Abstract
The thermal behavior of Al2O3/NH4HF2 mixtures with different mass ratios of NH4HF2:Al2O3 were analyzed by simultaneous thermogravimetry and differential thermal analysis (TG-DTA) and the critical temperature of DTA curve were determined. The morphologies and phases of the products obtained by direct thermal treatment before and after each critical temperature were further analyzed. The results show that the mass ratio has no influence on the critical reaction temperature and processes. The fluorination starts at room temperature with the formation of (NH4)3AlF6, which dominates at 162.3~162.8 °C and is completed around 180 °C. After further heat-treatment, (NH4)3AlF6 decomposes sequentially through a two-step decomposition reac-tion with the formation of NH4AlF4 at 249.8~250.1 °C and finally decomposes to β-AlF3 at 356.8~357.7 °C. The transformation of β-AlF3 to α-AlF3 occurs at 400~650 °C.
Science Press
Anhydrous AlF3 can be used as cathode materials for lithium battery[1,2], fluoride fiber materials[3] or raw materials related to aluminum electrolyte[4]. However, AlF3 prepared through hydrometallurgical process always contains crystal water. If heating aluminum fluoride containing crystal water, alumina will be formed because of the hydrolysis reaction of aluminum fluoride[5,6]. Therefore, it is important to develop a process for preparing pure AlF3 without water. Sublimation under vacuum is commonly used[7], but the experimental equipment is strictly required. Anhydrous fluorides can be prepared by a non-aqueous dry route through the fluorination of metal or oxides using fluorine gas (F2)[8], hydrogen fluoride gas (HF)[9,10], aqueous hydrofluoric acid (HF)[11,12], ammonium fluoride (NH4F)[13] and ammonium bifluoride (NH4HF2)[14,15]. Among them, fluorine, hydrogen fluoride and aqueous hydrofluoric acid are corrosive and poisonous gases, and thus are difficult to handle. NH4F is highly hygroscopic and there is a possibility for oxygen contamination[16,17]. Thus, a large quantity of NH4F should be added in order to produce a pure fluoride[13]. Therefore, NH4HF2 is considered as an appropriate fluorination agent for obtaining high pure fluoride because HF generated by the decomposition of NH4HF2 will inhibit the hydrolysis of aluminum fluoride. The melting and decomposition point of NH4HF2 is 126.8 and 238.8 °C, respectively. NH4HF2 is a solid without any environmental danger at room temperature; whereas it becomes a powerful fluorinating reagent when heated. It is reported that NH4HF2 can react with different Al2O3-containing minerals such as beryl[15], nepheline[18], non-bauxite[19], kyanite[20,21] and α-spodumene[22] to form (NH4)3AlF6 or NH4AlF4 according to following reactions:
After further heat-treatment, (NH4)3AlF6 starts to decom-pose and AlF3 forms through several steps at different temperatures[15,19-27], as listed in Table 1.
Table 1
Decomposition process of (NH4)3AlF6
| Mechanism | Reference | Reaction progress |
|---|
|
One-step |
[20, 21] |
(NH4)3AlF6AlF3 |
|
[19] |
(NH4)3AlF6AlF3 |
|
Two-step |
[22] |
(NH4)3AlF6 NH4AlF4AlF3 |
|
[15] |
(NH4)3AlF6 NH4AlF4AlF3 |
|
[18] |
(NH4)3AlF6NH4AlF4AlF3 |
|
[23] |
(NH4)3AlF6NH4AlF4AlF3 |
|
[24] |
(NH4)3AlF6NH4AlF4AlF3 |
|
Three-step |
[25] |
(NH4)3AlF6NH4AlF4(NH4F)0.69AlF3AlF3 |
|
Several step |
[26] |
(NH4)3AlF6NH4AlF4→AlF3·(0.8~0.9)NH4F→AlF3·(0.1~0.2)NH4F→AlF3·(0.02~0.06)NH4FAlF3 |
For example, Rimkevich et al[19-21] indicated that (NH4)3AlF6 formed by fluorination of kyanite or non-bauxite directly decomposed into AlF3 at 275~282 or 320 °C, respectively. However, Thorat[15], Makarov[18], Resentera[22], Kraidenko[23] and Shinn et al[24] indicated that a two-step progress occurs with the increment of temperature: (NH4)3AlF6→NH4AlF4→AlF3. Shinn et al[24] further pointed out that an intermediate phase with variable composition AlF3·(0.75~0.90)NH4F may exist between NH4AlF4 and AlF3. Hu et al[25] found that a three-step progress occurs: (NH4)3AlF6→NH4AlF4→(NH4F)0.69AlF3→AlF3. However, Menz et al[26] found that the decomposition of NH4AlF4 is a multi-stage process and the range of existence of intermediates is expanded with the increment of pressure: NH4AlF4→AlF3·(0.8~0.9)NH4F→AlF3·(0.1~0.2)NH4F→AlF3·(0.02~0.06)NH4F→AlF3. Resentera[22], Hu[25] and Menz[26] et al, further found that the increment of heating rate increases the corresponding decomposition tempe-rature. Therefore, the reaction process is quite complicated. Thus, the analysis of the fluorination process of Al2O3 by NH4HF2 and the decomposition of (NH4)3AlF6 is beneficial to achieving an effective fluorination process.
In this work, the possible reactions involved during fluorination progress with different mass ratios of NH4HF2 to Al2O3 were simultaneously analyzed by thermogravimetry (TG), derivative thermogravimetry (DTG) and differential thermal analysis (DTA). The critical reaction temperatures of DTA curves under different mass ratios of NH4HF2:Al2O3 were determined. In addition, the morphologies and phases of the products obtained before and after the critical temperatures were prepared using direct thermal treatment and further analyzed.
Commercially analytical grade Al2O3 (99.8wt%) and ammo-nium bifluoride (99.5wt%) used in this study are from Sinopharm Group (China). According to reaction (1) and (2), the mass ratio of NH4HF2:Al2O3 for completed fluorination is 3.3530 and 2.2353, respectively. In order to investigate the reaction progress, two mass ratios were chosen for investigation: one is 2.5, which is higher than the value for the formation of NH4AlF4 but lower than that of (NH4)3AlF6; the other is 3.5, which is higher than the value for the formation of (NH4)3AlF6.
Thermalgravimetric analysis (TGA)-differential thermal an-alysis (DTA) of Al2O3/NH4HF2 mixtures were carried out in a Shimadzu DTG-60 unit at a rate of 5 °C/min from 25 °C to 600 °C under N2 gas flow of 20 mL/min. DTG curves were obtained as the first derivative of the TGA curves.
After TGA-DTG-DTA analysis, the critical reaction temperatures of DTA curve were determined. In order to analyze the composition, morphologies and phases of products before and after each reaction stage, Al2O3 was firstly mixed with NH4HF2 (mass ratio of NH4HF2:Al2O3 is 2.5 or 3.5), put into a pure nickel crucible, and then placed in a modified tubular furnace consisting of a reactor and a two-zone condenser made from nickel (NP-2 grade) for fluorination test. The heating procedure was carried out at a rate of 5 °C/min under the flow of N2 (purity>99%). Once the selected temperature was reached, the sample remained isothermally for 1 h, and then was cooled down to room temperature for further characterization.
The morphologies, composition and phases of Al2O3 powder, NH4HF2 agent and products obtained were analyzed by Camscan MX2600FE type-scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) (Oxford Instruments, INCA) and D/Max-2500 pc type X-ray diffraction (XRD).
2.1 Microstructure of as-received coating
Fig.1a and Fig.1b show the SEM morphology and the corresponding EDS results of Al2O3 powders. Clearly, Al2O3 particles exhibit nearly spherical morphology with average particle size of 80~100 µm, which is consistent with particle size analysis, as shown in Fig.1c. EDS results in Fig.1b fur-ther indicate that only Al and O are detected. The atom ratio of Al:O is close to 2:3, which coincides with the chemical formula of Al2O3. XRD patterns in Fig.2 verify that it is Al2O3, a mixture of highly disordered α-Al2O3 and β-Al2O3. However, the broad peak shows that some Al2O3 particles are of micro- or nano-size dimension. Fig.3 shows the XRD pattern of NH4HF2 agent. The peaks are thin with a large ratio of peak to background, indicating that the structure is crystalline.
Fig.1 SEM image (a), corresponding EDS results of Al2O3 powder (b) and particle size analysis (c)
Fig.2 XRD patterns of Al2O3 powder
Fig.3 XRD patterns of NH4HF2 powder
2.2 Thermal analysis of fluorination of Al2O3 with NH4HF2
Fig.4 shows the TGA-DTG-DTA curves of Al2O3/NH4HF2 mixture with different mass ratios between 25 and 600 °C. Clearly, according to DTA curve, there are four apparent endothermic peaks between 25 and 600 °C and the mass ratio has no influence on the processes and the peak temperature: 126.8, 162.3~162.8, 249.8~250.1 and 356.8~357.7 °C. The first peak with minor mass loss at 126.8 °C is due to the melting of NH4HF2[22], which is partially overlapped with the second peak 2. Peaks 2~4 concede well with the peaks in the DTG curve with a large mass loss, especially for peaks 2~3. In this temperature range, Al2O3 does not decompose or change structurally[27]. Above results indicate that the formation of peaks 2~4 is due to the chemical reaction. From TGA and DTG curves, it can be further found that the mass loss begins at the room temperature with appreciable rate and sharply increases at 126.8 °C, and then reaches the highest value at peak 2, suggesting the chemical reaction between Al2O3 and NH4HF2 to form fluorides. The total mass loss after peak 2 for both mass ratio of NH4HF2:Al2O3 is 22%~23%. After peak 2, the mass loss slowly increases to 230 °C, and increases sharply at peak 3 and ends at 273 °C, suggesting that a new reaction occurs. The mass loss during the peak 3 is 20.2% and 25.8% for the mass ratio of 2.5 and 3.5, respectively. After 273 °C, the mass loss slowly increases up to peak 4. For peak 4 at about 356.8 °C, another mass loss peak occurs again, indicating another chemical reaction. The mass loss during peak 4 for both mass ratio of NH4HF2:Al2O3 is 13.4%~14.4%. After 360 °C, only minor mass loss occurs and the residues at 600 °C are 43.4% and 37.6% for mass ratio of 2.5 and 3.5, respectively.
Fig.4 TGA-DTG-DTA analysis of Al2O3/NH4HF2 mixtures with different mass ratios at 30~600 °C at a heating rate of 5 °C/min: (a) NH4HF2:Al2O3=2.5:1 and (b) NH4HF2:Al2O3=3.5:1
Based on above results, it can be found that three chemical reactions occur between 25 and 600 °C. The first chemical reaction starts at the room temperature, and sharply increases with the melting of NH4HF2, which dominates at 162.3~162.8 °C and is completed at 180 °C. The second chemical reaction starts at 235 °C, dominates at 249.8~250.1 °C and finishes at 273 °C. The third chemical reaction starts at 320 °C, dominates at 356.8~357.7 °C and is completed at 360 °C.
2.3 Characterization of the fluorination products of Al2O3
In order to identify and analyze the products involved in TGA-DTG-DTA curves of Fig.4, samples produced using a modified tubular furnace at different temperatures before and after each peak in the DTA curves were analyzed by XRD, as shown in Fig.5. Clearly, after 1 h heat treatment at 135 °C, (NH4)3AlF6 (#22-1036) with minor Al2O3 is observed for low mass ratio of 2.5:1; while (NH4)3AlF6 (#22-1036) with minor NH4HF2 (#12-0302) is observed for high mass ratio of 3.5:1. Between 150~180 °C, the peaks of Al2O3 and NH4HF2 at 135 °C significantly decrease and no new phases emerge under both mass ratios. At 270 °C, the peak of Al2O3 disappears and only NH4AlF4 is detected under both mass ratios. At 360 °C, the peak of NH4AlF4 disappears and only AlF3 (#43-0435) is identified.
Fig.5 XRD patterns of Al2O3/NH4HF2 mixtures after thermal treatment at different temperatures for 1 h: (a) NH4HF2:Al2O3=2.5:1 and (b) NH4HF2:Al2O3=3.5:1
Above results indicate that the mass ratio of NH4HF2:Al2O3 has no influence on the critical chemical reaction temperature and phases of products. In this case, only the products at the mass ratio of 3.5 after heat treatment at different temperatures for 1 h were chosen for SEM/EDS analysis. Fig.6a is the SEM image of Al2O3/NH4HF2 mixtures after thermal treatment at 135 °C for 1 h. Clearly, faceted-grain particles with a size of 0.5~3 µm are observed. The average size is ~2.5 µm. With the increment of heat treatment temperature, the size of faceted-grain particles at 150 °C decreases to a mean size of 2 µm, as shown in Fig.6c. However, some particles larger than 2.5 µm form again after heat treatment at 180 °C for 1 h. EDS results in Fig.6b, Fig.6d and Fig.6f further indicate that the fluorides consist of Al, F and N without H due to detection limit. However, the content of Al increases with the increment of heat treatment temperature, suggesting the decomposition of NH4HF2 or the loss of NH4F from fluoride between 135~180 °C, which needs further investigation.
Fig.6 SEM morphologies (a, c, e) and corresponding EDS results (b, d, f) of Al2O3/NH4HF2 mixtures with mass ratio of NH4HF2:Al2O3=3.5 after thermal treatment at different temperatures for 1 h: (a, b) 135 °C, (c, d) 150 °C, and (e, f) 180 °C
Fig.7a and Fig.7b show the SEM morphology and the corresponding EDS results of Al2O3/NH4HF2 mixtures after thermal treatment at 270 °C for 1 h, which is between peak 3 and peak 4 of DTA curve in Fig.4. Clearly, finer spherical particles with a mean size of 0.5 µm form. Fig.7b further indicates the fluorides consisting of Al, F and N, the same as fluorides produced at 135~180 °C. However, a high Al and low N content is observed, suggesting the further loss of NH4F from fluoride. Fig.7c shows the SEM morphology of Al2O3/NH4HF2 mixtures after thermal treatment at 360 °C for 1 h, which is after peak 4 of DTA curve in Fig.4. Clearly, the size of spherical particle is further decreased compared to fluorides produced at 270 °C. EDS results in Fig.7d show that only Al and F are detected and the atomic ratio of F:Al is close to 3, consistent with the formal ratios of AlF3.
Fig.7 SEM morphologies (a, c) and corresponding EDS results (b, d) of Al2O3/NH4HF2 mixtures with mass ratio of NH4HF2:Al2O3=3.5 after thermal treatment at different temperatures for 1 h: (a, b) 270 °C and (c, d) 360 °C
2.4 Structure transformation of AlF3
Fig.8 shows the XRD patterns of (NH4)3AlF6 mixtures after thermal treatment at 400 and 650 °C for 1 h. Clearly, two types of AlF3 form: β-AlF3 (#43-0435) at 400 °C and α-AlF3 (#44-0231) at 650 °C, indicating a structure transformation between 400~650 °C. From Fig.8, it can be further found that α-AlF3 (#44-0231) formed at 650 °C exhibits the highest intensity of the (012) orientation, indicating the formation of α-AlF3 (#44-0231) along (012) preferred orientation.
Fig.8 XRD patterns of (NH4)3AlF6 after thermal treatment at 400 and 650 °C for 1 h
Fig.9 shows the SEM morphology and the corresponding EDS results of AlF3 produced at 400 and 650 °C. Clearly, AlF3 particles formed at 400 °C are still spherical, the same as that at 360 °C, but with a coarsesize, as seen in Fig.8a. The results indicate the coarsening of β-AlF3 (#43-0435) particles. How-ever, after the structure transformation, large rod-like α- AlF3 particles form at 650 °C because of the growth of α-AlF3 along (012) preferred orientation, as seen in Fig.9c. Fig.9b and Fig.9d indicate that both types of AlF3 exhibit a comparable Al and F content with F:Al atomic ratio of 3.
Fig.9 SEM morphologies (a, c) and corresponding EDS results (b, d) of (NH4)3AlF6 after thermal treatment at different temperatures for 1 h: (a, b) 400 °C and (c, d) 650 °C
Results in Fig.4 indicate that the first chemical reaction dominates at 162.2~162.8 °C (peak 2) and completes at 180 °C. Fig.5 further indicate that the phases of products between 135~180 °C are (NH4)3AlF6 with similar faceted-grain morpho-logies (Fig.6). The mass ratio of NH4HF2:Al2O3 has no influence on the chemical reaction progress and the products. Consequently, the formation of peak at 162.3~162.8 °C is due to the fluorination of Al2O3 to form (NH4)3AlF6 according to chemical reaction (1)[15,19-22]. From Fig.4, it can be also found that the chemical reaction between NH4HF2 and Al2O3 solid powder begins at the room temperature. To confirm this assumption, an NH4HF2/Al2O3 mixture with mass ratio of 3.5 was prepared and analyzed by XRD right now or after one week at room temperature, as shown in Fig.10.
Fig.10 XRD patterns of Al2O3/NH4HF2 mixtures with the mass ratio of NH4HF2:Al2O3=3.5 at room temperature: (a) after mixing and (b) after mixing for one week
Clearly, no (NH4)3AlF6 is detected after being mixed immediately; while (NH4)3AlF6 besides NH4HF2 and Al2O3 form after being mixed for one week, the same as that at 135~180 °C (Fig.5). With the melting of NH4HF2 at 126.8 °C [22], the reaction rate increases sharply due to higher liquid-solid reaction rate compared to lower solid-solid reaction rate. According to reaction (1), the mass ratio of NH4HF2:Al2O3 for completed fluorination is 3.3530. Therefore, the fluorination is uncompleted for mass ratio of 2.5. That is why Al2O3 is detected in XRD pattern at 135 °C (Fig.5a). However, minor NH4HF2 is detected after 135 °C (Fig.5b), for mass ratio of 3.5 higher than 3.3530. With the increment of temperature, the decomposition and sublimation of NH4HF2 occur, so the XRD peak of NH4HF2 disappears at high temperature, as shown in Fig.5b.
Fig.4 indicates that the second chemical reaction starts at 235 °C, dominates at 249.8~250.1 °C and is completed at 273 °C, which coincides well with the value in Ref.[23] and is close to the value in Ref.[18]. Results in Fig.5 and Fig.7 further indicate that only NH4AlF4 (#20-0077) with finer grain size of 0.5 µm is observed at 270 °C, suggesting the decomposition of (NH4)3AlF6 to form NH4AlF4 at 249.8 °C through the following reaction:
For low mass ratio of 2.5, part of Al2O3 powder is not fluoridated. Thus, the residue consists of NH4AlF4 and original Al2O3 at 270 °C. The theoretical calculation indicates that the mass of residue is 57.8%, which coincides with the measured value of 56.8%, as shown in Fig.4a. However, for high mass ratio of 3.5, all Al2O3 powder is fluoridated and the residue at 270 °C is NH4AlF4. In this case, the mass of NH4AlF4 residue at 270 °C is 52.7%, which is close to the measured value of 52.2%, as shown in Fig.4b.
Based on TG-DTA results in Fig.4, the residue at 270 °C for low mass ratio of 2.5 should contain minor un-reacted Al2O3. However, the decomposition (NH4)3AlF6 will form NH4F, which can further react with un-reacted Al2O3 to form NH4AlF4 according to following reaction:
That is why no Al2O3 is detected from the XRD pattern at 270 °C (Fig.5b).
With further increase in temperature, the third chemical reaction starts at 320 °C, dominates at 356.8~357.7 °C and is completed at 360 °C, which coincides well with the value in Ref.[18, 23]. Fig.5 indicates that only AlF3 (#43-0435) is identified at 360 °C. Fig.7 shows that the grain size is further refined compared to NH4AlF4 formed at 270 °C. Based on above results, it can be concluded that the formation of peak at 356.8~357.7 °C is due to the decomposition of NH4AlF4 to form AlF3 through the following reaction:
Therefore, AlF3 forms at 360 °C. TG-DTA results in Fig.4 exhibit that AlF3 is stable without chemical change up to 600 °C; while the release of adsorbent such as F will cause minor mass loss. The fluorination is uncompleted for low mass ratio of 2.5, and then the residue at 600 °C is AlF3 with minor un-reacted Al2O3. The theoretical calculation indicates that the residue for low mass ratio of 2.5 is 42.3%, which coincides well with the measured value of 43.4%, as shown in Fig.4a. However, all Al2O3 powder is fluoridated for high mass ratio of 3.5, and the residue is AlF3. In this case, the calculated residue at 600 °C is 36.6%, which is close to measured value of 37.6%, as shown in Fig.4b. Even though no chemical reaction occurs for AlF3 at 360~600 °C, a structure transfor-mation from β-AlF3 (#43-0435) to α-AlF3 (#44-0231) occurs at 456 °C[28], which causes the growth of rod-like AlF3 (Fig.9) along (012) preferred orientation, as seen in Fig.8.
1) The mass ratio of NH4HF2:Al2O3 has no influence on the critical reaction temperature and reaction process.
2) The fluorination starts at room temperature with the formation of (NH4)3AlF6, dominates at 162.3~162.8 °C and is completed at 180 °C.
3) (NH4)3AlF6 starts to decompose sequentially through a two-step decomposition reaction with the formation of NH4AlF4 at 249.8~250.1 °C and finally decomposes to β-AlF3 at 356.8~357.7 °C.
4) The transformation from β-AlF3 to α-AlF3 occurs be-tween 400~650 °C.
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