly attributed to the conductive percolation phenomena of Al in the α-Cu₂V₂O₇/Al composites.     

Effects of Bi2O3–Li2CO3 additions on dielectric and pyroelectric properties of Mn doped Pb(Zr0.9Ti0.1)O3 thick films

Pb(Zr_0.9Ti_0.1)O₃ pyroelectric thick films adding various amounts of the sintering aids Bi₂O₃–Li₂CO₃ have been deposited on the substrates Al₂O₃ by the screen-printing process, and the dependence of microstructure, dielectric and pyroelectric properties on the content of sintering aids has been studied. When the amount of Bi₂O₃–Li₂CO₃ increases from 0 wt% to 5.4 wt%, the sintering temperature of the thick films decreases from 1100 °C to 900 °C, and the grain size and the lattice constant decrease either, but the density and the dielectric constant increase. The Pb(Zr_0.9Ti_0.1)O₃ thick film with 5.4 wt% of Bi₂O₃–Li₂CO₃ sintered at 900 °C has the maximum pyroelectric coefficient 10.51×10^?8 C/cm^?2 K^?1 and the highest figure-of-merit 10.58×10^?5 Pa^?0.5.     

High resolution optical microprobe investigation of surface grinding stresses in Al2O3 and Al2O3/SiC nanocomposites

It has previously been suggested that Al₂O₃/SiC nanocomposites develop higher surface residual stresses than Al₂O₃ on grinding and polishing. In this work, high spatial resolution measurements of residual stresses in ground surfaces of alumina and nanocomposites were made by Cr³⁺ fluorescence microspectroscopy. The residual stresses from grinding were highly inhomogeneous in alumina and 2 vol.% SiC nanocomposites, with stresses ranging from ～ ?2 GPa within the plastically deformed surface layers to ～ +0.8 GPa in the material beneath them. Out of plane tensile stresses were also present. The stresses were much more uniform in 5 and 10 vol% SiC nanocomposites; no significant tensile stresses were present and the compressive stresses in the surface were ～ ?2.7 GPa. The depth and extent of plastic deformation were similar in all the materials (depth ～ 0.7–0.85 μm); the greater uniformity and compressive stress in the nanocomposites with 5 and 10 vol% SiC was primarily a consequence of the lack of surface fracture and pullout during grinding. The results help to explain the improved strength and resistance to severe wear of the nanocomposites.     

Synthesis and thermal expansion behaviors of spin-coated Sc2Mo3O12 thin films

Orthorhombic Sc₂Mo₃O₁₂ films have been successfully prepared via spin coating technique followed by annealing at 500–750 °C. The phase composition, microstructure, morphology and negative thermal behavior of the synthesized Sc₂Mo₃O₁₂ films were investigated. XRD and XPS analysis indicate that as-deposited film is amorphous. Orthorhombic Sc₂Mo₃O₁₂ films can be prepared after post-annealing at 500–750 °C for 1 h. The crystallinity of Sc₂Mo₃O₁₂ films gradually improved with the increase of post-annealing temperature. SEM analysis shows as-deposited film is smooth and compact, and the grain size of Sc₂Mo₃O₁₂ film grows up as the post-annealing temperature increases. Variable temperature XRD analysis demonstrates that the synthesized orthorhombic Sc₂Mo₃O₁₂ films show stable thermo-chemical and anisotropic NTE property in 25–700 °C. The corresponding coefficients of thermal expansion (CTEs) of the orthorhombic Sc₂Mo₃O₁₂ film in a, b and c directions are ?6.68 × 10^?6 °C^?1, 5.08 × 10^?6 °C^?1 and ?4.76 × 10^?6 °C^?1, respectively. The whole unit cell of the orthorhombic Sc₂Mo₃O₁₂ film shrinks and the volumetric CTE of the Sc₂Mo₃O₁₂ thin film is ?6.36 × 10^?6 °C^?1, and the linear CTE is about ?2.12 × 10^?6 °C^?1 (α_v = 3α_l).     

Effect of high alumina cement on permeability and structure properties of ZrO2 composites

Porous ZrO₂ based ceramics are widely used for filtration/separation processes due to the good chemical and thermal stability. For these applications it is desirable that the material have a controlled porous structure in order to obtain good permeability. In this study Ca stabilized ZrO₂ composites were developed from a starting mixture of pure ZrO₂ containing different mole proportions of calcium aluminate cement. Ceramics disks were uniaxially pressed and subsequently sintered at 1300–1450 °C. The influence of process parameters such as chemical compositions and sintering temperature on textural characteristics (volume fraction of pores, pore size distribution) and permeability was followed by apparent density measurements, Hg porosimetry and N₂ permeation, respectively. Sintered microstructure was examined by scanning electron microscopy SEM. The XRD analysis showed that m-ZrO₂ transformed to tetragonal and/or cubic ZrO₂, these phases probably coexisted at relatively low CaO addition. For 30 mol% addition, amount of the cubic Ca_0.15Zr_0.85O_1.85 phase appreciably increased. At 50 mol% CaO, CA₂ was the major phase of the composite with minor CaZrO₃ formation whereas relative content c-ZrO₂ is slightly reduced.The composites had 30–40 vol% porosity with typical pore radius of 1–1.3 μm and the corresponding Darcian permeability k₁ values varied between 2 and 4 × 10^?14 m², such structure parameters slightly increased for high cement addition. The k₁ of ceramics produced from 50 mol% CaO composition remained nearly constant up to 1450 °C due to similar densification degree. The experimental permeability dependence on pore structure parameters as well as the comparison with the value estimated by Ergun's equation are showed.     

High-strength ZrB2-based ceramics prepared by reactive pulsed electric current sintering of ZrB2–ZrH2 powders

Fully densified ZrB₂-based ceramic composites were produced by reactive pulsed electric current sintering (PECS) of ZrB₂–ZrH₂ powders within a total thermal cycle time of only 35 min. The composition of the final composite was directly influenced by the initial ZrH₂ content in the starting powder batch. With increasing ZrH₂ content, ZrB₂–ZrO₂, ZrB₂–ZrB–ZrO₂ and ZrB₂–ZrB–Zr₃O composites were obtained. The ZrB₂–ZrB–ZrO₂ composite derived from a 9.8 wt% ZrH₂ starting powder exhibited an excellent flexural strength of 1382 MPa combined with a Vickers hardness of 17.1 GPa and a fracture toughness of 5.0 MPa m^1/2. The high strength was attributed to a fine grain size and the removal of B₂O₃ through reaction with Zr. Higher ZrH₂ content starting powders were densified through solution-reprecipitation resulting in the formation of coarser angular ZrB₂–ZrB composites with a Zr₃O grain boundary phase with a fracture toughness of 5.0 MPa m^1/2 and an acceptable strength in the 852–939 MPa range.     

High Li-ion conductivity of Al-doped Li7La3Zr2O12 synthesized by solid-state reaction

Li₇La₃Zr₂O₁₂ (LLZO) has cubic garnet type structure and is a promising solid electrolyte for next-generation Li-ion batteries. In this work, Al-doped LLZO was prepared via conventional solid-state reaction. The effects of sintering temperature and Al doping content on the structure and Li-ion conductivity of LLZO were investigated. The phase composition of the products was confirmed to be cubic LLZO via XRD. The morphology and chemical composition of calcined powders were investigated with SEM, EDS, and TEM. The Li-ion conductivity was measured by AC impedance. The results indicated the optimum sintering temperature range is 800–950 °C, the appropriate molar ratio of LiOH·H₂O, La(OH)₃, ZrO₂ and Al₂O₃ is 7.7:3:2:(0.2–0.4), and the Li-ion conductivity of LLZO sintered at 900 °C with 0.3 mol of Al-doped was 2.11×10⁻⁴ S cm⁻¹ at 25 °C.