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Thermodynamic Simulation of Gas Phase Hydrogen Reduction Synthesis of Nb3Sn Powder
Nb 3 Sn is a typical low-temperature superconducting material that maintains its superconducting properties at higher field strengths and higher temperatures than other materials, and its critical current density is as high as 3000 A·mm -2 (12T). , 4.2K), almost all industrial superconducting devices, such as high-energy physical devices, thermal fusion experimental devices, alternators and superconducting force transmission devices, etc., use Nb 3 Sn as a superconductor. However, Nb 3 Sn is brittle and very sensitive to stress and difficult to process. The most commonly used methods are bronze, internal tin method, powder method and MJR tubing method. If Nb 3 Sn powder can be synthesized first, then Nb 3 Sn superconducting wire can be manufactured by powder tube technology or sintered to form a Nb 3 Sn superconducting block, which has the advantages of uniform composition, small prestress, and fine grain. And it is possible to significantly reduce the heat treatment time.
Synthesis by chemical vapor deposition and metal alloy ultra-fine powder with high purity, fine grain size, particle size controllability, etc., and the method has a simple process, continuous production, short flow characteristics, which is widely attention. Recently, Zhu Jun et al. synthesized ultrafine Nb 3 Sn powder at 1000 ° C using hydrogen reduction ruthenium and tin chloride. In this method, the temperature of the two evaporation zones, the temperature of the reaction zone, the temperature of the collection zone, and the amount of different carrier gas Ar(g) and reducing gas H 2 (g) are the main experimental parameters, and the number of these parameters is numerous and affects each other. Experiments alone to determine reasonable synthesis conditions will require a lot of work. In this paper, the thermodynamic process model of synthesizing Nb 3 Sn powder using this method is established by using the principle of multivariate multiphase equilibrium calculation. The influence of these experimental parameters on product type, purity and metal recovery rate is calculated by thermochemical integration software FactSage, and then reasonable Nb is determined. 3 Synthesis conditions of Sn.
First, the experiment
The experimental setup is shown in Figure 1. The entire quartz reactor can be divided into three zones: NbCl 5 (s) and SnCl 2 (s).
After evaporating in the evaporation zones A1 and A2 to T A1 and T A2 , respectively, the resulting NbCl 5 (g) and SnCl 2 (g) are carried into the reduction reaction zone B by the carrier gas Ar (g); In the case where NbCl 5 (g) and SnCl 2 (g) are mixed, H 2 (g) is introduced at a high temperature T B , and Nb 3 Sn(s) generated by the reduction reaction enters the collection zone C under the action of the gas stream. In the collection zone C, the Nb 3 Sn(s) formed by the reaction was collected on a filter, and the exhaust gas was absorbed using pure water.
Figure 1 Schematic diagram of the experimental device
   Second, thermodynamic process model and thermodynamic data
For the above experimental apparatus and method, the following hypothesis is made: there are four local equilibriums in the evaporation zone A1, A2, reaction zone B and collection zone C, respectively. The species phase, number and phase composition in each zone can be balanced by multivariate multiphase Calculated. (1) In the evaporation zones A1 and A2, NbCl 5 (s) and SnCl 2 (s) and Ar(g) reach evaporation equilibrium at temperatures T A1 and T A2 ; (2) Evaporation equilibrium solid phase products remain in evaporation In zone A1, the gas phase product enters reaction zone B; (3) in reaction zone B, the vaporized equilibrium vapor phase product is mixed with H 2 (g) at a temperature T B to achieve a reduction equilibrium; (4) in the collection zone C, reduction equilibrium The gas phase product reaches a collection equilibrium at a temperature of T C. The equilibrium gas phase product exits the reactor as a final off-gas, and the solid phase product and the reduced equilibrium solid phase product are the final products of the entire process. The material flow and unit operation of the entire synthesis process can be expressed as the process model shown in FIG. 2.
Figure 2 Process model for the preparation of Nb 3 Sn by hydrogen reduction
In the above model, the four local balances are the multivariate multiphase equilibrium of the complex system. This paper is calculated by Equilib, the balance calculation module of the thermal chemical integration software FactSage. In the thermodynamic data used in the calculation, the Nb-Sn binary system was taken from the thermodynamic data rigorously evaluated by Toffolon et al. using the CALPHAD method, which consisted of three pure phase phases (BCT - A5 phase Sn, DIAMOND - A4 phase Sn and compound NbSn 2). And four solution phases (Nb-Sn liquid phase, N2 solid liquid of A2 structure, Nb 3 Sn phase of C15 structure and Nb 6 Sn 5 phase); the rest of the data were taken from the pure substance database of FactSage. The system includes five components such as Nb, Sn, Cl, H, Ar, etc. The phases consisting of these five components are: (1) 1 gas phase: containing H (g), H 2 (g), Cl ( g), Cl 2 (g), HCl (g), Ar (g), Nb (g), NbCl 4 (g), NbCl 5 (g), Sn (g), Sn 2 (g), SnH 4 ( g), 14 species such as SnCl 2 (g), SnCl 4 (g), (2) 14 solid phases: NbCl 2 , NbCl 3 , NbCl 4 , NbCl 5 , Nb 3 Cl 7 , Nb 3 Cl 8 , SnCl 2 , A5-Sn, A4-Sn, NbSn 2 , A2-Nb, Nb 3 Sn, Nb 6 Sn 5 ; (3) 4 liquid phases: NbCl 5 , SnCl 2 , SnCl 4 , Nb-Sn.
Third, the results and discussion
(1) Evaporation zone temperature and carrier gas Ar (g) dosage
In order to make the hydrogen reduction reaction product Nb 3 Sn powder, it is necessary to make the concentration ratio of NbCl 5 (g) and SnCl 2 (g) in the reaction zone B to 3:1, and NbCl 5 (g) and SnCl 2 in the reaction zone B. The concentration ratio of (g) is determined by the ratio of the amount of NbCl 5 (g) and SnCl 2 (g) entering the reaction zone from the evaporation zones A1 and A2 at each time, and is ultimately determined by the evaporation zone temperature T A1 , T A2 and the respective carrier gas Ar (g) ratio.
3(a) and (b) are the amounts of the evaporation zone temperature and the amount of carrier gas Ar(g) in the evaporation zones A1 and A2, respectively, for the amount of NbCl 5 (g) and SnCl 2 (g) entering the gas phase. influences. It can be seen from the figure that when the temperature of the evaporation zone is constant, as the amount of carrier gas Ar (g) increases, the amount of substances entering the gas phase of NbCl 5 (g) and SnCl 2 (g) increases until NbCl 5 (s) and SnCl 2 (s) disappear completely; and in the same carrier gas Ar (g) amount, as the evaporation temperature increases, the substances entering the gas phase of NbCl 5 (g) and SnCl 2 (g) The amount increases until NbCl 5 (s) and SnCl 2 (s) completely disappear. Therefore, increasing the evaporation temperature and increasing the amount of carrier gas Ar (g) contribute to the increase of the amount of NbCl 5 (g) and SnCl 2 (g) substances entering the reduction reaction zone, according to Fig. 3 (a) and Fig. 3 (b), A combination of a combination of evaporation temperature and carrier gas usage can be selected to achieve a ratio of the amount of material entering NbCl 5 (g) and SnCl 2 (g) in reaction zone B of 3:1. Under laboratory conditions, it is possible to maintain a lower amount of carrier gas and adjust the temperature of the two evaporation zones to achieve a ratio of NbCl 5 (g) to SnCl 2 (g) in the gas phase of 3:1. When the amount of the carrier gas and NbCl 5 (g) and SnCl 2 (g) ratio is 1:1 respectively, satisfy the gas phase NbCl 5 (g) and SnCl 2 (g) the molar ratio of 3:1 value, corresponding to the evaporation temperature See Table 1.
Figure 3 The amount of NbCl 5 (g) in the evaporation zone A1 at different temperatures (a) and different evaporation zones
Relationship between the amount of SnCl 2 (g) substance in A2 (b) and the amount of carrier gas Ar (g)
Table 1 The ratio of the amount of NbCl 5 (g) and SnCl 2 (g) substances is 3:1 corresponding evaporation temperature
Evaporation temperature of NbCl 5 (s)/°C
150
170
190
210
230
Amount of NbCl 5 (g)/mol
0.0171
0.0569
0.1838
0.6093
0.9999
Evaporation temperature of SnCl 2 (s)/°C
345
382
423
467
484
Amount of SnCl 2 (g)/mol
0.0057
0.0190
0.0613
0.2031
0.3333
(2) H 2 (g) dosage and reaction zone temperature
Since both Nb and Sn have chlorides of various valence states, the presence of these chlorides will make the amount of H 2 (g) as a reducing agent much larger than that of NbCl 5 (g) and SnCl 4 (g). The amount required for the alloy calculation. Therefore, in the reduction zone B, the reduction temperature and the amount of hydrogen will determine the completeness of the reduction reaction and the type of product formed, which has an important influence on the recovery rate and product purity of the entire process. When the evaporation temperature of NbCl 5 (s) is 230 ° C, the evaporation temperature of SnCl 2 (s) is 484 ° C, and the molar ratio of carrier gas Ar (g) to NbCl 5 (s) and SnCl 2 (s) is 1, According to Table 1, the amount of NbCl 5 (g) and SnCl 4 (g) substances entering the reaction zone at this time was 0.9999 mol and 0.3333 mol, and if completely reduced, 0.3333 mol of Nb 3 Sn(s) was formed. When the reduction reaction temperatures were 900, 1000, 1100 and 1200 ° C, the relationship between the amount of hydrogen used and the yield of Nb 3 Sn(s) was shown in Fig. 4. As can be seen from the figure, in order to convert NbCl 5 (s) and SnCl 2 (s) entering the reduction reaction zone into Nb 3 Sn(s) as much as possible, the amount of H 2 (g) used is excessively large, in order to achieve The recovery rate of 99% or more, the minimum H 2 (g) required for each of the above reaction temperatures is shown in Table 2.
Fig. 4 Relationship between conversion rate of Nb 3 Sn(s) and H 2 (g) usage in different reduction reaction temperatures
Table 2 H 2 (g) usage required for conversion of Nb 3 Sn(s) to 99% at different reduction reaction temperatures
Reduction temperature/°C
900
1000
1100
1200
Least amount of H 2 (g) / (mol·mol -1 Nb 3 Sn(s))
570
780
830
650
(3) Collection zone temperature
When the gas phase product of the reduction reaction zone enters the collection zone C, a solid product may be formed due to a decrease in temperature, and Nb 3 Sn(s) formed by the product reduction reaction is collected on the filter sheet, thereby reducing the purity of the final product. .
Compared with the initial conditions calculated in (b), when the reduction reaction temperature is 1000 ° C and the H 2 (g) amount is 250 mol, the reduction reaction will produce 254.83 mol of the gas phase product, the main component of which is 96.993% of H 2 ( g), 2.22% HCl (g), 0.78482% Ar (g) and a small amount of SnCl 2 (s), NbCl 5 (s) and other chlorides. When the above gases were collected at different temperatures, the amount of the solid phase product formed was shown in Table 3. It can be seen from the table that when the temperature of the collection zone is increased, it is beneficial to reduce the formation of new solid phase products in the collection zone. When the Nb 3 Sn(s) recovery rate of the reduction reaction is higher than 99%, the temperature of the collection zone is controlled to be 400. At °C, the molar content of Nb 3 Cl 8 (s) in the final product is 0.01% or less.
Table 3 Contents of solid impurities in final products at different collection temperatures
Temperature/°C
20
50
100
200
300
400
SnCl 2 (s)
0.00445
0.00445
0.0444
0.00247
0
NbCl 3 (s)
0.00059
0
0
0
0
0
Nb 3 Cl 8 (s)
0
0.00020
0.0020
0.0002
0.0002
0.0001
Fourth, the conclusion
The thermodynamic model of hydrogen reduction synthesis of Nb 3 Sn powder was developed based on the principle of multivariate multiphase equilibrium. The evaporation temperature, carrier gas Ar (g) dosage, reduction reaction temperature, H 2 (g) dosage, and collection were studied by thermochemical software FactSage. The influence of key key parameters such as zone temperature on the synthesis product. The calculation results show the minimum H 2 (g) amount required for the optimum evaporation zone evaporation temperature and carrier gas usage, 99% conversion at different reduction reaction temperatures, and the content of impurities in the final solid product at different collection zone temperatures. It provides the necessary thermodynamic basis for large-scale synthesis of Nb 3 Sn powder.