Low Temperature Oxidation of Titanium Nitride under High Oxygen Pressure by O2-HIP

Bo Wang, Koji Matsumaru, Hiroya Ishiyama, Jianfeng Yang, Kozo Ishizaki and Junichi Matsushita

Presented at the 2011 International Conference on Hot Isostatic Pressing Kobe, Japan, 12-14 April 2011.
Submitted: 12 April 2011, Accepted: 24 May 2011

Topics Covered

Abstract

Titanium nitride is a well-known structural ceramic. On the other hand, anatase phase of titanium oxide is a well-known photocatalyst. If we can form anatase phase on TiN, we may add an extra-function on TiN structure material. In normal oxidation process, TiN forms rutile phase as TiO₂. The present work demonstrates the possibility of forming anatase phase on TiN surface by using O₂-HIP (hot isostatic press). Anatase and rutile phases of TiO₂ on the surface of TiN ceramics were prepared by oxidizing TiN at low temperature 520 °C-550 °C under high oxygen partial pressure, which was obtained by O₂-HIP of 20 %O₂-Ar gas mixture. The oxidation behavior of TiN porous samples under O₂-HIPing treatment and the effects of gas pressure of 20 %O₂-Ar upon the formation of anatase-TiO₂ were investigated. The oxidation rate increased as the porosity and the temperature increased. The samples oxidized under ambient atmosphere have only rutile phase without anatase phase. On the other hand those oxidized under O₂-HIPing conditions contain TiO₂ with high ratio of anatase phase which can be obtained at low temperature under ~100 MPa of 20 % O₂-Ar gas mixture. The low temperature and the high oxygen partial pressure suppressed the formation of rutile-TiO₂ and accelerated the formation of anatase-TiO₂.

Keywords

TiO₂, Anatase, Rutile, Oxidation, Oxygen Partial Pressure, O₂-HIP

Introduction

Titanium nitride is of great interest for structural applications owing to its high wear resistance and good thermal stability associated with high hardness and high corrosion resistance[1]. On the other hand, in the last decade, as a kind of photocatalyst, TiO₂ has attracted much attention due to its photostability, chemical stability, nontoxicity, and a broad functionality[2, 3]. However, low efficiency of photocatalysts capability is the one of major factors that keeps it from large-scale applications. It is well known that the photocatalytic activity of TiO₂ mainly relies on its physicochemical properties, such as crystalline structure, surface area, porosity, crystallite size, and surface active sites[4-6]. Among them, crystalline structure is the most significant factor that influences the photocatalytic performance of TiO₂. Titania usually exists in three crystalline phase forms: anatase, rutile and brookite. Due to larger band gap and lower electron-hole recombination probability, the anatase phase is more active in photocatalysis than the rutile phase[7]. Much effort has been paid for growth of anatase TiO₂ thin films with enhanced photocatalytic activity. Several preparation methods for TiO₂ thin films are described in the literature, such as TiO₂ chemical vapor deposition (CVD)[8], ion-assisted techniques[9], pulsed-laser deposition (PLD)[10], and reactive RF sputtering[11]. However, most of these processes require high temperature oxidation annealing after TiO₂ film deposition in order to reduce the leakage current due to defects such as oxygen deficiency.

It is well known Titanium nitride can be oxidized to TiO₂ by high temperature treatment in air or oxygen atmosphere[12, 13]. If it is possible to form pure anatase-TiO₂ film on the surface of TiN by direct oxidation of TiN, the simple process provides an attractive technique to prepare an extra-function on TiN structural material. However, only rutile-TiO₂ on the surface of TiN is achieved by normal oxidation process. It is due to pure anatase phase has lower thermal stability than rutile, and it can be easily converted into rutile phase at 500–700 ºC.

The present work demonstrats the possibility of forming anatase-TiO₂ film on the surface of TiN is by using O₂-HIPing (hot isostatic pressing). In this process, TiN bulk was oxidized at low-temperature in 20 % O₂-Ar gas mixture atmosphere. The oxidation behavior under O₂-HIP treatment and the effect of total gas pressure of 20 %O₂-Ar upon the formation of anatase-TiO₂ were investigated.

Experimental

Three kinds of TiN porous ceramics with different relative density (SN1: 87 %; SN2: 66 %; SN3: 61%) were prepared by sintering the TiN powder at 1500 ºC for 1h under argon atmosphere. SN2 and SN3 samples were including small amount of Al₂O₃. Before oxidation, the surface of TiN ceramics were untrasonically cleaned and then in ethanol.

Isothermal oxidation tests were carried out on three kinds of TiN ceramics at 520, 550 ºC for 5 h under air atmosphere in an electrical furnace with a heating rate of 400 K/h. In contrast, the TiN ceremics were oxidized in HIP chamber (O₂-Dr. HIP, Kobe Steel Co., Ltd.) at 520, 550 ºC for 5 h with a heating rate of 400 K/h under pressures of 89, 92 and 200 MPa. Before increasing the temperature, the HIP chamber was vacuum purged and increased the HIP pressure to the final pressure backfilling with 20 % O₂-Ar gas. Each specimen was carefully weighted with 0.1 mg sensitivity before and after oxidation test, to determine the weight change caused by the oxidation process. The phase identification of the samples was performed by an X-ray diffractometer using Cu Kα radiation.

Results and Discussions

In order to obtain the oxidation process trends under different conditions, the three different conditions: 0.1 MPa air, ~100 MPa and 200 MPa 20 %O₂-Ar gases, were chosen to carry out the isothermal oxidation. The variation in weight gains is presented in Fig. 1(a) and (b) for temperature at 520 ºC and 550 ºC, respectively. Comparing Fig. 1(a) and (b), the weight gains increase as the treatment temperature increases. The SN3 samples have highest weight gain, and have the highest porosity, i. e., the largest surface area. The weight gain of the oxidation products under 20 %O₂-Ar gas atmosphere is lower than that of the oxidation products in 0.1 MPa air. It may be due to the different furnace structure. Cooling time of HIP equipment is shorter than that of normal furnaces.

Figure 1. The weight gain of TiN oxidized at (a)520 ºC, (b)550 ºC for 5 h in air and 20 % O₂-Ar atmosphere under different gas pressure.

Fig. 2(a) and (b) show X-ray diffraction patterns after oxidation at 520 ºC for 5 h under different gas atmosphere for different samples, and for SN2 under different gas pressures, respectively. Fig. 2(a) shows the XRD patterns of samples treated under 92 MPa of 20 %O₂-Ar gas mixture. Before oxidation, TiN peaks were detected and they still appeared after oxidation. A mixture of anatase and rutile TiO₂ was obtained. With increasing the porosity of the TiN ceramics (from SN1 to SN3), the peak intensity of anatile and rutile are increased. The Al₂O₃ phase is obviously appeared in the SN2 and SN3 samples. It could be seen that SN2 has relative high ratio of anatase/rutile phases. The dependence on the 20 %O₂-Ar gas pressure of phase of SN2 is shown in Fig. 2(b). The oxidized TiO₂ products contains only rutile TiO₂ phase when the sample oxidated under 0.1 MPa dry air at 520 ºC. In contrast, a mixture of anatase and rutile is obtained unde O₂-HIPing treatment at 520 ºC. Under 92 MPa 20 %O₂-Ar atmosphere，the anatase-TiO₂ peaks are strong and narrow, indicating that the anatase phase has reasonable crystallinity and crystallite size. However, as 20 %O₂-Ar gas pressure increases to 200 MPa, the TiO₂ peak intensity becames slightly lower, indicating suppressed oxidation of TiN to anatase TiO₂ phase.

Figure 2. XRD patterns of TiN ceramics after oxidation at 520 ºC for 5 h: (a)samples with different porosity under 92 MPa 20 %O₂-Ar gas; (b) sample SN2 oxidized under 0.1 MPa air and different gas pressure of 20 % O₂-Ar gas atmosphere.

Fig. 3 (a) and (b) show XRD patterns of different TiN ceramics after oxidation at 550 ºC for 5 h under 200 MPa for different samples and for SN2 under different gas atmosphere and different gas pressures, respectively. Comparing Fig. 2 and 3, the rutile-TiO₂ peaks become shaper as the temperature increases from 520 to 550 ºC. It means that the oxidation temperature determines the oxidation rate of rutile phase. Fig. 3(a) shows the XRD patterns of samples treated under 200 MPa of 20 %O₂-Ar gas mixture. Rutile-TiO₂ is detected on the surface of all samples. As the porosity increases, a weak trace of anatase-TiO₂ was detected on the surface of SN2 and SN3 sample. The anatase- and rutile-TiO₂ peak intensity of the oxidation products increase with increasing the porosity of TiN ceramics. The dependence of the peak of TiO₂ phase (SN2) on the 20 %O₂-Ar gas mixture is shown in Fig. 3(b). Under O₂-HIP condition with 20 %O₂-Ar gas, the rutile-TiO₂ peaks intensity decreases and the anatase-TiO₂ peaks intensity increases, indicating that the high pressure 20 %O₂-Ar suppresses the formation of rutile-TiO₂ and accelerates the oxidation of TiN to anatase-TiO₂.

Figure 3. XRD patterns of TiN ceramics after oxidation at 550 ºC for 5 h: (a) samples with different porosity under 200 MPa 20 %O₂-Ar gas; (b) sample SN2 oxidized under 0.1 MPa air and different gas pressure of 20 %O₂-Ar gas atmosphere.

Figure 4. The peak intensity (XRD) ratio of anatase and rutile TiO₂ phase of oxidized products under different conditions; TiN ceramics were oxidated at (a)520 ºC , (b)550 ºC for 5 h in air and 20 % O₂-Ar atmosphere under different gas pressure.

Fig.4 shows the peak intensity (XRD) ratio of anatase-TiO₂/TiN and rutile-TiO₂/TiN of different samples under different temperature. The ratio of the two different crystal forms was calculated according to the peak intensity ratio of IA/ITiN and IR/ITiN. As shown in the Fig. 4(a), when the TiN was treated at 520 ºC, the ratio of IR/ITiN was considerably decreased from 0.1 MPa air to 200 MPa 20 %O₂-Ar gas. The SN3 with porosity 62 % had highest ratio 0.71 of IR/ITiN. However, The peak intensity ratio of IA/ITiN was slight increased from 0.1 MPa air to 92 MPa 20 % O₂-Ar gas and then decreased from 92 MPa to 200 MPa 20 %O₂-Ar gas. The oxidation product TiO₂ containing 45.6 % of anatase and 54.4 % of rutile was obtained when the TiN ceramic was treated sample SN2 at 520 ºC for 5 h under 92 MPa of 20 %O₂-Ar gas. It suggested that high ratio of anatase-TiO₂ products could be obtained under appropriate gas pressure of 20 % O₂-Ar gas, which would show superior photocatalytic activity. The tendency of the ratio of IA/ITiN and IR/ITiN was similar for the samples oxidized at 550 ºC (as seen in Fig.4 (b)). However, the ratio of IR/ITiN was lower than the samples treated at 520 ºC, which indicated that a large amount of rutile-TiO₂ was formed at high temperature. It is in agreement with the weight gains of the oxidation products.

The Oxidation reaction of TiN is as following: TiN(s)+O₂(g)=TiO₂(s)+1/2N₂(g). The whole oxidation process can be considered a gas–solid reaction and the process can be separated into three stages: (i) The initial oxidation rate-controlling step of chemical reaction of thin oxidation layer; (ii) Oxygen transfer from the bulk of gas flow to the surface and the interface by diffusion, and then oxygen reacted with TiN produce TiO₂ and nitrogen; (iii) Nitrogen transfers from the interface through gas-solid boundary into the gas flow, or nitrogen dopes in TiO₂ [14]. All the diffusion and reaction process are associated with oxygen partial pressure, the total pressure and the porosity. In the initial stage of the reaction, high oxidation rate could be obtained under high oxygen partial pressure. In the middle stages, the oxidation process was controlled by mixed chemical reaction control and diffusion control. In the last stage, normally it is not considered the possibility of the total gas pressure influence. Atomic nitrogen diffuses in a solid. On the surfaces of solid, nitrogen atoms form nitrogen molecules. At this last stage of nitrogen gas formation, the high gas total pressures suppress the gas formation. When the porosity increases, the diffusion of oxygen and nitrogen also increases, and leads to high oxidation rate. As shown in the Fig.1, the weight gain of SN2 and SN3 is higher than that of SN1.

On the other hand, under high 20 %O₂-Ar gas pressure conditions, the total gas pressure has significant effect on the stability of a gas-solid reaction and the sintered product phase as an independent variable[15]. In combination of the results, it clearly demonstrated that the high 20 %O₂-Ar gas pressure suppresses the formation of rutile- TiO₂ and accelerates the oxidation of TiN to anatase-TiO₂. The high 20 %O₂-Ar gas pressure may contribute to the phase transformation of TiO₂ from rutile to anatase at low temperature. It reverses the results comparing to the oxidation behaviour under 0.1 MPa air. Usually, the anatase to rutile transformation would happen between 500 ºC and 600 ºC during the oxidation of titanium compounds under 0.1 MPa air[3].

Conclusions

TiO₂ with a mixed anatase/rutile phase on the surface of TiN ceramics are fabricated by treating TiN ceramics at low temperature 520 ºC-550 ºC under high gas pressure of 20 % O₂-Ar gas. The high 20 %O₂-Ar gas pressure can suppress the formation of rutile-TiO₂ and accelerate the oxidation of TiN to anatase-TiO₂. Comparing to the oxidation products in 0.1 MPa air, containing almost only rutile phase, high ratio of anatase/rutile phase TiO₂ can be obtained when the TiN is treated at 520ºC for 5 h under 92 MPa of 20 %O₂-Ar gas. The oxidation rate of TiN ceramics is increased with larger porosity and higher temperature.

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Contact Details

Bo Wang, Koji Matsumaru, Hiroya Ishiyama, and Kozo Ishizaki
Nagaoka University of Technology, Nagaoka, 940-2188, Japan

Jianfeng Yang
Xi’an Jiaotong University, Xianning West Road No. 28, 710049 Xi’an City, Shaanxi Province, China

Junichi Matsushita
Tokai University, 1117 Kitakaname, Hirastsuka 252-1292, Japan

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 13[1] (2011) 7-13.