| Tungsten boride (W2B5) powder was synthesized by element synthetic method in this experiment, and W2B5material was prepared with the addition of nickel (Ni) as sintering aids, the effect of sintering aids content and sintering temperature on the microstructure composites and room temperature mechanical properties of W2B5material were studied, high temperature oxidation properties of W2B5material and neutron shielding properties of W2B5composite were evaluated, tungsten boride powder prepared by self-propagating high temperature synthesis (SHS) metallurgy was preliminarily studied. The research has great significance of the development for tungsten boride material, it provides experimental data and technical support for studying the basic properties of tungsten boride material which will be used in the field of metallurgy, building materials, nuclear power, medical treatment, aviation, national defense and so on.W2B5powder was synthesized with tungsten (W) and boron (B) as raw materials by element synthetic method. Firstly, thermodynamic calculation and W-B binary phase diagram of the reaction were systematically studied. Secondly, the experimental results show that pure W2B5powder can be synthesized at the ratio of W and B is1to2.5about1500degrees Celsius (℃) for1h by the analysis of X-ray diffraction (XRD), mass transfer way of W-B system was controlled by the diffusion of boron atoms towards tungsten atoms at1500℃.W2B5material was prepared with nickel as sintering aids at high sintering temperature, the effect of sintering aids content and sintering temperature on the sintering properties of material were systematically studied. The experimental results show that the volume is shrinking, the porosity is reducing and relative density is increasing with Ni adding or sintering temperature increasing of the material, the best volume shrinkage, porosity and relative density of the material in turn is44.56%,2.87%and95.48%when it was sintered with3weight percentage (wt%) Ni at1500℃. XRD shows that it helps to prepare W2B5material without sintering aids by compare with adding sintering aids, product detection show there is only W2B5and Ni is not found when Ni adds to W2B5material. The content of Ni and sintering temperature have great impact on particle sizes and morphology, particles of the material are spherical with the size is about1micron (um) and loose porous between particles when it was sintered at1500℃without sintering aids; particles grow to6tolOμm in needle stick and combine closely when the nickel adds to material. With the sintering temperature increasing, pores between the particles of material are decreasing and sizes of particles are growing, SEM micrograph shows Ni can evenly distribute in W2B5base material at1500℃. Room temperature mechanical properties of W2B5material are enhancing by adding Ni and increasing sintering temperature, bending strength and Vickers micro-hardness of the material are240MPa and2563.06MPa respectively when it was sintered with3wt%Ni at1500℃, SEM micrograph shows that material fracture of bending strength is transgranular fracture, and Ni fills in W2B5base material dispersedly.With oxidation behavior experiment of W2B5material, it suggests that W2B5material will be oxidized at1000-1200℃by thermodynamic calculation, and it can be oxidized easier with oxidation temperature increasing. The results of TG-DSC show that the W2B5material is oxidized about750℃to800℃, and the oxidation reacts slowly when the temperature is below1000℃. The analysis of oxidation kinetic and XRD and SEM of oxidation products shows that W2B5material is oxidized to tungsten trioxide (WO3) and diboron trioxide (B2O3) with different oxidation modes in different temperature. Liquid-phase B2O3which formed at1000-1100℃covers around spherical WO3as protection films will prevent oxidation reaction; oxidation rate is controlled by the element diffusion through oxidation layer. It makes the size of WO3grow when liquid-phase B2O3vaporizes, the thickness of oxidation layer become120μm, and exfoliation phenomenon of oxidation layer have taken place at1200℃, oxidation rate is controlled by the chemical reaction on the oxidation boundary. Oxidation activation energy of W2B5material is93.07kJ/mol.Neutron shielding properties of W2B5composite were tested, the results show that the neutron shielding properties of W2B5composite are linear enhancement with W2B5content increasing; the neutron shielding rate is93.2%when the W2B5composite content is70wt%.Using SHS metallurgy to prepare tungsten boride powder has been systematically studied, adiabatic temperature (Tad) calculating results of Mg-B2O3-W system show that Tad of W system is2987K which is higher than1800K, and it indicates SHS reaction of W take place easily. Results suggest that tungsten boride (W2B5and WB) and magnesium oxide (MgO) may be generated in the system by calculating Gibbs energy of Mg-B2O3-W system. XRD results of combustion products which were produced by SHS show that products phases consist of mainly W2B5and MgO and little WB and Mg3B2O6, W2B5content increases with magnesium (Mg) and B2O3content appropriate increasing. By-product such as MgO and Mg3B2O6et al. have been removed completely by leaching through hydrochloric acid (HCl) for SHS combustion products, W2B5yield in the leaching product increases with Mg and B2O3content increase appropriately and particle size of tungsten boride decreases in the combustion process, it indicates that leaching products composition and particle-size of tungsten boride can be controlled by changing the content of reactants. |
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