Papers by Author: Yu Zhou

Abstract: Chromium powder reinforced geopolymer composite (Cr/geopolymer) was prepared in order to enhance its thermal conductivity and mechanical properties. The phase composition, microstructure and mechanical properties of Cr/geopolymer before and after heat treatment at 900, 1000, 1100 and 1200°C were investigated by the X–ray diffraction (XRD), scanning electron microscopy (SEM) and three–point bending test. With increasing heat treatment temperature from 900 to 1100°C, mechanical property of Cr/geopolymer increased gradually and at 1100°C flexural strength got the peak value, which was 325% higher than that of specimens without heat treatment. Cr/geopolymer treated at 900°C did not completely transform into crystalline phase, and many micro cracks around the chromium particles were observed. Cr/geopolymer treated at 1000–1100°C completely crystallized into leucite phase, and the metal chromium remained its original state. Meanwhile, most of the micro cracks were closed. When the temperature further increased to 1200°C, many visible defects were observed in Cr/geopolymer, and chromium oxide appeared in the interface of Cr/geopolymer, which had detrimental effect on the heat conduction and mechanical property of the composite.

Abstract: Nd--sialons with the stoichiometric composition and extra 2wt.% addition of Nd2O3 in Nd0.333Si10Al-2ON15 were synthesized by hot-press sintering. The longtime oxidation resistance of the two ceramics at high temperature was investigated. The oxidation process is very similar, i.e. the weight gain increases linearly at primary stage, and then tends to be constant with prolonging the oxidation time. The oxidation kinetic obeys the parabolic rate law. Inward diffusion of oxygen into ceramics and outward diffusion of metal cations is the main oxidation mechanism. Due to the variation of the content of the added rare earth oxide, the oxidation performance of the two Nd--sialon ceramics are also different.

Abstract: Mineralization is a powerful approach for fine inorganic minerals and organic/inorganic complexs with novel structure. The morphology of composites show the mineralized magnetite dispersed in chitosan hydrogel uniformly. The reason for uniform dispersion might be that the chitosan hydrogel restricted the moving ability of iron ions and the iron ions can only move to the nearest crystal nucleus. The magnetite content of magnetite/chitosan composite was approximately 12.6%. The mineralized magnetite have a significant interaction with chitosan which induced the magnetite/chitosan composite has a higher decomposition temperature than chitosan. After decomposition, the magnetite transformed into γ-Fe2O3 and α-Fe2O3, and a significant crystal growth happened.

Abstract: Polymers with negative charge groups, such as carboxylic or phosphatic groups, were frequently used to induce or promote the apatite deposition. However, chitosan with potentially chelated calcium ions, were ignored. Inspired by layer by layer technology, chitosan hydrogel without any surface modification process was used as framework to mineralize bone-like apatite. XRD and IR results shown that in situ synthesis bone-like apatite, similar to apatite in rib of rabbit, was carbonate ions partially substituted apatite and preferred growth orientation in direction of their c-axis. Bone-like apatite with nano-flake shape with size of 1um fully covered chitosan in short period of time. In case of chelation effect of amino groups, chitosan hydrogel provided heterogeneous apatite nucleation sites, which supported by fact that bone-like apatite tended to deposit on porous wall of chitosan, rather than grow in form of free-standing.

Abstract: The present investigation reports on the preparation of BiFeO3 nanopowders by a sol-gel method using acetylacetone as a stabilizer. Single-phase BiFeO3 nanopowders without any impurity or amorphous phases were obtained when the precursor was thermal treated at temperatures as low as 400 oC for 2 h. Acetylacetone (acac) plays an important role on lowering the formation temperature of pure phase BiFeO3 nanopowders. It is found that Bi/acac molar ratio of 1/30 was favorable for a stable sol and for the lowest crystallization temperature of pure BiFeO3 nanopowders. X-ray diffraction and Fourier transform infrared spectroscopy revealed that thermally induced crystallization process of BiFeO3 nanopowders from Bi-Fe polymeric precursor. When the thermal treated temperature was below 200 oC, only amorphous phase existed. With the temperature increasing up to 300 oC, crystallized phase, carbonate, were detected. After annealed at 400 oC, Bi-Fe precursor totally changed to rhombohedral BiFeO3 nanopowders. Scanning electron microscopy characterized morphologies of BiFeO3 nanopowders calcined at 400 oC and 500 oC. The ferroelectric transition of BiFeO3 nanopowders at 827 oC has been detected by differential thermal analysis.

Abstract: ZrO2(Y2O3)-Al2O3-SrSO4 nanocomposites incorporated with and without Ag addition have been fabricated by spark plasma sintering (SPS) to evaluate their friction and wear properties in sliding against alumina ball from room temperature to 600 oC. X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy were used to investigate microstructure and self-lubrication mechanisms of nanocomposites after wear tests at different temperatures. The ZrO2(Y2O3)-Al2O3-SrSO4 nanocomposite exhibits low and stable friction coefficients of 0.2 to 0.3 and wear rates in the order of 10-6 mm3/Nm at high temperatures. With the addition of Ag into the composite, the intermediate temperature lubricating property is greatly improved. Plastic deformation of SrSO4 and sliver during sliding plays an important role in formation of lubricating films on worn surfaces of nanocomposites. These lubricating films reduce the friction and wear of the ZrO2(Y2O3)-Al2O3 matrix composites.

Abstract: Nd--sialons with the stoichiometric composition of Nd0.333Si10Al¬2ON15 were obtained by hot-press sintering at 1800°C for 1h. The thermal shock behavior of the Nd--sialons was examined by a water-quenching technique. The influence of the thermal shock temperature difference (T) and cycle times on the residual strength was evaluated. Equiaxed -sialon grains formed together with a small amount of intergranular phase M (Nd2Si3-xAl¬xO3+xN4-x) and -sialon phase. The residual strength after a thermal shock tended to decrease gradually with increasing T above 500°C. However, the specimens exhibited an improved residual strength (~94% of the room temperature strength) after a thermal shock of T=1100°C. The residual strength presented a gradual decrease with increasing the thermal shock cycle times at T=1100°C, and was still remained ~55% of the room temperature strength after 11-time cycle. It is contributed to the surface oxidation which may results in the healing of surface cracks and the generation of surface compressive stresses.

Abstract: The ZrC-W composites with iron as sintering additive were fabricated by hot-press sintering. The densification, microstructure and mechanical properties of the composites were investigated. The incorporation of Fe beneficially promotes the densification of ZrC-W composites. The relative density of the composite sintered at 1900°C can attain 95.3%. W2C phase is also found in the ZrC-W composite sintered at 1700°C. The content of W2C decreases with the increase of sintering temperature. However, W2C phase is not identified in the composite sintered at 1900°C. The flexural strength and fracture toughness of the composites are strongly dependent on sintering temperature. The flexural strength and fracture toughness of ZrC-W composite sintered at optimized temperature of 1800°C are 438 MPa and 3.99 MPa·m1/2, respectively.

Abstract: In this work, porous titania-based coatings containing Ca and P ions were prepared on titanium alloy by micro-plasma oxidation (MPO) in an electrolyte containing nano-HA, calcium salts and phosphate under various applied voltages (200~450 V). The results showed that the MPO coatings formed at 250-450 V were composed of anatase and amorphous phase. With increasing applied voltage, the micropore size and thickness of the MPO coatings increase, while the micropore number decreases. Furthermore, the Ca and P concentrations, as well as atomic ratio of Ca/P are highly dependent on the applied voltage. In addition, the cross-sectional view results showed good adhesion between the MPO coatings and titanium alloy substrate at various applied voltages. In vitro experiments indicated that the MPO coating can induce apatite formation.

Abstract: The high-temperature friction and wear characteristics of different ceramics and ceramic matrix composites (CMCs) incorporated with various solid lubricants have been investigated from room temper- ature to 1000oC. The solid lubricants considered in this paper include representative precious metals, hexagonal boron nitride, graphite, fluorides, soft oxides, chromates, sulfates, and combinations of various solid lubricants. General design considerations relevant to solid lubrication were proposed on the basis of friction and wear data of self-lubricating CMCs. The self-lubricating composites incorporated with SrSO4 or/and CaSiO3 exhibits low and stable friction coefficients of 0.2 to 0.3 and small wear rates in the order of 10-6 mm3/Nm from room temperature to 800oC. The optimized composites appear to be promising can- didates for long-duration, extreme environment applications with low friction and small wear rate.