Papers by Author: De Chang Jia

Abstract: A mixture of cubic silicon powder, hexagonal boron nitride powder and graphite powder was mechanically alloyed for 30 hrs in argon. The as-milled 2Si-B-3C-N composite powder was heated up to 1900 °C in nitrogen, with a heating rate of 25 °C/min and under a pressure of 80 MPa. XRD and HRTEM results show that the as-milled 2Si-B-3C-N composite powder has a well amorphous structure. Under the current hot-pressing circumstances, the amorphous ceramic starts to crystallize at a temperature between 1800 °C and 1900 °C. Once the temperature is higher than crystallization temperature, crystallites appear in the amorphous matrix with a great nucleation rate, but a small growth rate. Hot pressed at 1900 °C for 0 mins or 10 mins, the prepared 2Si-B-3C-N bulk ceramic has an average grain size of 8.7 nanometers and 22.3 nanometers, respectively. After an intensive literature search, we believe the present work is the first one to make clear that it is possible to use the mechanical alloying route to prepare amorphous Si-B-C-N ceramic with such a high crystallization temperature.

Abstract: A mixture of the commercially available cubic silicon powder, hexagonal boron nitride powder and graphite powder was mechanically alloyed to prepare amorphous 2Si-B-3C-N composite powder. The amorphous powder was heated up to 1900°C in nitrogen, with a heating rate of 20°C/min and under a pressure of 80 MPa. Careful investigation was carried out on the densification curve, the microstructure and the mechanical properties of the prepared ceramics. Results show that the amorphous 2Si-B-3C-N powder mainly consists of near-spherical agglomerates, with an average size of 3.5±2.4 micrometers. When the amorphous powder was hot pressed, the densification process mainly included three stages, the denser packing of powder particles with the help of axial pressure, the initial sintering at about 1500-1800°C, and the rapid sintering at temperatures approximately higher than 1830°C. When the 2Si-B-3C-N ceramic was hot pressed at 1900°C for 10-30 mins, it exhibited large volume shrinkage, noticeable reduction of pores, and significantly improvement of density and mechanical properties. The applied high temperature and large pressure may give rise to severe plastic deformation, viscous flow and creep of powder particles, which greatly contribute to the rapid densification of the amorphous 2Si-B-3C-N powder.

Abstract: In this paper, the process of the transformation from kaolin to metakaolin was investigated. The kaolin was calcined at different temperatures and analyzed by Xray diffraction (XRD), Fourier transform infrared spectra (FTIR) and solid state nuclear magnetic resonance (NMR). The formation of metakaolin structure was based on the stacking polyhedrons changes, which originated from dehydroxylation of kaolinite. With increasing temperature, kaolin kept structure of kaolinite unchanged in the course of dehydroxylation and then structure of kaolinite transformed to metakaolin when the dehydroxylation was over. It was demonstrated that the essence of pozzolanic activity of metakaolin. The result revealed that the pozzolanic activity of metakaolin increased with increasing temperature at the range of 600~900 °C.

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: 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: ZnO mesoscale superstructures (ZMS) with the sizes of 2 to 6 µm were synthesized by a simple solution method without any additives. The novel ZMS were assembled by numerous nanoparticles with the diameter of less than 100 nm, and no pores were found. X-ray diffraction shows that the ZMS mainly evolved in solution. The formation process of the ZMS accords to the classical theory of nucleation and growth. Photoluminescence (PL) analysis shows a sharp UV emission peak at 392 nm and a weak broad green-yellow emission peak at about 600 nm.

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: A novel evaluation method of pozzolanic activity of metakaoline is studied. The pozzolanic activity can be evaluated by the measurement of the content of AlO4 tetrahedra. Base on this method, the influence of calcination time on pozzolanic activity of metakaoline is studied. The results indicate that the pozzolanic activity increases with the calcination time when calcination time is below 4 hours, and it decreases with the calcinations time increasing when the calcination time is 4-8 hours.

Abstract: Hollow carbon spheres containing Fe3N nanocrystals (Fe3N/HCSs) are synthesized by ferrocene and ammonium chloride in high pressure argon at 500 °C. The structure, morphology and properties of the products are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TG) and vibrating sample magnetometry (VSM). The hollow carbon spheres have diameters of 1-10 µm and shell thickness of hundreds of nanometers. The dimensions of the acicular Fe3N nanocrystals have diameters of ca. 100 nm and lengths of 600-800 nm. The weight percent of Fe3N nanocrystals in Fe3N/HCSs is about 37.8%. The saturation magnetization value of the hollow carbon spheres containing Fe3N nanocrystals is 10.61 emu/g.

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.