THERMODYNAMIC ANALYSIS OF INTERACTION OF CARBON WITH TiO 2 , ZrO 2 , AND B 2 O 3 AND OBTAINING OF MIXTURE OF BORIDES AND CARBIDES OF TITANIUM AND ZIRCONIUM

Metal borides and carbides, TiB2/TiC and ZrB2/ZrC are widely used nanostructured composite materials. A detailed thermodynamic analysis was performed to determine the formation conditions of titanium and zirconium borides and carbides in the Ti–B–O–C and Zr–B– O–C systems. The complete thermodynamic analysis was carried out in vacuum for the reactions 2TiO2 + B2O3 + 8C = TiB2 + TiC + 7CO and 2ZrO2 + B2O3 + 8C = ZrB2 + ZrC + 7СО. On the basis of the theoretically found results, experimental synthetic routes were developed to prepare TiB2/TiC and ZrB2/ZrC composite materials.


INTRODUCTION
The nanostructured composite materials, including metal borides and carbides containing ones like TiB2/TiC and ZrB2/ZrChave a unique set of physical and chemical properties (high hardness, heat resistance, high-temperature strength, electrical and thermal conductivities, and resistance to the molten metals in combination with low specific weight, corrosion-, radiation-and wear-resistance).
There are some known methods to obtain of these composites, 1-7 but a complete thermodynamical analysis 8 of the system to optimize their synthesis conditions have not been performed yet.In this work, the comprehensive thermodynamical analysis has been conducted to find optimal conditions for preparation of carbides and borides of zirconium and titanium.

EXPERIMENTALS
Computer calculations were carried out with the ASTRA-4 program described in [8]in the temperature range 500-2000K with the step of 50K for vacuum conditions.The mixture of ZrB2-ZrC was prepared by mixing the powders of ZrO2, B2O3 and C followed by 30h grinding in a high power (1000rpm) mill.The mill was a piece of special equipment designed and built in our laboratory.After briquetting, the obtained powder was sintered in a high vacuum oven under an argon atmosphere at ∼1400°C for 5 h.

System Ti-B-O-C
The thermodynamical analysis of Ti-B-O-C system in vacuum is carried out for the reaction: 2TiO2+B2O3+8C=TiB2+TiC+7CO The following species were considered as possible condensed and gaseous components: C, Ti, TiO, Ti2O3, TiO2, Ti3O5, Ti4O7, TiC, TiCO0.04,TiC0.10O,TiC0.40O0.60,TiC0.75O0.25,B, B2O3, B4C, TiB, TiB2, and Ar, O, O2, O3, C, C2, C3, C4, C5, CO, CO2, C2O, C3O2, Ti, TiO, TiO2, B, B2, BO, BO2, B2O, B2O2, B2O3and TiB,.The knowledge of reliable thermodynamic data of the reactive system components is necessary.Since some thermodynamic characteristics (∆H298, Tm., ∆Hm., Cp, and Cp(L)) of the abovementioned oxycarbides cannot be obtained from the literature, the corresponding thermodynamic constants of oxycarbides were calculated. 9e main results of the thermodynamic calculations for the Ti-B-O-C system are presented in Figure 1.It is evident that the reduction of TiO2 begins above 900 K, and Ti2O3, Ti3O5 and Ti4O7 are allocated in the condensed phase.Their amounts increase to ∼1000 K, but raising the temperature their amount started to decrease, and at∼1100 K all of the titanium oxides disappear entirely.It appears some amount of condensed carbon and simultaneous allocation of CO in the gas phase begin at temperatures higher than 900K.At ∼1200 K, the condensed carbon disappears and the amount of CO reaches its maximum which does not change further.At ∼1000 K appears the allocation of TiC which amount sharply increases to ∼1100 K and reaches its maximum (∼29 wt.%).Increasing the temperature, its amount decreases to ∼1200 K and its amount (∼18 wt.%) does not change until 1600 K.
The thermodynamic analysis showed that the experiments have to be conducted in vacuum at temperatures higher than 1200K to obtain the requested TiB 2-TiC mixtures.
The condensed carbon amount smoothly changes around ∼1100 K, but above this temperature its amount sharply decreases and completely disappears at ∼1350 K.The condensed zirconium carbide ZrC is allocated above 1300 K, its amount drastically increases up to ∼1350K reaching 25 wt.%, but above this temperature there are no further changes in its amount.Thermodynamic analysis of reaction 2 showed that the experiments should do above 1350 K.in vacuum to obtain the expected ZrB2 and ZrC,

RESULTS
Based on the results of thermodynamic calculations, some tests have been done to prepare TiB2-TiC mixtures from the mixture of TiO2 and B2O3 with C (graphite) or with ZrO2-B2O3/graphite mixtures in a high-temperature furnace in the argon atmosphere at ∼1400°C for 3h.The X-ray diffraction patterns of the prepared powder are given in Figure 3.As can be seen, the product is ca.1:1 mixture of TiB2 and TiC.

Figure 1 .
Figure 1.Dependence of components content on temperature for reaction (1) in vacuum (0.0001 atm) in temperature interval 700-1600 K.

Figure 2 .
Figure 2. Dependence of components content on temperature for reaction (2) in vacuum (0.0001 atm) in temperature interval 1100-1500 K.

Figure 3 .
Figure 3.The X-ray diffraction pattern of a mixture of TiB2 and TiC prepared from TiO2-B2O3/graphite mixture at 1400 °C in 3 h

Figure 4 .
Figure 4.The x-ray diffraction pattern of a mixture of ZrB2 and ZrC prepared from ZrO2-B2O3/graphite mixture at 1400 °C in 3 h