Study on the thermal decomposition reaction process and kinetics of SF6 and tungsten

The thermal reaction of SF 6 with tungsten powder was investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC), combined with the characterization of solid decomposition products of SF 6 and tungsten by SEM, EDS, XPS and Raman. Based on above experiments, a two-stage decomposition process of SF 6 with tungsten was proposed: the first stage is the surface vulcanization reaction W+SF 6 → WS 2 , mainly taken place at 600 ℃, with the activation energy valued at 152.8 kJ · mol -1 , according to Avrami-Erofeev equation; the second stage is the endothermic fluorination reaction WS 2 +SF 6 → WF 6 , mainly taken place at 750 ℃,, with the activation energy valued at 126.0 kJ · mol -1 , according to the Avrami-Erofeev equation.


Introduction
SF 6 is a synthetic inert gas, colorless, tasteless, non-toxic, with excellent insulation performance, and has been applied in many important fields including gas insulated switchgear, semiconductor processing, aerospace etc [1]. On the other hand, SF 6 is a greenhouse gas, whose threat to the environment is 23900 times that of CO 2 [2]. Therefore, its extensive use has attracted many researchers to understand its chemical properties. In contrast to the widely believed chemical inertia, SF 6 can react with some compounds like metal or low-valent complex under regular conditions. For example SF 6 can react with hot sodium film at 200 ℃ [3]. At -61 ℃, SF 6 can be reduced with element Cs to CsS 2 in liquid ammonia [4]. Basta et al. found that the reaction of organotitanium and organozirconium complexes with SF 6 could react quickly at room temperature or even below room temperature [5]. Metal tungsten is a key material in electronic and electrical equipment, widely used in high voltage circuit breakers and plasma etching. Due to close contact of tungsten with SF6 under these harsh conditions, decomposition of SF6 inevitably taken place. For example, SF6 corrodes copper-tungsten alloy and generates WF 6 under the action of arc [6]. SF 6 -O 2 plasma corrodes refractory metal W to generate WF 6 , WO 3 , and WOF 4 [7], and Peignon et al. believed that WS 2 was an important intermediate for further oxidation or fluorination to obtain aforementioned decomposition products [8]. Fluoridation, vulcanization and oxidation occur during tungsten and SF 6 plasma etching [8][9][10][11]. At present, although researchers have basically confirmed the decomposition products of SF 6 caused by tungsten, the relevant decomposition processes are still unclear. Thermal decomposition reaction is a classical method to study the transformation of substances, which can reveal the thermodynamics and dynamics of the transformation process. However, the apparent activation energy and thermal decomposition kinetics of SF 6 reaction with tungsten powder have not been reported. Herein we adopt synchronous thermogravimetric/differential scanning calorimetric technique investigated thermal decomposition reaction of SF 6 and tungsten powder. Based on the principle of thermal analysis kinetics, the reaction temperature of W powder in SF 6 atmosphere was determined, the apparent activation energy of the reaction was calculated, and the kinetic equation of thermal decomposition reaction was obtained. Meanwhile, the decomposition product and the change of microstructure and composition at high temperature was determined by means of environmental scanning electron microscope (SEM), X-ray energy spectrum analysis (EDS), X-ray photoelectron spectrometer (XPS) and Raman spectroscopy characterization.

Thermal decomposition reaction of SF6 and tungsten powder
The thermal decomposition behavior of SF 6 and tungsten was monitored by HCT-4 comprehensive thermal analyzer(Beijing Hengjiu Experimental Equipment Co., LTD.) to obtain the kinetic equation under following conditions: α-Al 2 O 3 crucible was used as the reference, the gas flow rate was 100 mL·min -1 , sample dosage was 30-50 mg, the heating rate was 5, 10, 20 ℃ꞏmin -1 , and the sample pool was α-Al 2 O 3 , the temperature range was 25~900℃, and the temperature is ventilated at room temperature for 60 min to ensure that the furnace is filled with SF 6 gas and then the samples are tested by TG and heat flow at different heating rates.

Preparation of thermal decomposition reaction products
Solid decomposition products were obtained under constant temperature: under the atmosphere of SF 6 , Al 2 O 3 crucible was used as the reference, the flow rate was 100 mL · min -1 , and the furnace was ventilated at room temperature for 60 min to ensure that the furnace was filled with SF 6 gas, and the heating rate was 30 ℃·min -1 to 615 ℃ for 60 min, to make sure SF 6 and tungsten powder react completely. Solid samples were collected for further characterization after the reaction.

Characterization of solid decomposition products
The morphology and size of decomposition products were characterized by Quanta 200 environmental scanning electron microscope and EDS spectroscopy, with the parameter accelerating voltage at 20 kV and accelerating current at 2 μA. The chemical composition and element binding state of the products were analyzed by Axis UltraDLD X-ray photoelectron spectroscopy (Shimadzu, Japan), and the energy reference was stained carbon C1s (Eb=284.6 eV). The chemical composition was identified by a Reflex laser Raman spectrometer with a transmittance efficiency greater than 30%, scanning range 100-4000 cm -1 , signal to noise ratio of silicon third-order peak was better than 22:1 and spectral resolution as visible full spectrum ≤1 cm -1 .

Thermal decomposition reaction of SF6 and tungsten powder
High voltage circuit breaker contacts are mainly composed of tungsten, but the research on the chemical reaction of tungsten in SF 6 atmosphere is not clear, and current research mainly focuses on the plasma etching of SF 6 and tungsten. In order to study the reaction mechanism of SF 6 and tungsten at high temperature, a thermal decomposition reaction model of SF 6 and tungsten powder was established to study the reaction process.

Fig.1 Thermal decomposition reaction of W powder
in SF 6 atmosphere It can be seen from Fig 1 that under SF 6 atmosphere, when heated at a heating rate of 5 ℃·min -1 , the weight gain of the substance occurs in the range of 457-598 ℃ with a weight gain rate of 2.6%, and the weight loss of the substance occurs in the range of 598-678 ℃ with a weight loss rate of 2.6%. The weight gain rate is basically the same as the weight loss rate. The maximum exothermic peak appears on the DSC curve at 609 ℃, which may be due to the sulfide reaction W+SF 6 →WS 2 on the surface, and the heat release is 1728.1 J·g -1 . Substantial weight loss occurs at 678-754 ℃, with a loss rate of 83.3%. At 702 ℃, an endothermic peak appears on the DSC curve, which may be caused by surface fluoridation WS 2 +SF 6 →WF 6 , the heat absorption is 3126.9 J·g -1 .

Characterization of solid products of thermal decomposition of SF6 and tungsten powder
The morphology and elements of raw materials of tungsten powder and solid products after thermal decomposition reaction of SF 6 and tungsten powder were analyzed by environmental scanning electron microscopy (SEM) energy spectrum analyzer. The results are shown in Fig 2. Fig 2a shows the raw material of 4-6 μm tungsten powder, which is distributed in irregular polygons. Fig. 2b shows the scanning electron microscopy of solid decomposition products of 4-6 μm tungsten powder at 615 ℃ for 60 min. Compared with unprocessed tungsten powder, smaller particles in μm magnitude are generated on the surface, indicating that tungsten powder has undergone chemical reaction with SF6. Fig. 2c shows the EDS spectra of 4-6 μm tungsten powder raw material, containing only W element. Fig. 2d shows the EDS diagram of tungsten powder at 615 ℃ for 60 min, which shows that the measured sample contains W, S and O elements, and their mass ratios are 76.91%, 17.06% and 6.03%, respectively. It is speculated that oxides and sulfides may be generated.   [16][17][18]. It is speculated that the formation of WS 2 is caused by the reaction between W and SF 6 , and a small amount of WO 3 may be caused by the reaction between solid decomposition products and O 2 in the air during the cooling process, but F element is not detected, which is speculated to be caused by the volatilization of WF 6 . 6 atmosphere a) Full spectrum; b) W4f; c )S2p

Kinetic analysis of thermal decomposition reaction between SF6 and tungsten powder
In order to further study the thermal decomposition reaction between SF 6 and tungsten powder and determine the most probable mechanism function F (α), the nonisothermal thermal decomposition kinetics of tungsten powder under SF 6 atmosphere was analyzed on a comprehensive thermal analyzer with different heating rates. The kinetic parameters of thermal decomposition of SF 6 with tungsten powder were calculated by using the Kissinger equation and the Flynn-Wall-Ozawa equation. Table 1 shows the kinetic parameters of the thermal decomposition of SF 6 and tungsten powder calculated at different heating rates (β). It can be seen from Table 1 that the activation energy of the first stage of the sulfurization reaction of SF 6 and tungsten powder is 152.8 kJ·mol -1 , and the activation energy of the second stage of the fluoridation reaction is 126.0 kJ·mol -1 .
Ti was obtained from DSC curves of thermal decomposit ion reaction between SF 6 and tungsten powder at differen t heating rates (β), and the apparent activation energy E α corresponding to the reaction fraction α was calculated b y Ozawa equation. α and E α were used to draw α~T and E α ~α curves, and the results were shown in Fig.5.    Table 2 shows that each G(α) and F (α) in 41 mechanism functions [19] are substituted into the General Integral, Mac-Callum-Tanner, Statava-Sestak and Agrawal equations [20]. The kinetic parameters of thermal decomposition reaction between SF 6 and tungsten powder were determined by linear regression and logical selection. By comparison and analysis of the value of activation energy E 0 , the most possible mechanism functions can be obtained as the No.

Conclusion
In summary, thermal decomposition reaction of tungsten powder with SF 6 was studied through synchronous thermogravimetric/differential scanning calorimetric technique. In addition, environmental scanning electron microscope (SEM), X-ray energy spectrum analysis (EDS), X-ray photoelectron spectrometer (XPS) and Raman spectroscopy characterization was carried out for solid breakdown products under the condition of high temperature, obtaining the change of microstructure, elements, chemical composition and structure. Based on the principle of thermal analysis kinetics, the reaction temperature of W powder in SF 6 atmosphere was determined, the apparent activation energy of the reaction was calculated, and the kinetic equation of thermal decomposition reaction was obtained. The main conclusions are as follows: (1) Tungsten powder will interact with SF 6 at 600 ℃, and will interact with SF 6 to form WS 2 and WO 3 at 615 ℃. When the temperature reaches 750 ℃, that is, before reaching the melting and boiling point, it will evaporate completely in the form of gas WF 6 .
(2) The reaction between SF 6 and tungsten can be divided into two stages. The first stage is exothermic reaction, W+SF 6 → WS 2 , ΔH = -1728.1 J · g -1 . The thermal decomposition kinetic equation of sulfide reaction is as follows: dα/dt=10 6