Frontiers of Materials Science (v.4, #3)

Preface by Jie-Cai Han (225-225).

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.
Keywords: non-Fourier heat conduction; thermal wave model; finite element method; micro/nanoscale

The coefficient of thermal expansion of biomimetic composites by Hai-Jun Lei; Bin Liu; Dai-Ning Fang (234-238).
The excellent mechanical properties of biocomposites has attracted a lot of research attention, and people have started attempting to fabricate biomimetic staggered composites. In this paper, the relationship between the equivalent coefficient of thermal expansion (CTE) and the microstructure of a biomimetic staggered composite is investigated. A shear-lag based thermalelastic analytical model is developed and is found to agree well with the finite element simulations. It is found that besides the volume fraction and the material constants of the constituent phases, the aspect ratio of the hard platelet plays an important role in the CTE of biocomposites. Hence, there are additional design parameters in staggered composites that can be used to adjust the CTE, which makes this type of composite promising in thermalelastic loading.
Keywords: biomimetic composites; thermalelastic properties; microstructure; the equivalent CTE

This paper introduces a numerical method for predicting the effective elastic module of ZrB2-based composites, which includes a random generation-growth method for generating microstructures of ZrB2-based composites based on existing statistical macroscopic geometrical characteristics and a highly efficient lattice Boltzmann algorithm for solving the governing equations on the multiphase microstructures. The effective elastic module of random ZrB2-based materials is analyzed with different parameters by the present method, including effects of component size and material anisotropy. The simulation results indicate this numerical method’s effectiveness and robustness by comparing the predictions with experimental data and other theoretical models.
Keywords: ZrB2 ; lattice Boltzmann method; elastic property; Young’s modulus; multiphase composites; random structure

Calculation of crystal cohesive energy of ZrB2 compound by Jin-Ping Li; Shan-Liang Dong; Song-He Meng; Xiao-Guang Luo; Yu-Min Zhang (245-250).
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.
Keywords: ZrB2 ; empirical electron theory (EET); valence electron structure (VES); crystal cohesive energy (CCE)

The physical mechanisms that determine the retained strength of ceramics after thermal shock are studied by measuring experimentally and statistically the density and depth of cracks produced in the interior of the ceramics. The analysis indicates that the key factor controlling the retained strength is the maximum depth of cracks rather than the density of cracks in the ceramics. The result presented here forms a basis to further understand the thermal shock behavior of ceramics.
Keywords: crack depth; crack density; retained strength; thermal shock

Damage mode effects on fracture strength of ultra-high temperature ceramics by Wei-Guo Li; Ding-Yu Li; Xue-Feng Yao; Dai-Ning Fang (255-258).
Improving the fracture strength of ceramics has been one of the most important concerns in the ceramics field, and highly accurate evaluation of the fracture strength of ceramics is the right foundation of this topic. In this paper, we analyze fracture strength by using a previously established temperature-damage-dependent strength model. Sensitivities of fracture strength to relative parameters are analyzed, and the influences of different failure modes and typical sizes of different micro-structural characteristics on the fracture strength of ultra-high temperature ceramics (UHTCs) are discussed. This study can provide a theoretical basis and technical platform for the design, application and reliability assessment of UHTCs in applications.
Keywords: ultra-high temperature ceramic (UHTC); thermal-damage; high-temperature strength; sensitivity; micro-structure

Assessment of failure temperature of ultra-high temperature ceramic plates by Tao Zeng; Shi Yan; Dai-Ning Fang; Yu Gao (259-261).
A theoretical model is presented to assess the failure temperature for ultra-high temperature ceramics (UHTCs). The parameters in the present work, such as thermal expansion coefficient and Young’s modulus, are considered functions of temperature to calculate the stress field of UHTCs under high temperature conditions. The critical elevated temperature for failure is calculated by using the Maximum Principle Strength theory. By establishing the relation between the temperature and the mechanical properties of the UHTCs, it is found that the failure behavior of UHTCs is affected by initial temperature.
Keywords: ultra-high temperature ceramics (UHTCs); mechanical properties; stress field; failure temperature

Effect of the La addition content on valence electron structure and properties of ZrB2 ceramics by Jin-Ping Li; Yan Wang; Qing Liu; Xiao-Guang Luo (262-265).
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.
Keywords: ZrB2 ; La substitution; valence electron structure (VES); properties

A novel ultra-high temperature oxidation technique in flowing gas with controlled oxygen partial pressure by Jing-Jun Xu; Mei-Shuan Li; Xue-Liang Fang; Zhong-Wei Zhang; Zheng-Hui Xu; Jun-Shan Wang (266-270).
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.
Keywords: induction heating; ultra-high temperature oxidation; oxidation kinetics

Preparation and mechanical properties of ZrB2-based ceramics using MoSi2 as sintering aids by Cui-Wei Li; Ya-Mei Lin; Ming-Fu Wang; Chang-An Wang (271-275).
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.
Keywords: zirconium diboride (ZrB2); MoSi2 ; sintering aids; mechanical properties

Preparation and characterization of ZrB2-SiC ultra-high temperature ceramics by microwave sintering by Hai-Long Wang; Chang-An Wang; De-Liang Chen; Hong-Ling Xu; Hong-Xia Lu; Rui Zhang; Lun Feng (276-280).
ZrB2-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB2-SiC composites and almost fully dense ZrB2-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB2-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m1/2, respectively.
Keywords: ultra-high temperature ceramics (UHTCs); zirconium diboride; silicon carbide; mechanical properties; microwave sintering

Preparation of B4C-ZrB2-SiC eutectic ceramics by arc melting method by Qi-Long Guo; Jun-Guo Li; Ai-Yi Peng (281-284).
The B4C-ZrB2-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m1/2. The eutectic mole composition is 0.39B4C-0.25ZrB2-0.36SiC, and the eutectic lamellar microstructure is composed of B4C matrix with the lamellar ZrB2 and SiC grains.
Keywords: B4C-ZrB2-SiC; arc melting; eutectic composition

Synthesis of ultrafine ZrB2 powders by sol-gel process by Li-Juan Yang; Shi-Zhen Zhu; Qiang Xu; Zhen-Yu Yan; Ling Liu (285-290).
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.
Keywords: ultrafine ZrB2 powders; sol-gel method; synthesis

Migration of specific planar grain boundaries in bicrystals: application of magnetic fields and mechanical stresses by Dmitri A. Molodov; Tatiana Gorkaya; Christoph Günster; Günter Gottstein (291-305).
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 〈10 $$ ar 1 $$ 0〉 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 noncoincidence 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.
Keywords: grain boundary; migration; magnetic driving force; stress; shear strain

Effects of dispersed medium systems on substitution pattern and solution performance of carboxymethyl cellulose by Bo Li; Zi-Qiang Shao; Jie-Min Hong; Fei-Jun Wang; You-De Zhang; Bing Liao (306-313).
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.
Keywords: degree of substitution; alkalization; substitution uniformity; molecular interaction; aggregation

Preparation of PLGA/β-TCP composite scaffolds with supercritical CO2 foaming technique by Chang Yang; Yun-Qing Kang; Xiao-Ming Liao; Ya-Dong Yao; Zhong-Bing Huang; Guang-Fu Yin (314-320).
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.
Keywords: scaffold; poly(DL-lactide-co-glycolide) (PLGA); β-tricalcium phosphate (β-TCP); supercritical CO2 foaming technique

Effects of applied magnetic field on twin-wire indirect arc shapes by Shun-Shan Zhang; Zeng-Da Zou (321-324).
Research on regularity of indirect arc shapes change with variation of applied magnetic field is studied. Results show that indirect arc would be elongated or compressed in XOZ plane with variation of applied transverse magnetic field’s direction and intensity, while the indirect arc would be deflected with the application of longitude magnetic field in YOZ plane, and the deflection degree and direction will be also changed by the variation of longitude magnetic field’s intensity and direction. It is considered that change of arc shapes is caused by variation of arc forces. The influence of Ampere force on indirect arc deformation and deflection is analyzed in this paper.
Keywords: metallurgy; indirect arc shape; magnetic field; twin-wire welding