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Self-propagating rapid synthesis and characterization of LaCrO3 powder
Journal Pre-proofs Self-propagating rapid synthesis and characterization of LaCrO3 powder Dayan Xie, Kuibao Zhang, Weiwei Li, Baozhu Luo, Haibin Zhang PII: DOI: Reference:
S0167-577X(19)31505-8 https://doi.org/10.1016/j.matlet.2019.126873 MLBLUE 126873
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
20 June 2019 8 October 2019 26 October 2019
Please cite this article as: D. Xie, K. Zhang, W. Li, B. Luo, H. Zhang, Self-propagating rapid synthesis and characterization of LaCrO3 powder, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.126873
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Self-propagating rapid synthesis and characterization of LaCrO3 powder Dayan Xiea, Kuibao Zhanga, b*, Weiwei Lia, Baozhu Luoa, Haibin Zhangc a
State Key Laboratory of Environment-friendly Energy Materials, Southwest University of
Science and Technology, Mianyang, Sichuan, 621010, China. b
National Defense Key Discipline Lab of Nuclear Waste and Environmental Safety, Southwest University of Science and Technology, Mianyang, 621010, China.
c
Institutes of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang,
621900, China. Abstract: Lanthanum chromite (LaCrO3) is a promising functional material because of its high melting point, high thermal conductivity and excellent fire resistance. In this study, LaCrO3 was successfully prepared by self-propagating high-temperature synthesis (SHS) method using CrO3 and Cr as the oxidant and reductant. The optimum mole ratio of La2O3, Cr and CrO3 was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The result shows that the LaCrO3 powder with acceptable purity and good morphology was obtained when Cr and CrO3 are 10% excess over La2O3. The real temperature of SHS system reaction was recorded by paperless recorder. The particle size distribution of the SHS prepared LaCrO3 powder demonstrates median diameter of 1040 nm according to laser particle size spectrometer. Keywords: LaCrO3 powder; Self-propagating high-temperature synthesis; Particle size distribution Submitted to: Materials Letters *Corresponding Author: [email protected]
1. Introduction LaCrO3 belongs to the composite oxide of perovskite-type complex oxide (ABO3) with high melting point (2490 °C). It can exist in three different solid phases (orthorhombic, rhombohedral and cubic) depending on the temperature [1]. LaCrO3 has been extensively studied for solid oxide fuel cells (SOFCs), high-temperature heating elements, interconnect materials as well as catalysts for the treatment of automobile exhaust due to its superior electronic conductivity and high temperature stability [2-5]. In addition, the substitution on A and B sites with metal elements have been attracted considerable interest in the scientific research [6,7]. Self-propagating high-temperature synthesis (SHS), also known as combustion synthesis, is an attractive and efficient method for the preparation of materials, which was firstly proposed by Merzhanov and Borovinskaya [8]. In SHS reaction, the reactant mixture contains reductants and oxidants, the reaction among which produces the heat needed to sustain the self-propagating combustion front [9,10]. The method has been widely study because of rapid, simple process, low time consumption and low cost. Application of the SHS method to obtain complex oxides was started in the seventies of the last century. In the early researches, scientists obtained complex oxides in the implementation of redox reactions in the combustion of mixtures of simple oxides, which demonstrated the possibility of carrying out self-propagating high-temperature synthesis in mixtures of oxides [11,12]. SHS technique has also been proposed as a candidate approach for environmental protection, such as stabilization of radioactive and toxic wastes [13-15]. At present, there is almost no report about the redox reaction preparation of pure LaCrO3 powder in the combustion of simple oxide mixtures by SHS scheme. Q. Ming and A. A. Shiryaev et al. proposed that Sr was doped to substitute the La site of LaCrO3 to preparing La1-xSrxCrO3 via SHS [10,16]. According to M. V. Kuznetsov and LP. Parkin et al., lanthanide orthochromites of general formula LnCrO3 (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu) have been prepared by SHS [17,18]. Various SHS reactions were used, including the combustion of La2O3-Cr-CrO3 mixtures with sodium or barium perchlorates, as well as the combustion of La2O3Cr mixture in pure oxygen, for the synthesis of LaCrO3. However, these SHS approaches have complexity and explosion hazard due to the use of perchlorates and pure oxygen. The SHS
approach, proposed in this work to burn mixtures of La2O3-Cr-CrO3, is more simple and less dangerous. Commonly, LaCrO3 powder is prepared by traditional sintering method, such as solid-phase method and liquid-phase method. Q. Zhang et al. synthesized mechanochemically LaCrO3 powder by room temperature grinding of La2O3 and hydrous amorphous chromium oxide powder [19]. However, the obtained sample consists of agglomerates and large particles due to the water existing in the starting hydrous oxide. K. Azegami et al. developed preparation method of LaCrO3 using hydrazine monohydrate. The method is complicated and difficult to control because a feature of this method is the formation of a bridged ligand originated from bidentate hydrazine [20]. Moreover, solution combustion synthesis (SCS) was adopted for producing LaCrO3 powder with fine and homogeneous particles. This is a new technique to obtain nanoscale powders based on SHS [21-24]. However, the SCS process is not easy to control and formation of hard agglomerates of irregular shapes. Compared with above methods, the preparation way of SHS for producing complex oxides, including LaCrO3, has the advantages of simple preparation process, strong operability and easy realization of large-scale production. In the present study, we provide self-propagating rapid synthesis LaCrO3 powder within a short time. The sample was directly obtained by sintering a mixture of starting materials. The temperature of SHS reaction system was recorded by paperless recorder. The phase composition of as-prepared sample was explored by XRD. SEM illustrates shape and microstructure of powder. The particle sizes of the synthesized LaCrO3 powder were investigated with the help of PSD. 2. Experiment details Lanthanum oxide (La2O3), chromium (Cr) and chromium trioxide (CrO3) with purity higher than 99 wt.% were used as raw materials, which were purchased from Shanghai Aladdin Biochemical Technology. In the SHS method, appropriate amounts of La2O3, Cr, and CrO3 powders were mixed by mechanical grinding, which Cr and CrO3 were employed as oxidant and reductant, respectively. The SHS reactions are designed as follows: La2O3+Cr+CrO3=2LaCrO3
(1)
About 20 g of powder reactants were mechanically ground and mixed in agate mortar. Subsequently, the mixtures were dried in Electric blast drying oven (WGLL-125BE, Guangzhou Bubuhong Scientific Instrument Equipment Co., Ltd.) at 60 °C for 2 h. Then the completely mixed powder reactants were pressed into tablet at 20 MPa using Powder press (PC-15B, Tianjin Pinchuang Technology Co., Ltd.). The SHS process was conducted as illustrated in Fig. 1(a). The green body of reactants was ignited by a tungsten wire, which was located at one side with close contact of the pellets. The other side of the tungsten wire is connected to a voltage transformer. The cylindrical green body was ignited when the DC current was elevated to about 30 A. The real temperature of reaction was measured by W/Re 5/26 thermocouple located at the center of the cylindrical green bodies. And then the temperature trend curve was recorded through using a XME2002/U paperless reorder. The sintered sample was ground into powder for following research. An X-ray diffractometer (XRD: PANalytical B.V., Netherlands) using Cu Kα radiation and operating at 45 kV and 40 mA was used to identify the phase composition in the products. XRD datas were obtained using an angular range of 20-80° with a step size of 0.02° (2θ). The microstructure of sample was analyzed by field-emission scanning electron microscopy (FESEM; Zeiss Ultra-55, Oberkochen, Germany). Particle size of the sample powder was tested by laser particle size spectrometer (90-plus, Brookhaven Instruments Corp, US.). 3. Results and discussions 3.1 Temperature and powder XRD analysis According to the designed as mentioned above, the temperature of SHS reaction can reach 2010 °C, where the ratio of Cr and CrO3 is 10% excess over La2O3 because Cr exhibits a high vapor pressure at the high temperature. The target product can be obtained after 15 s combustion process. The temperature tendency curve of SHS reaction is shown in Fig 1(b). According to the result of Fig. 1(b), the highest temperature (2010 °C) in the SHS exceeded traditional solid phase sintering temperature (1627 °C) since the amount of oxidant and reducing agent is higher than the stoichiometric ratio. Actually, the maximum temperature of combustion will exceed the temperature of solid-phase sintering with the stoichiometric ratio. Since the combustion reaction
heat is released very quickly (Fig. 1(b)), only a small part of it has time to be transferred to the environment in contrast to the slow process of solid-phase sintering. Moreover, the excess of oxidant and the reducing agent over the stoichiometric ratio increases even more the maximum temperature of combustion. In this figure, the time-temperature curve rises sharply and then drops slowly. There was an abrupt rise in temperature as a result of the heat released by the reaction. The temperature immediately reached to its maximum, which is the actual temperature of the reaction. However, the peak temperature of the reaction is often lower than the theoretical temperature due to heat loss. The product of SHS reaction was analyzed with the XRD patterns presented in Fig. 2. The phase structure of sample is in consistent with the characteristics of LaCrO3 (PDF No.71-1231). Fig. 2(a) is the XRD results of a stoichiometric composition of the reactants. It is known from (a) that there were still reactants of La2O3 in the product. High combustion temperature cause volatilizing of starting reagents because the vapor pressure of Cr is high at high temperature. This would lead to the lack of oxygen in the product compared to the stoichiometric content. In order to avoid this situation, the powders of Cr metal were added to the initial mixture and adding equal amounts of CrO3 to balance the redox reaction. When Cr and CrO3 were simultaneously increased by 10%, the target products can be obtained. The XRD results from Fig. 2(b) depict that the phase composition of the products is single phase. In addition, the composition of the whole sintered sample is LaCrO3. However, Q. Ming et al. applied SHS to produce La0.8Sr0..2CrO3 containing different components in different sample areas [10]. Therefore, it indicates that the selfpropagating reaction as designed above is feasible. 3.2 The microstructure of sample The sintered samples were ground into powders, followed by ball milling in a planetary ball mill for a period of time using an agate ball as ball and alcohol as ball milling medium. SEM was carried out to gain direct information about microstructure of LaCrO3 sample, which image was illustrated in Fig. 3. The SEM reveals that the LaCrO3 powder prepared by the SHS method have a large particle size and many small particles were attached to the surface of large particles. In addition, it can be seen in Fig. 3 that LaCrO3 powder particles are non-uniform and irregular in
shape. The particle size and shape of samples present no change despite varied conditions. Most of the particles are micron-sized powders with particle sizes ranging from a few hundred nanometers to 2 microns. It corresponds to the SHS characteristics as mentioned above. In terms of particle size and shape, they are different from the powder prepared by other methods [20,23,25]. In particular, in the phase purity of the overall sintered product, this is differ from synthesis of La1xSrxCrO3
via SHS way because the microstructure of the product varies with the distance the
combustion wave spreads[10]. The grain is homogeneous and clean within the field of view. Thus it can indicated that the product is pure. The agglomeration of the particles is not obvious. This is in consistent with the results of XRD. 3.3 Particle size of the powder sample The powder was dissolved in deionized water to prepare a suspension with 1 wt.% for particle size analysis. Fig. 4 shows the powder particle size distribution as the median diameter is 1040 nm (d50=1040 nm) and the mean diameter is 1173 nm. The particle size is mainly distributed between 500 nm and 2000 nm. Particles less than 1000 nm in particles size account for about half of the total (d50 = 1040 nm). The coarse end particle size of the powder is about 2300 nm (d95 = 2330 nm), and the particles with d>2000 nm have only a small distribution. In general, the particle size of the product is not uniform because the product particles prepared by the SHS method are not uniform, and the particle size of the raw materials are also large. This is in agreement with the results of SEM. 4. Conclusions In this study, the SHS method was employed to prepare LaCrO3 powder. The product exhibits a simple phase composition when Cr and CrO3 are 10% excess over La2O3 from XRD and SEM. Compared with other methods such as liquid-phase and solid-phase methods, the SHS product particles are irregular and larger in diameter and no agglomeration from SEM and PSD results. After ball milling and desiccation, LaCrO3 powder was obtained with bimodal particle size distribution of 540 nm and 2000 nm. Assuredly, large-scale production can be carried out by using the SHS technique, which is not available for other methods. Generally speaking, it is significant that that SHS technique is a potential approach for producing LaCrO3 powder.
Acknowledgements: We sincerely appreciate the projects supported by the National Natural Science Foundation of China (Grant No.51672228), Key R&D Program for Civil-Military Integratio of Sichuan province (2018GZ0519), Science Development Foundation of China Academy of Engineering Physics and the Postgraduate Innovation Fund Project by Southwest University of Science and Technology (No.19ycx0016). References [1] T. Hashimoto, N. Tsuzuki, A. Kishi, et al., Solid State Ionics, 132 (2000) 181-188. [2] W. Feduska, A.O. Isenberg , J. Power Sources, 10 (1983) 89-102. [3] M.B. Phillipps, N.M. Sammes, O.J. Yamamoto, J. Mater. Sci., 31 (1996) 1689-1692. [4] X. Ding, Y. Liu, L. Gao, J. Alloy. Compd., 425 (2006) 318-322. [5] W.Y.H. Enciso, M.N. Tsampas, C. Zhao, et al., Catal. Today, 258 (2015) 525-534. [6] B. Moreno, E. Chinarro, J.R. Jurado, J. Eur. Cermic Soc., 28 (2008) 2563-2566. [7] N. Sakai, T. Kawada, H. Yokokawa, et al., J. Mater. Sci., 25 (1990) 4531-4534. [8] A.G. Merzhanov, Ceram. Int., 21 (1995) 371-379. [9] A.G. Merzhanov, J. Mater. Process. Technol., 56 (1996) 222-241. [10] Q. Ming, M. Nersesyan, K. Ross, J.T. Richardson, D.A.N. Lusst, Combust. Sci. Technol., 128 (1997) 279-294. [11] V.V. Aleksandrov, V.I. Smirnov, V.V.Boldyrev, Combust. Explos. Waves., 15 (1979) 330334. [12] V.I. Smirnov, V.V. Aleksandrov. J. Eng. Phys. Thermophys., 65 (1993) 1117-1120. [13] L. Peng, K. Zhang, Z. He, D. Yin, et al., J. Adv. Cermic., 7 (2018) 41-49. [14] K. Zhang, Z. He, J. Xue, et al., J. Nucl. Mater., 507 (2018) 93-100. [15] K. Zhang, G. Wen, H. Zhang, Y. Teng, J. Nucl. Mater., 465 (2015) 1-5. [16] A.A. Shiryaev, M.D. Nersesyan, Q. Ming, J. Mater. Synth. Technol., 7 (1999) 83-90. [17] M.V. Kuznetson, Inorg. Mater., 34 (1998) 1065-1067. [18] M.V. Kuznetson, I.P. Parkin. Polyhedron, 17 (1998) 4443-4450. [19] Q. Zhang, J. Lu, F.J. Saito, Powder Technol., 122 (2002) 145-149. [20] K. Azegami, M. Yoshinaka, K. Hirota, Mater. Res. Bull., 33 (1998) 341-348.
[21] L.A. Chick, L.R. Pederson, G.D. Maupin, et al., Mater. Lett., 10 (1990) 6-12. [22] K.C. Patil, S.T. Aruna, Curr. Opin. Solid State Mater. Sci., 6 (2002) 507-512. [23] K.P. Hong, Y.S. Han, D.K. Kim, et al., J. Mater. Sci. Lett., 17 (1998) 785-787. [24] A. Varma, A.S. Mukasyan, A.S. Rogachev, et al., Chem. Rev., 116 (2016). [25] Y. Jiang, J. Gao, M. Liu, et al., Mater. Lett., 61 (2007) 1908-1911.
Figure captions: Figure 1. (a) Schematic diagram of the designed SHS process, (b) the temperature curve of LaCrO3 sample during SHS reaction. Figure 2. X-ray diffraction patterns: (a) La2O3: Cr: CrO3=1:1:1, (b) the ratio of Cr and CrO3 is 10% excess over La2O3. Figure 3. The SEM images of LaCrO3 under different ball milling conditions: (a) powder without ball mill, (b) ball milled for 5 hours, (c) ball milled for 10 hours, (d) ball milled for 15 hours. Figure 4. The particle size distribution of LaCrO3 powder sample.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitte.
• LaCrO3 powder was readily prepared via SHS within 2 minutes. • The SHS method exhibits advantages in low cost and high efficiency. • The obtained powder shows bimodal particle size distribution of 0.50 μm and 2.00 μm.
S0167-577X(19)31505-8 https://doi.org/10.1016/j.matlet.2019.126873 MLBLUE 126873
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
20 June 2019 8 October 2019 26 October 2019
Please cite this article as: D. Xie, K. Zhang, W. Li, B. Luo, H. Zhang, Self-propagating rapid synthesis and characterization of LaCrO3 powder, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.126873
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Self-propagating rapid synthesis and characterization of LaCrO3 powder Dayan Xiea, Kuibao Zhanga, b*, Weiwei Lia, Baozhu Luoa, Haibin Zhangc a
State Key Laboratory of Environment-friendly Energy Materials, Southwest University of
Science and Technology, Mianyang, Sichuan, 621010, China. b
National Defense Key Discipline Lab of Nuclear Waste and Environmental Safety, Southwest University of Science and Technology, Mianyang, 621010, China.
c
Institutes of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang,
621900, China. Abstract: Lanthanum chromite (LaCrO3) is a promising functional material because of its high melting point, high thermal conductivity and excellent fire resistance. In this study, LaCrO3 was successfully prepared by self-propagating high-temperature synthesis (SHS) method using CrO3 and Cr as the oxidant and reductant. The optimum mole ratio of La2O3, Cr and CrO3 was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The result shows that the LaCrO3 powder with acceptable purity and good morphology was obtained when Cr and CrO3 are 10% excess over La2O3. The real temperature of SHS system reaction was recorded by paperless recorder. The particle size distribution of the SHS prepared LaCrO3 powder demonstrates median diameter of 1040 nm according to laser particle size spectrometer. Keywords: LaCrO3 powder; Self-propagating high-temperature synthesis; Particle size distribution Submitted to: Materials Letters *Corresponding Author: [email protected]
1. Introduction LaCrO3 belongs to the composite oxide of perovskite-type complex oxide (ABO3) with high melting point (2490 °C). It can exist in three different solid phases (orthorhombic, rhombohedral and cubic) depending on the temperature [1]. LaCrO3 has been extensively studied for solid oxide fuel cells (SOFCs), high-temperature heating elements, interconnect materials as well as catalysts for the treatment of automobile exhaust due to its superior electronic conductivity and high temperature stability [2-5]. In addition, the substitution on A and B sites with metal elements have been attracted considerable interest in the scientific research [6,7]. Self-propagating high-temperature synthesis (SHS), also known as combustion synthesis, is an attractive and efficient method for the preparation of materials, which was firstly proposed by Merzhanov and Borovinskaya [8]. In SHS reaction, the reactant mixture contains reductants and oxidants, the reaction among which produces the heat needed to sustain the self-propagating combustion front [9,10]. The method has been widely study because of rapid, simple process, low time consumption and low cost. Application of the SHS method to obtain complex oxides was started in the seventies of the last century. In the early researches, scientists obtained complex oxides in the implementation of redox reactions in the combustion of mixtures of simple oxides, which demonstrated the possibility of carrying out self-propagating high-temperature synthesis in mixtures of oxides [11,12]. SHS technique has also been proposed as a candidate approach for environmental protection, such as stabilization of radioactive and toxic wastes [13-15]. At present, there is almost no report about the redox reaction preparation of pure LaCrO3 powder in the combustion of simple oxide mixtures by SHS scheme. Q. Ming and A. A. Shiryaev et al. proposed that Sr was doped to substitute the La site of LaCrO3 to preparing La1-xSrxCrO3 via SHS [10,16]. According to M. V. Kuznetsov and LP. Parkin et al., lanthanide orthochromites of general formula LnCrO3 (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu) have been prepared by SHS [17,18]. Various SHS reactions were used, including the combustion of La2O3-Cr-CrO3 mixtures with sodium or barium perchlorates, as well as the combustion of La2O3Cr mixture in pure oxygen, for the synthesis of LaCrO3. However, these SHS approaches have complexity and explosion hazard due to the use of perchlorates and pure oxygen. The SHS
approach, proposed in this work to burn mixtures of La2O3-Cr-CrO3, is more simple and less dangerous. Commonly, LaCrO3 powder is prepared by traditional sintering method, such as solid-phase method and liquid-phase method. Q. Zhang et al. synthesized mechanochemically LaCrO3 powder by room temperature grinding of La2O3 and hydrous amorphous chromium oxide powder [19]. However, the obtained sample consists of agglomerates and large particles due to the water existing in the starting hydrous oxide. K. Azegami et al. developed preparation method of LaCrO3 using hydrazine monohydrate. The method is complicated and difficult to control because a feature of this method is the formation of a bridged ligand originated from bidentate hydrazine [20]. Moreover, solution combustion synthesis (SCS) was adopted for producing LaCrO3 powder with fine and homogeneous particles. This is a new technique to obtain nanoscale powders based on SHS [21-24]. However, the SCS process is not easy to control and formation of hard agglomerates of irregular shapes. Compared with above methods, the preparation way of SHS for producing complex oxides, including LaCrO3, has the advantages of simple preparation process, strong operability and easy realization of large-scale production. In the present study, we provide self-propagating rapid synthesis LaCrO3 powder within a short time. The sample was directly obtained by sintering a mixture of starting materials. The temperature of SHS reaction system was recorded by paperless recorder. The phase composition of as-prepared sample was explored by XRD. SEM illustrates shape and microstructure of powder. The particle sizes of the synthesized LaCrO3 powder were investigated with the help of PSD. 2. Experiment details Lanthanum oxide (La2O3), chromium (Cr) and chromium trioxide (CrO3) with purity higher than 99 wt.% were used as raw materials, which were purchased from Shanghai Aladdin Biochemical Technology. In the SHS method, appropriate amounts of La2O3, Cr, and CrO3 powders were mixed by mechanical grinding, which Cr and CrO3 were employed as oxidant and reductant, respectively. The SHS reactions are designed as follows: La2O3+Cr+CrO3=2LaCrO3
(1)
About 20 g of powder reactants were mechanically ground and mixed in agate mortar. Subsequently, the mixtures were dried in Electric blast drying oven (WGLL-125BE, Guangzhou Bubuhong Scientific Instrument Equipment Co., Ltd.) at 60 °C for 2 h. Then the completely mixed powder reactants were pressed into tablet at 20 MPa using Powder press (PC-15B, Tianjin Pinchuang Technology Co., Ltd.). The SHS process was conducted as illustrated in Fig. 1(a). The green body of reactants was ignited by a tungsten wire, which was located at one side with close contact of the pellets. The other side of the tungsten wire is connected to a voltage transformer. The cylindrical green body was ignited when the DC current was elevated to about 30 A. The real temperature of reaction was measured by W/Re 5/26 thermocouple located at the center of the cylindrical green bodies. And then the temperature trend curve was recorded through using a XME2002/U paperless reorder. The sintered sample was ground into powder for following research. An X-ray diffractometer (XRD: PANalytical B.V., Netherlands) using Cu Kα radiation and operating at 45 kV and 40 mA was used to identify the phase composition in the products. XRD datas were obtained using an angular range of 20-80° with a step size of 0.02° (2θ). The microstructure of sample was analyzed by field-emission scanning electron microscopy (FESEM; Zeiss Ultra-55, Oberkochen, Germany). Particle size of the sample powder was tested by laser particle size spectrometer (90-plus, Brookhaven Instruments Corp, US.). 3. Results and discussions 3.1 Temperature and powder XRD analysis According to the designed as mentioned above, the temperature of SHS reaction can reach 2010 °C, where the ratio of Cr and CrO3 is 10% excess over La2O3 because Cr exhibits a high vapor pressure at the high temperature. The target product can be obtained after 15 s combustion process. The temperature tendency curve of SHS reaction is shown in Fig 1(b). According to the result of Fig. 1(b), the highest temperature (2010 °C) in the SHS exceeded traditional solid phase sintering temperature (1627 °C) since the amount of oxidant and reducing agent is higher than the stoichiometric ratio. Actually, the maximum temperature of combustion will exceed the temperature of solid-phase sintering with the stoichiometric ratio. Since the combustion reaction
heat is released very quickly (Fig. 1(b)), only a small part of it has time to be transferred to the environment in contrast to the slow process of solid-phase sintering. Moreover, the excess of oxidant and the reducing agent over the stoichiometric ratio increases even more the maximum temperature of combustion. In this figure, the time-temperature curve rises sharply and then drops slowly. There was an abrupt rise in temperature as a result of the heat released by the reaction. The temperature immediately reached to its maximum, which is the actual temperature of the reaction. However, the peak temperature of the reaction is often lower than the theoretical temperature due to heat loss. The product of SHS reaction was analyzed with the XRD patterns presented in Fig. 2. The phase structure of sample is in consistent with the characteristics of LaCrO3 (PDF No.71-1231). Fig. 2(a) is the XRD results of a stoichiometric composition of the reactants. It is known from (a) that there were still reactants of La2O3 in the product. High combustion temperature cause volatilizing of starting reagents because the vapor pressure of Cr is high at high temperature. This would lead to the lack of oxygen in the product compared to the stoichiometric content. In order to avoid this situation, the powders of Cr metal were added to the initial mixture and adding equal amounts of CrO3 to balance the redox reaction. When Cr and CrO3 were simultaneously increased by 10%, the target products can be obtained. The XRD results from Fig. 2(b) depict that the phase composition of the products is single phase. In addition, the composition of the whole sintered sample is LaCrO3. However, Q. Ming et al. applied SHS to produce La0.8Sr0..2CrO3 containing different components in different sample areas [10]. Therefore, it indicates that the selfpropagating reaction as designed above is feasible. 3.2 The microstructure of sample The sintered samples were ground into powders, followed by ball milling in a planetary ball mill for a period of time using an agate ball as ball and alcohol as ball milling medium. SEM was carried out to gain direct information about microstructure of LaCrO3 sample, which image was illustrated in Fig. 3. The SEM reveals that the LaCrO3 powder prepared by the SHS method have a large particle size and many small particles were attached to the surface of large particles. In addition, it can be seen in Fig. 3 that LaCrO3 powder particles are non-uniform and irregular in
shape. The particle size and shape of samples present no change despite varied conditions. Most of the particles are micron-sized powders with particle sizes ranging from a few hundred nanometers to 2 microns. It corresponds to the SHS characteristics as mentioned above. In terms of particle size and shape, they are different from the powder prepared by other methods [20,23,25]. In particular, in the phase purity of the overall sintered product, this is differ from synthesis of La1xSrxCrO3
via SHS way because the microstructure of the product varies with the distance the
combustion wave spreads[10]. The grain is homogeneous and clean within the field of view. Thus it can indicated that the product is pure. The agglomeration of the particles is not obvious. This is in consistent with the results of XRD. 3.3 Particle size of the powder sample The powder was dissolved in deionized water to prepare a suspension with 1 wt.% for particle size analysis. Fig. 4 shows the powder particle size distribution as the median diameter is 1040 nm (d50=1040 nm) and the mean diameter is 1173 nm. The particle size is mainly distributed between 500 nm and 2000 nm. Particles less than 1000 nm in particles size account for about half of the total (d50 = 1040 nm). The coarse end particle size of the powder is about 2300 nm (d95 = 2330 nm), and the particles with d>2000 nm have only a small distribution. In general, the particle size of the product is not uniform because the product particles prepared by the SHS method are not uniform, and the particle size of the raw materials are also large. This is in agreement with the results of SEM. 4. Conclusions In this study, the SHS method was employed to prepare LaCrO3 powder. The product exhibits a simple phase composition when Cr and CrO3 are 10% excess over La2O3 from XRD and SEM. Compared with other methods such as liquid-phase and solid-phase methods, the SHS product particles are irregular and larger in diameter and no agglomeration from SEM and PSD results. After ball milling and desiccation, LaCrO3 powder was obtained with bimodal particle size distribution of 540 nm and 2000 nm. Assuredly, large-scale production can be carried out by using the SHS technique, which is not available for other methods. Generally speaking, it is significant that that SHS technique is a potential approach for producing LaCrO3 powder.
Acknowledgements: We sincerely appreciate the projects supported by the National Natural Science Foundation of China (Grant No.51672228), Key R&D Program for Civil-Military Integratio of Sichuan province (2018GZ0519), Science Development Foundation of China Academy of Engineering Physics and the Postgraduate Innovation Fund Project by Southwest University of Science and Technology (No.19ycx0016). References [1] T. Hashimoto, N. Tsuzuki, A. Kishi, et al., Solid State Ionics, 132 (2000) 181-188. [2] W. Feduska, A.O. Isenberg , J. Power Sources, 10 (1983) 89-102. [3] M.B. Phillipps, N.M. Sammes, O.J. Yamamoto, J. Mater. Sci., 31 (1996) 1689-1692. [4] X. Ding, Y. Liu, L. Gao, J. Alloy. Compd., 425 (2006) 318-322. [5] W.Y.H. Enciso, M.N. Tsampas, C. Zhao, et al., Catal. Today, 258 (2015) 525-534. [6] B. Moreno, E. Chinarro, J.R. Jurado, J. Eur. Cermic Soc., 28 (2008) 2563-2566. [7] N. Sakai, T. Kawada, H. Yokokawa, et al., J. Mater. Sci., 25 (1990) 4531-4534. [8] A.G. Merzhanov, Ceram. Int., 21 (1995) 371-379. [9] A.G. Merzhanov, J. Mater. Process. Technol., 56 (1996) 222-241. [10] Q. Ming, M. Nersesyan, K. Ross, J.T. Richardson, D.A.N. Lusst, Combust. Sci. Technol., 128 (1997) 279-294. [11] V.V. Aleksandrov, V.I. Smirnov, V.V.Boldyrev, Combust. Explos. Waves., 15 (1979) 330334. [12] V.I. Smirnov, V.V. Aleksandrov. J. Eng. Phys. Thermophys., 65 (1993) 1117-1120. [13] L. Peng, K. Zhang, Z. He, D. Yin, et al., J. Adv. Cermic., 7 (2018) 41-49. [14] K. Zhang, Z. He, J. Xue, et al., J. Nucl. Mater., 507 (2018) 93-100. [15] K. Zhang, G. Wen, H. Zhang, Y. Teng, J. Nucl. Mater., 465 (2015) 1-5. [16] A.A. Shiryaev, M.D. Nersesyan, Q. Ming, J. Mater. Synth. Technol., 7 (1999) 83-90. [17] M.V. Kuznetson, Inorg. Mater., 34 (1998) 1065-1067. [18] M.V. Kuznetson, I.P. Parkin. Polyhedron, 17 (1998) 4443-4450. [19] Q. Zhang, J. Lu, F.J. Saito, Powder Technol., 122 (2002) 145-149. [20] K. Azegami, M. Yoshinaka, K. Hirota, Mater. Res. Bull., 33 (1998) 341-348.
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Figure captions: Figure 1. (a) Schematic diagram of the designed SHS process, (b) the temperature curve of LaCrO3 sample during SHS reaction. Figure 2. X-ray diffraction patterns: (a) La2O3: Cr: CrO3=1:1:1, (b) the ratio of Cr and CrO3 is 10% excess over La2O3. Figure 3. The SEM images of LaCrO3 under different ball milling conditions: (a) powder without ball mill, (b) ball milled for 5 hours, (c) ball milled for 10 hours, (d) ball milled for 15 hours. Figure 4. The particle size distribution of LaCrO3 powder sample.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitte.
• LaCrO3 powder was readily prepared via SHS within 2 minutes. • The SHS method exhibits advantages in low cost and high efficiency. • The obtained powder shows bimodal particle size distribution of 0.50 μm and 2.00 μm.