| Boron carbide(B4C)has many excellent properties,such as low density(2.52 g/cm3),high hardness(9.36,second only to diamond and cubic boron nitride),high fusion point(2450℃),good abrasion resistance,high neutron capture cross section and good chemical stability.It is widely used in bulletproof materials,nozzle materials and nuclear neutron absorbing materials.However,there are some defects in the sintering process of B4C ceramics,such as low fracture toughness and high covalent bond content,which are difficult to densify.The existence of these defects greatly limits its application in a wider range of fields.How to improve the defects and properties of boron carbide ceramics has been explored.In this paper,several groups of pre-experiments in the smelting process of smelting conditions and reaction mechanism of research,to achieve a better recovery of crystal silica mortar cutting waste.It not only protects the environment and saves resources,but also brings huge economic benefits.The main contents and results of this paper are as followts:(1)The raw materials used in the experiment are analyzed and characterized.the particle size of boric acid is 80 μm,wt(boric acid)≥99.9%;Boron carbide particle size D50=2.673 μm,wt(B4C)≥95%;SiC particle size D50=7.209 μm,wt(SiC)≥99.5%;The particle size of petroleum coke powder D50=9.168 μm,wt(C)=87.8%;The particle size of silica powder D50=4.509 μm,wt(SiO2)≥99%;The particle size of crystal silica mortar cutting waste D50=2.68 μm,where wt(SiC)=24.67%,wt(Si)=24.59%,wt(SiO2)=39.1%,wt(metal and oxide impurities)=11.64%.(2)The phase diagrams of B2O3-C and SiO2-C systems were plotted by factsage software.The impacts of temperature to C/(C+B2O3)and C/(C+SiO2)were studied.The possible reactions of B-C-O and Si-C-O systems in smelting process and the initial temperatures were calculated.The processes of boric acid dehydration,boron carbide synthesis and silicon carbide synthesis were analyzed.At last,that boron carbide formation reaction is obtain:2B2O3(1)+7C(s)=B4C(s)+6CO(g),2B2O2(g)+5C(s)=B4C(s)+4CO(g);Silicon carbide formation reaction:SiO2(s)+3C(s)=SiC(s)+2CO(g),SiO(g)+2C(s)=SiC(s)+CO(g).(3)B4C and SiC were mechanically mixed and pressed into samples and smelted at 1850℃for 50 min in a medium frequency induction furnace to prepare SiC/B4C composite ceramic powder as a blank control experiment.The effects of different SiC content on the phase composition,microstructure and particle size distribution of smelting products were studied.The results of XRD analysis showed that only the diffraction peaks of B4C and SiC phases were detected,and the purity of the product was high.With the increase of SiC content,the intensity of diffraction peaks of B4C phase decreased gradually.The results of SEM/EDS analysis show that the B4C and SiC grains still maintain their original morphology,and their grain boundaries are distinct,and they exist separately from each other.The results of grain size analysis show that the grain size of each group is approximately normal distribution,and the grain size of the whole group tends to decrease at first and then increase.When the content of silicon carbide is 10%,the diffraction peak intensity of the phase is appropriate,the particle size is the finest,and the optimum ratio is obtained.(4)Mix B4C,silica powder and petroleum coke evenly;H3BO3,petroleum coke and SiC are uniformly mix;H3BO3,petroleum coke and silica pow are uniformly mixed;H3BO3,petroleum coke and crystal silica mortar cut waste were uniformly mix and pressed into sample,smelted at 1850℃ for 50 min in an intermediate frequency induction furnace,and SiC/B4C composite ceramic powder was synthesised in situ by carbothermal reduction method.The results of XRD analysis show that the diffraction peak of B4C phase increases at first and then decreases with the increase of silica source ratio in the four groups of experiments.The results of SEM/EDS analysis show that three elements,B,C and Si,are detected at one point,indicating that B4C and SiC grow together at one point and that the matrix is closely bound to the second phase.The results of particle size analysis showed that the particle size of each group was approximately normal distribution,and the particle size of each group decreased at first and then increased as a whole.When the content of silica powder was 13.01%in the blending of B4C,silica powder and petroleum coke;When the content of SiC in H3BO3,petroleum coke and SiC is 1.52%;When the content of silica powder was 2.22%in the mixture of H3BO3,petroleum coke and silica powder;In the mixture of H3BO3,petroleum coke and crystalline silica mortar,when the content of cutting waste is 1.49%,the peak intensity of phase diffraction is higher and the grain growth is better,and the D50 is 5.227,4.631,5.332 and 7.321 μm,respectively,which is the best ratio of H3BO3,petroleum coke and crystalline silica mortar.(5)Two new groups were added including B4C,silica powder,petroleum coke group and H3BO3,petroleum coke,SiC group.The obtained SiC/B4C composite ceramic powders prepared by the optimum ratio of H3BO3,petroleum coke and silica micropowder in the three groups of experiments were studied through analysis and comparison.The results showed that there was no significant difference in particle size among the three groups,and the D50 was about 5 μm.In the mixture of H3BO3,petroleum coke and silica powder,when the content of silica powder is 2.22%,the diffraction peak of carbon phase is relatively high,but the diffraction peak intensity of B4C and SiC phase is more reasonable,boron and silica are more suitable and the grain growth condition is the best,which is the optimal ratio.(6)Combing B4C and SiC and comparing the obtained SiC/B4C composite ceramic powders prepared by the optimum ratio of H3BO3,petroleum coke and silica powder in the two groups of experiments.The results showed that there was no significant difference in particle size between the two groups,and the D50 of the two groups was about 5 μm.The SiC/B4C composite ceramic powders prepared by in-situ synthesis of H3BO3,petroleum coke and silica powders are closely bound to the second phase and have good growth conditions.The diffraction peak intensity of the phase is higher and the ratio of boron to silica is appropriate,so it is the optimal ratio of SiC/B4C composite ceramic powders.(7)The growth mechanism of B4C and SiC in the process of smelting and preparing SiC/B4C composite ceramic powder was studied.The formation of B4C can be divided into two parts:decomposition of H3BO3 by heat;The reductant carbon reacts with the decomposed product.The results of EDS analysis show that all the grains contain Si element,which may be caused by the diffusion of SiO gas.It is inferred that SiO gas is an important intermediate product in the production of SiC.During the smelting process,B2O3,B2O2,SiO,SiO2 will diffuse to the surface of petroleum coke to form target products B4C and SiC by carbothermal reduction reaction,and the micromorphology of the smelted product is lamellar and retains the original morphology of petroleum coke. |
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