Preparation and properties of polyimide nanocomposites with negative thermal expansion nanoparticle filler

Nanocomposites with tunable coefficient of thermal expansion (CTE) were prepared by incorporating cubic zirconium tungstate (ZrW₂O₈) nanoparticles at various volume percentages in a polyimide (PI). Rod-shaped nanoparticles of cubic ZrW₂O₈, which has isotropic negative thermal expansion, were synthesized using a hydrothermal method. The interfacial interaction between the PI and ZrW₂O₈ was enhanced by covalently bonding different organic moieties, including a short aliphatic silane and PI oligomer, to the surface of ZrW₂O₈. Structure–property relationships for the PI–ZrW₂O₈ nanocomposites were investigated for thermal degradation, glass transition, tensile and thermal expansion properties. Addition of ZrW₂O₈ nanoparticles did not alter the thermal degradation and glass transition temperature of the base PI. The addition of ZrW₂O₈ nanoparticles increased the Young's modulus of the polymer, indicating stiffening of the polyimide matrix. The increase was higher for nanocomposites with engineered interfaces due to the efficient load transfer achieved through the presence of linker groups. The addition of ZrW₂O₈ reduced the in-plane CTE of the base PI at all loadings studied. The CTE of the base PI was reduced by around 22% with the addition of ZrW₂O₈ at 15 volume% loading.     

Preparation and thermal properties of zirconium tungstate/copper composites

ZrW₂O₈/Cu composites were prepared by the powder metallurgy method. Electroless plating was used to deposit copper on ZrW₂O₈ powder before sintering. The thermal expansion and thermal conductivity of composites were measured in the temperature range from 25 ^oC to 200 ^oC and compared with those predicted from various theoretical models. The results show that the coefficient of thermal expansion of ZrW₂O₈/ Cu composites with a different volume fraction of ZrW₂O₈ is greater than the theoretically calculated value. The thermal conductivities of ZrW₂O₈/ Cu composites increase with a higher copper content and decrease upon elevated temperature. The thermal conductivity of composites with a different volume fraction of ZrW₂O₈ is lower than the theoretically calculated value.     

ZrW2O8-doped epoxy as low thermal expansion insulating materials for superconducting feeder system

Epoxy resin insulating materials used in superconducting feeder system of fusion device are required to be low thermal expansion coefficient (TEC). In this paper, negative thermal expansion (NTE) material ZrW₂O₈ filled epoxy resins were fabricated. To improve the dispersion of fillers in epoxy matrix, plasma polymerization was performed on the surface of ZrW₂O₈ powders. Transmission electron microscope (TEM) and surface wettability analysis were performed before and after the surface modification of ZrW₂O₈ powders. The TEC of ZrW₂O₈/epoxy composites were measured from 77 K to room temperature. The results show the doping of ZrW₂O₈ can significantly reduce the TEC of epoxy resins. The sedimentation rate of ZrW₂O₈ before and after modified in epoxy was compared by density measurement. It can be seen that the ZrW₂O₈ surface modified by plasma polymerization can enhance its dispersion properties in epoxy matrix.     

The cryogenic thermal expansion and mechanical properties of plasma modified ZrW2O8 reinforced epoxy

In this article, epoxy resin reinforced by negative thermal expansion material, ZrW₂O₈, was fabricated. The surface modification of ZrW₂O₈ particles was performed via plasma enhanced chemical vapor deposition (PECVD) process. As a result, a thin film was uniformly deposited on the surfaces of the ZrW₂O₈ particles, leading to an improvement of compatibility and dispersion of ZrW₂O₈ fillers inside epoxy matrix. Moreover, the coefficients of thermal expansion (CTEs) of the composite material containing 0-40 vol.% fillers were studied under cryogenic temperatures. The results showed a significant reduction in thermal expansion with increasing ZrW₂O₈ content. The cryogenic mechanical properties of ZrW₂O₈/epoxy composites were also investigated, showing the properties were improved by adding ZrW₂O₈ to certain content. In addition, the mechanical strength and modulus of the composite were observed significantly higher at cryogenic temperature than that at room temperature because of the thermal shrink effect and the frozen epoxy matrix.     

Pressureless sintering of negative thermal expansion ZrW2O8/Zr2WP2O12 composites

Negative thermal expansion material ZrW₂O₈/Zr₂WP₂O₁₂ composite was prepared by liquid phase sintering. The apparent density of ZrW₂O₈ without any sintering additive was about 3.7 g/cm³, corresponding to about 73% of its theoretical density. However, the relative density of the samples, sintered with more than 5 mol% P₂O₅ was about 90%. The identified phases were mainly ZrW₂O₈ with small amounts of WO₃, ZrO₂ and Zr₂WP₂O₁₂ by XRD. The intensity of Zr₂WP₂O₁₂ peaks increased with increasing P₂O₅ content. It was surmised that the melting of ZrO₂-P₂O₅ resulted in liquid phase formation, which is then converted to Zr₂WP₂O₁₂ on the final stage of sintering. Therefore, Zr₂WP₂O₁₂ phase was observed at the gap between the ZrW₂O₈ grains and at the triple junctions. The ceramics sintered with 20 mol% P₂O₅ showed a negative thermal expansion coefficient of − 4.0 × 10^− 6 °C^− 1.     

The effect of phase transformation on the thermal expansion property in Al/ZrW2O8 composites

This article studied the effect of phase transformation on the thermal expansion property in Al/ZrW₂O₈ composites. The Al/ZrW₂O₈ composites of low-thermal expansion were fabricated by a squeeze casting method. The coefficient of thermal expansion (CTE) of as-made composites was discovered sharply increased at around 130 °C. The X-ray diffraction (XRD) spectra showed the existence of high-pressure γ-phase in the as-made composites. This high-pressure γ-phase was considered to be induced by the compressive residual stress originated from the thermal mismatch between Al matrix and ZrW₂O₈ particles. The in situ high-temperature XRD and the differential scanning calorimetry technique were used to study this thermally expanded abruption phenomenon. It was found that the phase transformation from high-pressure γ-phase to the low-pressure phases (α/β phase) in the composites should be responsible for fluctuation in the CTE of composites. Furthermore, using a proper heat treatment to eliminate the high-pressure phase in the composite, the Al/ZrW₂O₈ composites of low and uniform CTE (from 20 to 200 °C) could be achieved. And when temperature increased again, the thermal mismatch stresses between the metal matrix and ceramic particles in the composite were not large enough to re-induce the α-γ transformation.     

Hydration of ZrW2O8 nanopowders under ambient conditions

We report hydration of ZrW₂O₈ under ambient conditions, and its effect on the negative coefficient of thermal expansion (CTE) of ZrW₂O₈. On storing under ambient conditions for six months, about 66% of the outer annular volume was hydrated to ZrW₂O₈·0.35H₂O, while after one year of storage the sample was hydrated to ZrW₂O₈·0.72H₂O. The CTE was determined by in-situ high temperature X-ray diffraction (XRD) measurements in the temperature range from 25 to 200 °C. XPS and TGA were used to characterize the nature of bonding of the water in the ZrW₂O₈ structure. The negative CTE behavior of partially hydrated ZrW₂O₈·0.35H₂O remained intact, while on further hydration to ZrW₂O₈·0.72H₂O, negative CTE was not observed. The bonding of the water molecules to the ZrW₂O₈ structure were stronger than those of adsorbed water molecules but weaker than those of the structural hydroxyl ions.     

Low-temperature sintering of ZrW2O8–SiO2 by spark plasma sintering

Amorphous ZrW₂O₈ powder and amorphous SiO₂ powder were prepared by a sol–gel process as raw materials, and high-density ZrW₂O₈–SiO₂ were successfully prepared at a much lower temperature of 923 K for a much shorter holding time of 10 min by spark plasma sintering (SPS) method rather than by conventional melt-quenching method. The relative densities of 0.85ZrW₂O₈–0.15SiO₂ and 0.70ZrW₂O₈–0.30SiO₂ were 99.4% and 96.6%, respectively. The combined technique of a sol–gel process and SPS should enable us to prepare the varied types of high-density composites of ZrW₂O₈ without severe thermal cracking caused by melt-quenching. The thermal expansion properties and dielectric properties of ZrW₂O₈–SiO₂ were also investigated.     

Preparation of ZrO2-Al2O3 powder by thermal decomposition of gels produced from an aluminium chelate compound and zirconium butoxide

Organic precursors containing Al and Zr atoms were synthesized from an aluminium chelate compound and zirconium n-butoxide. A ZrO₂-Al₂O₃ composite powder was prepared by the thermal decomposition of these precursors. An amorphous phase exists to higher temperatures for this ZrO₂-Al₂O₃ powder than for a comparable powder prepared from aluminium sec-butoxide and zirconium n-butoxide. In addition the tetragonal ZrO₂ phase was stabler in this ZrO₂-Al₂O₃ powder than in a comparison powder. The ZrO₂ grains were 50–500 nm in diameter and were homogeneously dispersed in the Al₂O₃ matrix after heating at 1400 °C.     

Negative thermal expansion materials

Materials exhibiting negative thermal expansion through room temperature have a variety of applications, mainly in controlling the overall thermal expansion of various composites. Several materials showing negative thermal expansion have recently been identified. The most dramatic of such behavior is exhibited by ZrW₂O₈, which shows strong isotropic thermal expansion from 20 to 425K.     

Synthesis and characterization of hollow glass–ceramics microspheres

Hollow glass–ceramics microspheres (HGCMs), with the diameter from 10 to 60 μm and the shell thickness less than 2 μm, were successfully fabricated by a simple technique using polyacrylamide microspheres (PAMs) as template. The corresponding HGCM were obtained after a thermal treatment of the core–shell microspheres, which were synthesized with organic template method. The size, morphology and phase composition of synthesized products were determined via XRD, SEM, TGA. The effects of the amount of glass powder, the Hydrophile–Lipophile Balance (HLB) value, the sintering temperature, and the ratio of pre-adsorbed water to the water in the slurry on the morphologies of HGCM have been investigated. The results showed that the agglomeration of HGCM can be reduced by adjusting the HLB value. In addition, the amount of solid beads decreases obviously by reducing ratios and adjusting the HLB value. As the sintering temperature increases, the surface of the HGCM becomes smooth and compact. Meanwhile, computational investigations are carried out to better understand the strengthen effect of taking glass–ceramics materials in the system MgO–Al₂O₃–SiO₂ (MAS) as shell materials.     

Influence of fabrication method on the structure and thermal expansion property of ZrWMoO<Subscript>8</Subscript> and its composites

We report the preparation of ZrWMoO₈ and its composites with near zero thermal expansion property using in situ solid state reaction or co-precipitation route. The aim of this study is to compare the influence of the fabrication method on the structure and thermal expansion of the products. The composition of the obtained powders was characterized by X-ray diffraction (XRD) and thermal expansion property was investigated by Thermomechanical Analysis (TMA), respectively. The results indicate that the room temperature structure of ZrWMoO₈ depends on the preparation method, and the structure can be indexed either as α-ZrW₂O₈ structure for in situ solid state reaction or as β-ZrW₂O₈ structure for co-precipitation route. The participation of the water in the reaction process was speculated to be the reason for the difference structure. However, no matter which structure ZrWMoO₈ adopted, it exhibits excellent negative thermal expansion property and can be utilized to decrease the thermal expansion of ZrO₂. The only difference is that the CTE curves of ZrO₂ composites with ZrWMoO₈ adopting α-ZrW₂O₈ structure have a discontinuity at about 150 °C due to the α–β phase transition while the other curves are linear.     

Synthesis and thermal properties of zirconium ortho- and diphosphates

Single phase sodium zirconium orthophosphates [NaZr₂(PO₄)₃ and Na₂Zr(PO₄)₂] and zirconium diphosphate (ZrP₂O₇) have been synthesized. The measured linear expansion coefficient of NaZr₂(PO₄)₃ was rather small only reaching a value of 0.16% at 800 °C. The expansion coefficients of Na₂Zr(PO₄)₂ and ZrP₂O₇ were also rather small and were 1.4 and 1.0% respectively at 800 °C. These phosphates showed different behaviour depending on the type of thermal pretreatment before the measurement of the expansion coefficient.     

Development of ZrO2 interfaces for all-oxide composites

Development of a fibre coating process based on sol–gel synthesis for depositing ZrO₂ interfacial layers on Al₂O₃ fibres is described. The sol employed exhibited a shear-thinning behaviour and was used to infiltrate Nextel™ 610 fibre tows, forming minicomposites that are used as reinforcements in glass matrix composites (GMCs). The sol–gel method was investigated with respect to the rheological properties of the sol, and the thermal and sintering behaviour of the deposit. The dip-coating method was then optimised and uniformly coated fibres were obtained, which exhibited sufficient retained tensile strength (>50%) to be used as reinforcement in GMCs.     

Structural model for thermal expansion in MZr2P3O12 (M=Li,Na, K,Rb, Cs)

A structural model is proposed to describe the highly anisotropic thermal expansion in the sodium zirconium phosphate NaZr₂P₃O₁₂ structure as a result of the thermal motion of the polyhedra in the structure. In the proposed model the rotations of the phosphate tetrahedra are coupled to the rotation of the zirconium octahedra. Of the two versions considered, the first one allows angular distortions to occur only in the ZrO₆ octahedra; the second one permits all polyhedra to be distorted.     

Properties of nanostructured oxide formed during oxidation of a zirconium wire by supercritical water

Synthesis of ZrO₂ during oxidation of a zirconium wire by supercritical water at P = 25 MPa and T = 500 or 525°C has been investigated. It is established that an inhomogeneous nanostructured ZrO₂ layer is formed as a result of oxidation. Rate of 〈Zr〉 oxidation, oxide porosity and morphology, and average size and structure of crystallites are determined. The thermal conductivity of the synthesized ZrO₂ layer in supercritical water and in nitrogen is determined by pulsed electric heating of a partially oxidized wire. The low values of thermal conductivity (about 0.2 W/(m K)) correspond to a layered structure of porous material, with layers orientated parallel to the oxidized-metal surface.     

Synthesis of monodisperse spherical nanometer ZrO2 (Y2O3) powders via the coupling route of w/o emulsion with urea homogenous precipitation

Using xylol as the oil phase, span-80 as the surfactant, and an aqueous solution containing zirconium (3 mol% Y₂O₃) and urea as the water phase, tetragonal phase ZrO₂ nano-powders have been prepared via the coupling route of w/o emulsion with urea homogenous precipitation. The effects of the zirconium concentration, the reaction temperature and the urea content on the average size of the products have been examined. The as-prepared ZrO₂ powders and the precursor powders were characterized by TGA–DTA, XRD, TEM and BET. Experimental results indicate that ZrO₂ powders prepared via the coupling route of w/o emulsion with urea homogenous precipitation possess some excellent characteristics, such as well-rounded spherical shape and excellent dispersing.     

Infrared and microwave absorbing properties of BaTiO3/polyaniline and BaFe12O19/polyaniline composites

BaTiO₃/polyaniline and BaFe₁₂O₁₉/polyaniline composites were synthesized by in situ polymerization and introduced into epoxy resin and polyethylene to be microwave and infrared absorber. The spectroscopic characterizations of the formation processes of BaTiO₃/polyaniline and BaFe₁₂O₁₉/polyaniline composites were examined using Fourier transform infrared, ultraviolet–visible spectrophotometer, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and electron spin resonance. Microwave absorbing properties were investigated by measuring reflection loss in the 2–18 and 18–40 GHz microwave frequency range using the free space method. The thermal extinction measurements in the 3–5 and 8–12 μm were done to evaluate the shielding affectivity of infrared. The results showed that the BaTiO₃/polyaniline and BaFe₁₂O₁₉/polyaniline composites have good compatible dielectric and magnetic properties and hence the microwave absorbency show broad frequencies absorbing properties. Moreover, the infrared thermal image testing that the detecting ability of infrared thermal imaging was decreased when the BaFe₁₂O₁₉ and BaTiO₃ was coating with polyaniline.     

Investigation on the phase stability,sintering and thermal conductivity of Sc2O3–Y2O3–ZrO2 for thermal barrier coating application

Rare earth oxides co-doped zirconia have been developed for application in thermal barrier coating systems to promote the performance and durability of gas turbines. 8 mol%Sc₂O₃, 0.6 mol%Y₂O₃–stabilized ZrO₂ (ScYSZ) powder was synthesized by chemical co-precipitation method. The phase stability, sintering resistance and thermo-physical properties of ScYSZ and 8 wt%Y₂O₃ stabilized ZrO₂ (8YSZ) were investigated. The results indicated that both ScYSZ and 8YSZ show single tetragonal phase before heat treatment. After heat treating at 1500 °C for 300 h, ScYSZ exhibits excellent phase stability with 100% metastable tetragonal (t′) phase, whereas the content of monoclinic phase in 8YSZ reached 49.4 mol%. ScYSZ also exhibits higher sintering resistance and lower thermal conductivity than 8YSZ. ScYSZ can be considered to be explored as candidate material for TBC application.     

Preparation and properties of binary oxide bioceramics

Candidate inert bioceramics based on Al₂O₃ and ZrO₂ and on SiO₂-TiO₂ were prepared via slip casting and sol-gel/hot-pressing techniques, respectively. Their properties relevant to applicability in biomedicine-microstructure, microhardness and coefficient of thermal expansion-were determined. The affinity of the oxides with body-liquids was evaluated by wetting experiments at 37°C. High-quality materials were achieved due to the advantages offered by the preparation techniques employed. The Al₂O₃ and ZrO₂ based ceramics have high hardness, a constant coefficient of thermal expansion within a wide temperature range and low adhesion with biological liquids. The SiO₂-TiO₂ samples, the crystallinity of which depends on the preparation conditions, have lower hardness and lower coefficient of thermal expansion, which in the case of crystalline samples considerably changes at low temperatures, and display good affinity with biological liquids, strongly affected by the presence of glassy phase in the oxide.