Thermodynamic Study of Titanium Oxycarbide - Springer Link

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Feb 8, 2012 - The combustion enthalpy and specific heat capacity of oxycarbide were measured by differential ... reaction 0.5TiO + 0.5TiC = TiC0.5O0.5, consequently, was calculated based on the preceding ... material attracts considerable interest due to its special .... TiO2 (Anatase, 99.5 pct, Sinopharm Group Chemical.
Thermodynamic Study of Titanium Oxycarbide BO JIANG, KAI HUANG, ZHANMIN CAO, and HONGMIN ZHU Titanium oxycarbide was synthesized through carbothermic reduction of TiO2 and sintering of TiO and TiC. The combustion enthalpy and specific heat capacity of oxycarbide were measured by differential scanning calorimetry (DSC). The mixing enthalpy and Gibbs free energy of the reaction 0.5TiO + 0.5TiC = TiC0.5O0.5, consequently, was calculated based on the preceding measured data. The Gibbs free energy obtained was in good agreement with those results obtained by the equilibrium measurements. DOI: 10.1007/s11661-011-1032-1  The Minerals, Metals & Materials Society and ASM International 2012

I.

INTRODUCTION

MANY studies on the preparation and applications of titanium oxycarbide were published during the preceding decades.[1–3] Titanium oxycarbide is usually obtained through the carbothermic reduction of TiO2.[4–6] This material attracts considerable interest due to its special physical properties such as high melting points, high conductivity, high hardness, etc. Recently, the consuming anode for titanium metal extraction through the electrolysis process was extensively developed.[7,8] It is regarded as a solid solution of titanium carbide (TiC) and titanium monoxide (TiO), generally denoted as TiCxO1–x, where 0 £ x £ 1, with the same face-centered-cubic structure of TiC. However, the formation mechanism of TiCxO1–x is still not clear, and the thermodynamic measurement of titanium oxycarbide is the concern. Ouensanga reported that the CO equilibrium pressure over the condensed phases b-Ti3O5, TiCxO1–x, and C(graphite) was obtained by the reduction of rutile TiO2 with carbon in a graphite furnace.[9,10] The reduction of TiO2 by graphite produces the oxycarbide TiC0.67O0.33 in equilibrium with b-Ti3O5, CO, and C at 1580 K (1307 C), and the standard free energy of the obtained TiC0.67O0.33 was calculated to be DGof (1580 K) = 254 kJ mol–1. Hashimoto also measured the CO pressures in equilibrium with TiCxO1–x specimens, which were prepared from TiO2 and C in the temperature region from 1573 K to 2200 K (1300 C to 1927 C).[11] He summarized an equation for the standard Gibbs energy of formation of TiCxO1–x as the function of temperature and x value. Both of the preceding studies obtained the thermodynamic data directly by establishing the equilibrium of CO and TiCxO1–x, while there are some problems with this method. First, the measurement is usually feasible only at high temperatures, because at low temperatures, it will take a very long time to reach equilibrium; for BO JIANG, Postdoctoral Student, KAI HUANG and ZHANMIN CAO, Associate Professors, and HONGMIN ZHU, Professor, are with the Department of State Key Laboratory of Advanced Metallurgy, University of Science and Technology, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted January 10, 2011. Article published online February 8, 2012 3510—VOLUME 43A, OCTOBER 2012

example, at 1580 K (1307 C), it takes more than 10 days to reach the equilibrium of CO pressure.[9–11] Second, the measured composition of the oxycarbide is the value in equilibrium at the temperature, so it usually has a high carbon content (x > 0.5) because the equilibrium is under the saturation of carbon or pure TiC. The highest oxygen value Hashimoto reached in TiCxO1–x was x = 0.9. However, for the application of the consuming anode, the oxygen value is required to be equal to the carbon value, which is C/O  1.[7,8] Therefore, it is more worthwhile to measure the thermodynamic data of the titanium oxycarbide with a high oxygen value, especially around TiCxO1–x (x = 0.5). Maitre obtained the combustion enthalpies of TiC (DH695K(422C) = 1180 kJ mol–1) and TiC0.5O0.5 (DH735K(462C) = 636 kJ mol–1) using differential scanning calorimetry (DSC).[12] He also measured the specific heat capacities of TiC and TiC0.5O0.5 in the temperature range of 298 K to 1800 K (25 C to 1572 C) and gave the thermodynamics values of TiC and TiC0.5O0.5. However, the values of DH and Cp of TiC obtained by Maitre exhibited distinct differences from the JANAF data (DH695K(422C) = 1148.9 kJ mol–1),[13] and the formation Gibbs free energy of TiC0.5O0.5 calculated based on the combustion enthalpy presented a large difference from those results obtained by equilibrium measurements.[9–11] According to our knowledge, one of the reasons for the error of the data might be the impurity in samples. The sample used in the Maitre’s article was prepared by carbothermic reduction from TiO2. Through this method, it is usually difficult to obtain completely pure oxycarbide. So in this study, TiO and TiC were recommended as the reagents to synthesize TiC0.5O0.5, and this could prepare a much purer TiC0.5O0.5 than Maitre’s method, since a much easier and more homogeneous diffusion reaction occurred during the sintering at high temperature. The TiC0.5O0.5 thus obtained was used as the sample for measurement of its combustion enthalpy and the specific heat capacity in the present study.

II.

EXPERIMENTAL

The samples of titanium oxycarbide (TiC0.5O0.5) were prepared through two different routes: (1) carbothermic METALLURGICAL AND MATERIALS TRANSACTIONS A

reduction from TiO2 and (2) sintering from TiC and TiO at high temperature. TiO2 þ 2C ¼ TiC0:5 O0:5 þ 3=2COðgÞ

½1

0:5TiC þ 0:5TiO ¼ TiC0:5 O0:5

½2

Route (1) is the most conventional process adopted for the preparation of titanium oxycarbide, and the sample prepared by this route usually contains some unreduced titanium oxide if the reaction temperature is not high enough. Route (2) is the direct solid dissolution of TiC and TiO and is relatively easier to completely. TiO2 (Anatase, 99.5 pct, Sinopharm Group Chemical Reagent Co., Ltd.) and high-purity graphite (99.5 pct, Beijing Sanye Carbon Company) were used as received without any further treatment. Titanium carbide (99.9 pct) and titanium oxide (99.99 pct) were used as the raw reactants supplied by the Beijing Mountain Technical Development Center. The raw materials were first mixed according to the stoichiometric ratios through ball milling for more than 12 hours. Then the mixtures were pressed into a pellet of 10 mm in diameter and 5 mm in height and placed in a corundum crucible, sintering at 1873 K (1600 C) for more than 4 hours in argon atmosphere. The reaction extent was evaluated by the mass loss before and after sintering. The reaction products were analyzed by X-ray diffraction (XRD, M21X, MAC Science Co., Ltd., Tokyo, Japan), and the morphology of the products was examined by scanning electron microscopy (SEM, ZEISSEVO 18, Oberkochen, Germany). The combustion enthalpy and specific heat capacity were measured by DSC (STA409, NETZSCH, Selb, Germany). To obtain the combustion enthalpy (DH), a small amount of sample (ca. 10.0 mg) was put in a platinum crucible after calibrating the DSC by standard substance. The calorimeter was vacuumed to 10–2 Pa and then filled with high-purity argon. The temperature scanning rate was fixed at 1 K min–1, and oxygen was flown in with a fixed flow rate from 1073 K to 1173 K (800 C to 900 C). The measurement was repeated more than 3 times for each material. The specific heat capacity (Cp) was also measured in the temperature region from 373 K to 1273 K (100 C to 1000 C) under vacuum by DSC. The combustion enthalpy values (DHo) of TiC, TiO, and TiC0.5O0.5 can be described as the enthalpy change of the following reaction equations: TiC þ 2O2 ðgÞ ¼ TiO2 þ CO2 ðgÞ

½3

TiO þ 0:5O2 ðgÞ ¼ TiO2

½4

TiC0:5 O0:5 þ 1:25O2 ðgÞ ¼ TiO2 þ 0:5CO2 ðgÞ

½5

The mixing enthalpy (DHom) of the equation for Reaction [2] can be calculated as follows: DHom ¼ DHoðTiC0:5 O0:5 Þ  ð1=2DHoTiC þ 1=2DHoTiO Þ

METALLURGICAL AND MATERIALS TRANSACTIONS A

½6

where DHoTiC, DHoTiO, and DHo(TiC0.5O0.5) are the combustion enthalpies of TiC, TiO, and TiC0.5O0.5, respectively.

III.

RESULTS AND DISCUSSION

The X-ray diffraction spectra of titanium oxycarbide prepared by the two preceding routes are shown in Figure 1. It can be seen that the products have the same face-centered-cubic structure as that of TiC and TiO, suggesting that the main phase produced by both routes is titanium oxycarbide. However, as shown in Figure 1(b), the close-up of XRD spectra, weak peaks of Ti2O3 appeared in the route (1) products; Figure 1(c) also illustrates that the two main peaks of TiCxO1–x obtained from route (1) locate slightly in the lower angle than those from route (2). This means that the TiC content in products by route (1) is higher than that by route (2). The mass loss during the reaction also indicated that the reaction rate of route (1) is 98 pct approximately. All of these results indicate that the reduction Reaction [1] was incomplete, resulting in the remaining Ti2O3 and carbon-rich oxycarbide (TiCxO1–x, x > 0.5) instead of TiC0.5O0.5. During the sintering of samples prepared by route (2), no weight change was found, indicating that the reaction proceeded as designed. Figure 2 presents the comparison of morphologies for the samples prepared by preceding two routes, and it is shown that TiC0.5O0.5 prepared by route (2) has uniform and larger particle morphology than route (1). All of the preceding results indicate that the pure TiC0.5O0.5 was prepared by using high-purity TiC and TiO as the raw materials through reaction route (2). Therefore, only samples prepared through route (2) were used for the further thermodynamic measurements. TiC, TiO, and TiC0.5O0.5 prepared by route (2) were burned and tested in the DSC system. Figure 3 shows the DSC measurement results of TiC0.5O0.5 prepared by route (2). During the measurement, oxygen was introduced at 1073 K (800 C), and the heat emission reached the peak around 1100 K (827 C). Therefore, the combustion enthalpies obtained by this study are the values around 1100 K (827 C). Table I shows the results of the combustion enthalpies of TiO, TiC, and TiC0.5O0.5. In the same table, the JANAF data and also the Maitre’s data of TiC and TiC0.5O0.5 were listed for comparison. The value tested in this study for both TiO and TiC are slightly lower compared to JANAF, although the deviation is less than 2 pct. One of the reasons may be ascribed to the heat loss carried by the gas fluid. However, for each compound, the results were well reproduced within the error range of ±0.5 pct, and this can be accepted as the systematic error. Therefore, it is reasonable to calculate the mixing enthalpy of titanium oxycarbide by Eq. [6] to be DHom(TiC0.5O0.5) = 24.7 ± 4 kJ mol–1. The specific heat capacities of TiC, TiO, and TiC0.5O0.5 were measured under optimal conditions by DSC, respectively, in the temperature region of 373 K to

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Fig. 2—SEM micrographs of TiCxO1–x prepared by (a) route (1) and (b) route (2).

Fig. 3—DSC result of TiC0.5O0.5 powder combusted in flowing oxygen atmosphere.

Table I.

Fig. 1—(a) XRD spectra of TiCxO1–x prepared through routes (1) and (2). (b) Close-up of XRD patterns from the 2 theta angle of 30 to 55 deg and (c) from 35 to 44 deg.

1273 K (100 C to 1000 C). The results were compared with the reported ones given by Maitre and JANAF. It can be seen from Figures 4 through 6 that the present study results are in good agreement with the JANAF data, but are largely different from Maitre’s data. In Figure 6, the heat capacity of Cp(TiC0.5O0.5) is also 3512—VOLUME 43A, OCTOBER 2012

Combustion Enthalpies DHo of TiO, TiC, and TiC0.5O0.5, kJ mol–1

Compound

This Work

Maitre

JANAF

TiO TiC TiC0.5O0.5

–395.1 –1123.0 –734.4

— –1180 –636

–401.1 –1148.9 —

compared with the value of (Cp(TiC) + Cp(TiO))/2, indicating the additive feature of the heat capacity of titanium oxycarbide. This result exactly corresponds with the empirical Neumann–Kopp additive rule (NKR), which is most universal.[14] The NKR was originally applied to most alloys and currently extends to other compounds such as mixed oxides and some solid solutions in the case of no phase transformation in structure and chemical reactions. METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 4—Specific heat capacity of TiC.

Fig. 5—Specific heat capacity of TiO.

Fig. 6—Specific heat capacity of TiC0.5O0.5.

The mixing Gibbs free energy of Reaction [2] was also evaluated and compared with the literature data. Since the additive property was conformed in the heat capacity of titanium oxycarbide, it can be assumed that the mixing enthalpy is constant regardless of the temperature variation and the heat capacity of Reaction [2] DCp  0. The titanium oxycarbide was considered as random and continuous solid solution of TiC and TiO METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 7—Mixing Gibbs free energy of 0.5TiC + 0.5TiO = TiC0.5O0.5.

at different stoichiometric ratios, which are very similar in structure, as well as some physical properties, so it can be assumed that the titanium oxycarbide can be regarded as the regular solution.[15] The regular solution has a nonzero mixing enthalpy in contrast to the case of ideal solutions, and its mixing entropy is equal to that of an ideal solution with the same composition due to the random mixing without strong specific interactions, i.e., DSm(TiCxO1–x) = R[xÆln(x)+(1  x)ln(1  x)], where R is the gas constant and x is 0.5 in this work. So the standard mixing Gibbs free energy of Reaction [2] as DGom(TiC0.5O0.5) = DHom(TiC0.5O0.5) - TDSom(TiC0.5O0.5). Figure 7 presents the mixing Gibbs free energy of Reaction [2] obtained through this equation. The results reported by Maitre et al.[6] with the same method and those reported by Hashimoto[11] through direct measurement of the equilibrium CO pressure are also compared in the same figure. As can be seen in Figure 7, the data of Maitre show a wide-ranging variation from 400 K to 1300 K (127 C to 1027 C), which should be attributed to the variation of DHTiC and DHTiC0.5O0.5, especially for the variation of DCp shown in Figures 4 and 6. Another error in their data probably came from their samples of the titanium oxycarbide, since they used the samples prepared through the thermoreduction of TiO2 as Reaction [1]. As described in the sample preparation section, trace of Ti2O3 was always detected during the carbon reduction from TiO2 and the reaction hardly occurred completely. The mixing Gibbs free energy reported by Hashimoto is in good agreement with our results in several kJ mol–1, but his data show a positive temperature dependence, which is not natural for a mixing reaction of two same structure materials. This deviation might come from the extrapolation of his experimental data. Hashimoto’s measurements only covered a high carbon range of titanium oxycarbide. The highest molar ratio of TiO in TiCxO1–x was 0.1 in his equilibrium measurements. For further comparison, the Gibbs free energies of titanium oxycarbide with x = 0.95, 0.9, and 0.85 were calculated assuming the function of mixing enthalpy, DHom = kx(1 – x). It can be seen from Figure 8 that our results of the Gibbs free energy were consistent with those results obtained by the equilibrium measurement by Hashimoto at high carbon content where their experiments were conducted. The VOLUME 43A, OCTOBER 2012—3513

0.5TiC = 2TiC0.5O0.5 was correspondingly calculated as DHom(TiC0.5O0.5) = 24.7 ± 4 kJ mol–1 on the basis of the combustion enthalpy values (DH) of TiO, TiC, and TiC0.5O0.5, and the mixing Gibbs free energy of the solid solution TiC0.5O0.5 was calculated according to the regular solid solution model. The obtained results are in agreement with those obtained by the equilibrium measurements in the literature.

ACKNOWLEDGMENTS

Fig. 8—Comparison of mixing Gibbs free energy of TiCxO1–x (x = 0.95, 0.9, and 0.85) in Hashiomoto’s data and the present study.

value of the mixing Gibbs free energy, as well as its negative dependence on temperature obtained by this study, gives a good description for those experimental phenomena that the TiCxO1–x is easier to obtain at higher temperatures in the preparation through carbothermic reduction. It also confirms the rationalization of the regular solid solution model in this system.

IV.

CONCLUSIONS

The mixing enthalpy of TiC0.5O0.5 prepared by using high pure TiO and TiC as the raw materials, instead of the conventional synthesis process of TiO2 and carbon, was systematically investigated in this study. The combustion enthalpy (DH) and specific heat capacity (Cp) of the TiC, TiO, and TiC0.5O0.5 were measured by DSC, and the tested results were compared with the literature data. It is found that the Neumann–Kopp rule could be applied to the heat capacity description of the additive feature of the measured heat capacities of TiC0.5O0.5 in the temperature range of 373 K to 1273 K (100 C to 1000 C), so the TiC0.5O0.5 could be assumed as a regular solution with no heat capacity change DCp  0. The mixing enthalpy of the reaction 0.5TiO +

3514—VOLUME 43A, OCTOBER 2012

The authors gratefully acknowledge the National Natural Scientific Foundation Key Research Program (50934001/E041203), the National Research Program of China (973 Project, 2007CB613301), and the program for Changjiang Scholars and the Innovative Research Team in the University (PCSIRT, No. IRT0708) for financial support of this research work.

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METALLURGICAL AND MATERIALS TRANSACTIONS A