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A finite element method for non-Fourier heat
conduction in strong thermal shock environments
Bao-Lin WANG, Jie-Cai HAN,
Front. Mater. Sci.. 2010, 4 (3): 226-233.
https://doi.org/10.1007/s11706-010-0090-4
Non-Fourier effect is important in heat conduction in strong thermal environments. Currently, generally-purposed commercial finite element code for non-Fourier heat conduction is not available. In this paper, we develop a finite element code based on a hyperbolic heat conduction equation, which includes the non-Fourier effect in heat conduction. The finite element space discretization is used to obtain a system of differential equations for the time. The transient responses are obtained by solving the system of differential equations, based on the finite difference, mode superposition, or exact time integral. The code is validated by comparing the numerical results with exact solutions for some special cases. The stability analysis is conducted and it shows that the finite difference scheme is an ideal method for the transient solution of the temperature field. It is found that with mesh refining (decreasing mesh size) and/or high-order elements, the oscillation in the vicinity of sharp change vanishes, and can be essentially suppressed by the finite difference scheme. A relationship between the time step and the space length of the element was identified to ensure that numerical oscillation vanishes.
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Calculation of crystal cohesive energy of ZrB 2 compound
Jin-Ping LI, Shan-Liang DONG, Song-He MENG, Xiao-Guang LUO, Yu-Min ZHANG,
Front. Mater. Sci.. 2010, 4 (3): 245-250.
https://doi.org/10.1007/s11706-010-0082-4
The valence electron structure (VES) of ZrB2 was set up with the bond length difference (BLD) method based on empirical electron theory (EET) of solids and molecules, and there were 43 potential hybridization combinations. Based on the calculation result of the melting point, the 16th hybridization step of Zr atom and fifth hybridization step of B atom are ascertained as the final hybridization combination. Therefore, the covalent electron number and the bonding energy of the strongest bond (the B–B bonds), the theoretical melting point, and the crystal cohesive energy of the ZrB2 compound can be figured out.
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Effect of the La addition content on valence
electron structure and properties of ZrB 2 ceramics
Jin-Ping LI, Yan WANG, Qing LIU, Xiao-Guang LUO,
Front. Mater. Sci.. 2010, 4 (3): 262-265.
https://doi.org/10.1007/s11706-010-0083-3
ZrB2 is a combined bonding compound composed of strong covalent bonds which make it difficult to sinter and densify. Thus, rare earth or other metal elements are usually used to be sintering additives to improve its sintering properties. To forecast properties of ZrB2 solid solutions with addition of lanthanum, their valence electron structure (VES) was calculated by using the empirical electron theory (EET) of solids and molecules, and the effect of lanthanum with various proportion on the VES and properties of ZrB2 ceramics has been studied. The results show that with the increase of the lanthanum addition content, the hybridization steps of Zr and B atoms of ZrB2 solid solutions are still A16 and 5, respectively. The hybridization step of lanthanum is always A1. The covalent electron numbers and bonding energy of the strongest bonds of the ZrB2 matrix decrease with the lanthanum addition content increase. These suggest that the addition of lanthanum will improve the fracture toughness and decrease the hardness, crystal cohesive energy and melting point of ZrB2. In a word, its sintering properties are improved, and its densities are increased.
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A novel ultra-high temperature oxidation technique
in flowing gas with controlled oxygen partial pressure
Jing-Jun XU, Mei-Shuan LI, Xue-Liang FANG, Zhong-Wei ZHANG, Zheng-Hui XU, Jun-Shan WANG,
Front. Mater. Sci.. 2010, 4 (3): 266-270.
https://doi.org/10.1007/s11706-010-0086-0
For the purpose of investigating ultra-high temperature oxidation, a novel induction heating facility has been established. The oxidation kinetics of several typical ultra-high temperature materials (UHTMs), including two graphite-based composites (C/C and ZrB2/C) and two ternary Zr-Al-C ceramics (Zr2Al3C4 and Zr2[Al(Si)]4C5), were tested by utilizing this facility. It has been identified that the tested cylindrical samples with dimensions of Φ 20mm × 20 mm can be oxidized uniformly. The maximum temperature of 2450°C can be achieved on graphite-based composites, and the oxygen partial pressure can be controlled in the range of 102–105Pa. This novel technique exhibits many advantages, such as an extremely high heating rate of about 20°C/s, easy controlling of temperature and gas pressure, low energy consumption, low cost, and high efficiency. Therefore, it provides a potential way for profoundly investigating the ultra-high temperature oxidation behaviors of UHTMs.
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Preparation and mechanical properties of ZrB 2 -based ceramics using MoSi 2 as
sintering aids
Cui-Wei LI, Ya-Mei LIN, Ming-Fu WANG, Chang-An WANG,
Front. Mater. Sci.. 2010, 4 (3): 271-275.
https://doi.org/10.1007/s11706-010-0084-2
ZrB2 (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi2 as sintering aids. The effects of sintering condition and MoSi2 content on densification behavior, phase composition, and mechanical properties of SiCw/ZrB2 composites were studied. Nearly, fully dense materials (relative density>99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB2 phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi2 addition sintered by HP at 1750°C displayed the best mechanical properties.
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Synthesis of ultrafine ZrB 2 powders by sol-gel process
Li-Juan YANG, Shi-Zhen ZHU, Qiang XU, Zhen-Yu YAN, Ling LIU,
Front. Mater. Sci.. 2010, 4 (3): 285-290.
https://doi.org/10.1007/s11706-010-0094-0
Ultrafine zirconium diboride (ZrB2) powders have been synthesized by sol-gel process using zirconium oxychloride (ZrOCl2·8H2O), boric acid (H3BO3) and phenolic resin as sources of zirconia, boron oxide and carbon, respectively. The effects of the reaction temperature, B/Zr ratio, holding time, and EtOH/H2O ratio on properties of the synthesized ZrB2 powders were investigated. It was revealed that ultrafine (average crystallite size between 100 and 400 nm) ZrB2 powders can be synthesized with the optimum processing parameters as follows: (i) the ratio of B/Zr is 4; (ii) the solvent is pure ethanol; (iii) the condition of carbothermal reduction heat treatment is at 1550°C for 20 min.
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Migration of specific planar grain boundaries
in bicrystals: application of magnetic fields and mechanical stresses
Dmitri A. MOLODOV, Tatiana GORKAYA, Christoph GüNSTER, Günter GOTTSTEIN
Front. Mater. Sci.. 2010, 4 (3): 291-305.
https://doi.org/10.1007/s11706-010-0080-6
Recent research on the dynamics of planar grain boundaries is reviewed. Novel measuring techniques developed for in situ observation and recording of magnetically and stress driven grain boundary migration are presented. The results of migration measurements obtained on bismuth, zinc and aluminum bicrystals are addressed. The experiments revealed that the inclination of a 〈112〉 tilt boundary in Bi has a very strong influence on its mobility. The migration of planar tilt grain boundaries with different misorientation angles was measured in situ in bicrystals of high purity zinc. The results proved that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The shear stress induced migration of planar symmetric 〈100〉 tilt boundaries in aluminum bicrystals was observed to be accompanied by a lateral translation of the adjacent grains. The coupling between boundary motion and shearing is not confined to low angle and some low Σ high angle boundaries, but occurs also for non-coincidence high angle 〈100〉 tilt boundaries. It has been found that also for stress induced grain boundary motion there is a misorientation dependence of the migration activation parameters. Lower values of the activation enthalpy and the pre-exponential mobility factor can be associated with boundaries with tilt angles close to low Σ CSL orientation relationships.
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Effects of dispersed medium systems on substitution
pattern and solution performance of carboxymethyl cellulose
Bo LI, Zi-Qiang SHAO, Jie-Min HONG, Fei-Jun WANG, You-De ZHANG, Bing LIAO,
Front. Mater. Sci.. 2010, 4 (3): 306-313.
https://doi.org/10.1007/s11706-010-0097-x
Effects of three dispersed medium systems consisting of isopropyl alcohol (IPA), ethyl alcohol (EtOH) and toluene (TOL) on the substitution patterns of carboxymethyl cellulose (CMC) were studied, and the corresponding influences on solution performances were investigated on a rheometer. In EtOH-IPA system, the structure of higher average substitution degree and enlarged partial substitution degrees disparity (determined by 1H nuclear magnetic resonance) but lower distribution uniformity along molecular chains (speculated from static/dynamic light scattering) were characterized by which the thixotropy and apparent viscosity of solution decreased due to the aggregation of longer unsubstituted segments. For the phase separation (identified by gas chromatography) of TOL-IPA system, considerable unsubstituted regions in the structure aggregated into hydrophobic centers to form swollen macrogel particles in solution, leading to the sharp rise in apparent viscosity and almost constant flow-behavior index with hardly any thixotropic behaviors presented.
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Preparation of PLGA/β-TCP composite scaffolds
with supercritical CO 2 foaming technique
Chang YANG, Yun-Qing KANG, Xiao-Ming LIAO, Ya-Dong YAO, Zhong-Bing HUANG, Guang-Fu YIN,
Front. Mater. Sci.. 2010, 4 (3): 314-320.
https://doi.org/10.1007/s11706-010-0081-5
A high porosity scaffold with suitable compressive strength prepared by a gentle method has become a pressing need. To meet this demand, poly(DL-lactide-co-glycolide) (PLGA) and β-tricalcium phosphate (β-TCP) were designed to prepare composite scaffolds by the supercritical technique. The preparation process consisted of three units: the mixing of PLGA and β-TCP, compression molding of the mixture, and the foaming process. Six influencing factors — temperature, pressure of the scCO2 system, maintaining time of scCO2, the ratio of β-TCP to PLGA, the rate of depressurization, and the molecular weight — were investigated. The results collectively indicated that the optimized conditions for the foaming process were that CO2 pressure and temperature be 8MPa and 39°C, respectively, which should be kept for 8h; the content of β-TCP in the mixture should be 25% and the depressurizing rate be 0.1 MPa/s, using PLGA of an 80kDa molecular weight. Scaffolds with a porosity of 65.47% and a compressive strength of 4.76 MPa could be obtained. The pore size ranged around 100 µm. The material’s use as tissue engineering scaffolding is expected.
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