Papers by Author: Tai Qiu

Abstract: Fully dense (ZrB₂+ZrC)/Zr₃[Al (Si)]₄C₆ composites with ZrB₂ content varying from 0 to 15 vol.% and fixed ZrC content of 10 vol.% were successfully prepared by in situ hot-pressing in Ar atmosphere using ZrH₂, Al, Si, C and B₄C as raw materials. With the increase of ZrB₂ content, both the bending strength and fracture toughness of the composites increase and then decrease. The synergistic action of ZrB₂ and ZrC as reinforcements shows significant strengthening and toughing effect to the Zr₃[Al (Si)]₄C₆ matrix. The composite with 10 vol.% ZrB₂ shows the optimal mechanical properties: 516 MPa for bending strength and 6.52 MPa·m^1/2 for fracture toughness. With the increase of ZrB₂ content, the Vickers hardness of the composites shows a near-linear increase from 15.3 GPa to 16.7 GPa. The strengthening and toughening effect can be ascribed to the unique mechanical properties of ZrB₂ and ZrC reinforcements, the differences in coefficient of thermal expansion and modulus between them and Zr₃[Al (Si)]₄C₆ matrix, fine grain strengthening and uniform microstructure derived by the in situ synthesis reaction.

Abstract: Microstructures and microwave dielectric properties of Ca(Sm_0.5Nb_0.5)O₃ ceramics, prepared by a conventional solid-state reaction method, were systematically investigated by varying calcining temperature, sintering temperature and cooling rate. The XRD result showed that a single Ca(Sm_0.5Nb_0.5)O₃ phase could be synthesized at a calcining temperature of 1200 °C. Optimized combination of microwave dielectric properties of ε_r = 22.36, Q×f = 18030 GHz and τ_f = -31.2 ppm/°C was obtained for furnace-cooled Ca(Sm_0.5Nb_0.5)O₃ ceramics sintered at 1550 °C for 4 h. However, some microcracks were found from the microstructures of the furnace-cooled specimens. Further, the Q×f value could be increased by controlling the cooling rate during the sintering process due to the disappearance of microcracks in the final material. With a cooling rate of 2 °C/min, Ca(Sm_0.5Nb_0.5)O₃ceramics exhibited an enhanced Q×f value of 37130 GHz.

Abstract: Single spinel phase nanocrystalline ferrite (Ni_0.5Zn_0.5)_1-XCo_xFe₂O₄was prepared using the spraying co-precipitation method. The coercivity of prepared (Ni_0.5Zn_0.5)_1-XCo_xFe₂O₄ increases with an increasing content of cobalt. The enhancement of the saturation magnetization was observed up to the maximum value of 102.1emu/g as x was 0.5. This can be attributed to the substitution of the Ni₂₊ and Zn₂₊ ions with Co₂₊ ions. This changed the net magnetic moment between the sublattices of structure.

Abstract: This Magnesium-doped calcium polyphosphate (MCPP) porous bioceramics of different magnesium content were prepared by the method of solid reaction sintering. The effect of magnesium on the structure and density of magnesium-doped calcium polyphosphate bioceramics was studied. Phases, cross section morphologies and porosity of MCPP bioceramics were analyzed with X-ray diffraction(XRD) and scanning electron microscopy (SEM). The results show that MCPP ceramics were successfully prepared by the solid reaction sintering, and the bending strength of ceramics began to increase and then to decrease with increasing amounts of magnesium content apart with the improvement of the stability improved.

Abstract: Nanocrystalline CoFe₂O₄ ferrite have been synthesized by a spraying-coprecipitation method. X-ray diffraction (XRD) and Transmission electron microscope (TEM) confirmed the formation of single-phase CoFe₂O₄ ferrite nanoparticles in the range 10 nm ~ 100 nm depending on the calcining temperature. The magnetic measurements show that specific saturation magnetization of nanocrystalline CoFe₂O₄ ferrite increases from 36.1 A•m2•kg-1 to 88.6 A•m2•kg-1 as grain size increases from 12 nm to 98 nm. The coercivity shows a peak with grain size, peaking at around 43 nm. The magnetic behaviour of nanocrystalline CoFe₂O₄ ferrite might be attributed to the effects of surface and random anisotropy.

Abstract: To obtain novel intermediate temperature alloy solders with melting temperature of 400~600°C, (Ag-Cu28)-25Sn alloy ribbons were prepared by high frequency induction melting and melt spinning at different quenching linear speed. The effect of the development of solidification structure on melting properties and microhardness of the ribbons were investigated. The XRD results show that the as-prepared alloy ribbons have the same phase composition as the master alloy, which consists of Ag4Sn and Cu3Sn. With the quenching linear speed increasing, the solidification structures are refined and change from dendritic crystals to uniform granular crystals. As the quenching linear speed increases up to 32.25m/s, the grain size of the alloy ribbon has a distribution ranging from submicron to about 2μm. The DSC results indicate that the melting properties of alloy ribbons strongly depend on the solidification structure, and the melting temperature of alloy ribbons decreases with the quenching linear speed increasing. The lowest liquidus points of the alloy ribbon prepared at linear speed of 32.25m/s are located at 473.6°C and 524.7°C, respectively. The refined solidification structure notably increases the microhardness of the alloy ribbons, and the largest hardness value of 396HV is obtained for the alloy ribbon prepared at linear speed of 32.25m/s, which increases 27.0% compared with the master alloy.

Abstract: Ba₄Sm_9.33Ti₁₈O₅₄ (BST) ceramics were prepared by conventional solid-state reaction method. The effect of CuO-Bi₂O₃ co-doping on the sintering behavior and microwave dielectric properties of BST ceramics has been investigated. The results indicated that when the total addition of CuO-Bi₂O₃ doping agent was 1.0wt%, the sintering temperature of BST ceramics was reduced to 1240°C as a result of liquid-phase sintering effect derived from CuBi₂O₄. When the total addition of CuO-Bi₂O₃ was fixed at 1.0wt%, the microwave dielelctric properties of BST were improved by adjusting the mass percentage of Bi₂O₃. With the increase of Bi₂O₃ amount from 20wt% to 80wt%, the dielectric constant (ε_r) decreased, the quality factor (Q•f) increased and the temperature coefficient of resonant frequency (τ_f) shifted from negative value to positive value. The excellent microwave dielectric properties of ε_r=82.51, Q•f=7878GHz and τ_f=+6.02ppm/ were obtained for the sample with Bi₂O₃ amount of 80wt% when sintered at 1240°C for 3h.

Abstract: Calcium borosilicate (CaO-B2O3-SiO2, CBS) glass based glass-ceramic composites were prepared by introducing borosilicate glass. The effects of borosilicate glass and firing temperature on the microstructure and properties of the glass-ceramic composites were investigated. The results showed that the composites containing 0~40% (in mass fraction, the same below) borosilicate glass can be sintered at 850°C. The dielectric constant (εr) decreases with the increase of borosilicate glass content and can be adjusted in the range of 5.6~6.6. The coefficient of thermal expansion (CTE) increases with the increase of borosilicate glass content. Increasing sintering temperature favors the precipitations of crystal phases, which have lower εr than CBS glass, resulting in the decrease of εr for the composites.

Abstract: Nanocrystalline Ni1-xZnxFe2O4 ferrites with 0 ≤ x ≤ 1 were prepared by sprayingcoprecipitation method. The microstructure was investigated by TG-DSC, XRD, SEM, TEM and BET. Magnetic properties were measured with vibrating sample magnetometer at room temperature. The results showed that uniform and fine nanocrystalline Ni1-xZnxFe2O4 ferrites are obtained. The grain size of sample calcined at 600°C for 1.5h is about 30nm. There are a few agglomerates with average sizes below 100nm. The specific saturation magnetization, Ms, of the sample increases with increasing Zn2+ concent x at room temperature, and the maximum Ms is 66.8 A·m2·kg-1 as the Zn2+ content x is around 0.5mol. As calcining temperature increased from 400°C to 1050°C, the Ms of Ni0.5Zn0.5Fe2O4 ferrite increases from 40.2 A·m2·kg-1 to 75.6 A·m2·kg-1. The coercivity maximum is about 5.97 kA·m-1 as its critical grain size is about 62.0nm. The relation between coercivity and grain size for nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite may be explained based on random anisotropy theory.

Abstract: TiO2/(O′+β′)-Sialon multiphase ceramics with different phase composition of TiO2 were prepared by pressureless sintering under high-purity N2 atmosphere with (O′+β′)-Sialon powder and nano TiO2 (anatase) powder as raw materials, Yb2O3 or Tb2O3 as additive. For each sample, the weight percentage of anatase in TiO2 was calculated from XRD data and the kinetics of anatase-rutile transformation was investigated, wherein the emphasis was placed on the influence of Yb2O3 and Tb2O3. The results indicate that the added Tb2O3 and Yb2O3 serve the significant function of inhibition and promotion on the phase transformation, and the effects are enhanced and attenuated with increasing additive content, respectively. For the sample without additive, the transformation process follows apparent first-order kinetic model. The addition of Yb2O3 or Tb2O3 results in completely different transformation kinetic law. For the samples with Yb2O3 added, the transformation is an apparent second-order reaction, whereas a unique kinetic model, CA=kt1/2+C, is valid for the samples containing Tb2O3. In the two cases, the effect of the additive content on the transformation can be perfectly reflected by the apparent rate constant.