机械铣削制备高锰硅化物的表征

I. Chicinaș, V. Popescu, T. Marinca, V. Cebotari, F. Popa
{"title":"机械铣削制备高锰硅化物的表征","authors":"I. Chicinaș, V. Popescu, T. Marinca, V. Cebotari, F. Popa","doi":"10.21741/9781945291999-9","DOIUrl":null,"url":null,"abstract":"The mechanical milling of manganese and silicon powder in a planetary ball mill up to 18 h was performed. In the X-ray diffraction pattern recorded after 18 hours of milling the MnSi phase and Mn15Si26 compound are detected. The agglomeration of powders after complete reaction of the elements was observed by scanning electron microscopy. Heating up at 1000 °C, an unreacted sample, milled 4 hours, has found to have the effect of completing the reaction of elements, but forms oxides. Handling of the powder during sampling, without protective atmosphere was found to form oxides. The oxidation of the samples was evidenced by FTIR analysis. Introduction The modern society has the tendency to increase the quantity of hydrocarbons which are transformed into energy, with negative effects on the environment. To reduce this impact alternatives are searched. Thermoelectric materials represent a solution to improve the quality of the environment by reducing the combustion product gases. These materials are able to convert the thermal energy directly into electrical energy and vice versa. The quality of a thermoelectric material can be estimated by the figure of merit ZT=SσT/k where: S is the Seebeck coefficient, σ is electrical conductivity, T is temperature and k is thermal conductivity [1]. Thermoelectric materials can convert heat from a different source such as solar heat, geothermal heat or exhaust gases [2]. From the studied thermoelectric materials, those based on silicon, especially High Manganese Silicide (HMS) is friendly with the environment and considered as promising candidates. HMS is chemically stable [3] and are preferred in detriment of those based on Pb-Te which operate in the same range of temperature. The HMS materials are nontoxic as well as their constituent chemical elements [4]. HMS are thermoelectric compounds with p-type conduction, having general formula MnSix where the x value ranges from 1.67 up to 1.87 [5] and with an energy gap of 0.77 [eV] [6]. HMS system contains four compounds, Mn4Si7, Mn11Si19, Mn15Si26, and Mn27Si47, all with the same electronic structure [7]. Crystallographic structure of HMS compounds belongs to Nowotny chimney ladder (NCL) phases, where manganese is located in the corners of tetragon and silicon are arranged inside in the form of a spiral [8]. MnSi1.75 compound presents the largest ZT, while the MnSi1.77 compound has the smallest value. The low value for the figure of merit is the effect of Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 81 a large thermal conductivity [6]. Problem with HMS is that obtaining method influence the final phase. Based on the preparation method it is possible to obtain different compounds: by vacuum levitation melting Mn15Si26 is obtained, Mn4Si7 may obtain by vacuum levitation-induction melting and by dry milling [9-11]. Preparation by melting leads to an inhomogeneous structure and coarse microstructure [12]. The obtaining by mechanical alloying has the advantage of obtaining a small crystallite size which leads to lower thermal conductivity [11]. Also, dry milling leads to the decrease of the quantity of MnSi secondary phase, which reduces the thermoelectric proprieties. In the milling experiments, using different process control agents (PCA) it is possible to control the MnSi phase. Hexane conducts to the formation of 38.8% of MnSi phase, acetone to 8.7% and ethanol to 5.3%. Milling without any PCA leads to the formation of 49.5% MnSi/HMS phase [10]. In order to obtain the proper HMS, the conditions can be summarized to be small milling time and high rotation speed according to [6, 13]. Prolonged milling conducts to the decomposition of HMS compound in MnSi phase as a result of the excess energy which is generated by collisions [14]. The increase in the thermoelectric properties can be achieved by doping. Adding Yb, the carrier concentration increases, and MnSi phase quantity decreases [14]. By doping with Co a homogenous microstructure is obtained and the ZT increases proportionally to the concentration of Co [12]. Other chemical elements that are studied for increasing the thermoelectric proprieties are Cr, Ti, Fe, Al, and Ge. The doping increases the thermoelectric proprieties only if the concentration of elements does not exceed the limit of solubility because the doped elements are located at Mn sites [15-18]. The present paper is focused on the synthesis of HMS with the chemical composition MnSi1.75. The formation of this compound by mechanical milling is studied as a function of the milling time. The paper presents the evolution of the powder morphology and the distribution of the chemical elements in the samples after milling. The thermal stability of powders is also presented and discussed. Experimental The thermoelectric material has been obtained starting from elemental powders of manganese with purity of 99.3% (-325 mesh, Alfa Aesar) and silicon with purity of 99.9% (-100 mesh, Alfa Aesar), in a stoichiometric ratio corresponding to MnSi1.75 compound formula. The powder mixture was loaded into the vial with grinding media after the prior homogenisation of the elemental powders. The mechanical milling was made in a planetary ball mill Fritsch Pulverisette 6 using a ball to powder mass ratio (BPR) of 10:1, and a 400 rpm rotational speed of vial. The vial and balls are of stainless steel with a diameter of 14 mm. For the protection of powders which are subjected to milling process from oxidation, the milling was done under argon atmosphere. The milling was conducted up to 18 hours, and sampling was done after the following milling times: 0, 1, 2, 4, 6, 8, 10, 14 and 18 hours. The structural study and phases composition of the samples were investigated by X-ray diffraction using an INEL 3000 Equinox diffractometer using Kα radiation of Co (λ = 1.79026 Å). To study the morphology of the powders and the local chemical homogeneity a JEOLJSM 5600 LV electron microscope equipped with EDX spectrometer (Oxford Instruments, INCA 2000 soft) was used. The thermal stability of the samples was investigated by differential scanning calorimetry (DSC), using a LabSys-Setaram apparatus. The DSC investigations were performed in an argon atmosphere, up to 1000 °C, with heating/cooling rate of 10 °C/min using alumina as a reference sample. The presence of oxide inside the probe was investigated by the Fourier Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 82 Transform Infrared (FTIR) technique using Spectrum BX II apparatus. The experiment was realised by embedding the Mn-Si powder into the potassium bromide pellet. Results and discussion X-ray diffraction patterns of the mechanically milled powders are presented in Fig. 1. In diffraction pattern of the starting sample are identified the peaks corresponding to the used elemental powders. In the diffraction patterns corresponding to the sample milled for one hour is observed a reduction of the peaks intensity and a pronounced broadening. This is assigned to the reduction of crystallite size and an increase of the internal stresses [19]. The sample milled for 2 hours presents similar behavior. A new MnSi phase appears after 4 hours of milling. The formation of the HMS compound begins after 6 hours of mechanical milling. The complete reaction of the elements is observed after 18 hours. Fig. 1. X-ray diffraction of elemental powder mixture corresponding to chemical composition MnSi1.75 at different milling times. The evolution of morphology and the distribution map of chemical elements was analyzed and is presented in Fig. 2. The SEM image presented in Fig. 2a is recorded on starting powders mixture. The particles present polyhedral irregular shapes. The distribution map for starting mixture reveals a good homogenisation of particles before loading in the vial for the milling process. A good homogenisation of powders is necessary to reduce as much as possible the silicon deposit on the balls, being more ductile than manganese, this can lead to the increase of the mechanical alloying duration as has been already reported in [20]. The image of powder milled for 4 hours (Figure 2 b), presents agglomeration of powder particles. After 4 hours of milling, manganese is more homogenously distributed as compared with the starting sample, due to initiation of the alloying process by milling. Powder milled 18 hours presents an irregular shape, Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 83 the dimension of the particles is small than 50 μm and much smaller as compared to particles of the starting sample. The sample milled for 18 hours shows particles with a size of less than 1 μm and particles with a size of a few micrometers that are composed of fine particles that are welded together. After 18 hours of mechanical milling, the distributions maps of manganese and silicon are uniform. Fig. 2. SEM images for probe milled a) 0h; b) 4h; 18h. Map distribution of manganese is presented in d, e, f, and for silicon in g, h, i for initial mixture; and 4 hours; and respectively 18 hours.â Fig. 3. DSC analysis of the sample milled for 4 hours. Heating was made up to 1000 C with a heating rate of 10 C / min. Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 84 DSC analyses of the sample milled for 4 hours is shown in Fig. 3. The DSC curve on heating up to 1000 C present 5 distinct phenomena but the curve on cooling does not present any phenomena. To identify the phase transition after each event, X-ray d","PeriodicalId":20390,"journal":{"name":"Powder Metallurgy and Advanced Materials","volume":null,"pages":null},"PeriodicalIF":0.0000,"publicationDate":"2018-11-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Caracterisation of high manganese silicides prepared by mechanical milling\",\"authors\":\"I. Chicinaș, V. Popescu, T. Marinca, V. Cebotari, F. Popa\",\"doi\":\"10.21741/9781945291999-9\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"The mechanical milling of manganese and silicon powder in a planetary ball mill up to 18 h was performed. In the X-ray diffraction pattern recorded after 18 hours of milling the MnSi phase and Mn15Si26 compound are detected. The agglomeration of powders after complete reaction of the elements was observed by scanning electron microscopy. Heating up at 1000 °C, an unreacted sample, milled 4 hours, has found to have the effect of completing the reaction of elements, but forms oxides. Handling of the powder during sampling, without protective atmosphere was found to form oxides. The oxidation of the samples was evidenced by FTIR analysis. Introduction The modern society has the tendency to increase the quantity of hydrocarbons which are transformed into energy, with negative effects on the environment. To reduce this impact alternatives are searched. Thermoelectric materials represent a solution to improve the quality of the environment by reducing the combustion product gases. These materials are able to convert the thermal energy directly into electrical energy and vice versa. The quality of a thermoelectric material can be estimated by the figure of merit ZT=SσT/k where: S is the Seebeck coefficient, σ is electrical conductivity, T is temperature and k is thermal conductivity [1]. Thermoelectric materials can convert heat from a different source such as solar heat, geothermal heat or exhaust gases [2]. From the studied thermoelectric materials, those based on silicon, especially High Manganese Silicide (HMS) is friendly with the environment and considered as promising candidates. HMS is chemically stable [3] and are preferred in detriment of those based on Pb-Te which operate in the same range of temperature. The HMS materials are nontoxic as well as their constituent chemical elements [4]. HMS are thermoelectric compounds with p-type conduction, having general formula MnSix where the x value ranges from 1.67 up to 1.87 [5] and with an energy gap of 0.77 [eV] [6]. HMS system contains four compounds, Mn4Si7, Mn11Si19, Mn15Si26, and Mn27Si47, all with the same electronic structure [7]. Crystallographic structure of HMS compounds belongs to Nowotny chimney ladder (NCL) phases, where manganese is located in the corners of tetragon and silicon are arranged inside in the form of a spiral [8]. MnSi1.75 compound presents the largest ZT, while the MnSi1.77 compound has the smallest value. The low value for the figure of merit is the effect of Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 81 a large thermal conductivity [6]. Problem with HMS is that obtaining method influence the final phase. Based on the preparation method it is possible to obtain different compounds: by vacuum levitation melting Mn15Si26 is obtained, Mn4Si7 may obtain by vacuum levitation-induction melting and by dry milling [9-11]. Preparation by melting leads to an inhomogeneous structure and coarse microstructure [12]. The obtaining by mechanical alloying has the advantage of obtaining a small crystallite size which leads to lower thermal conductivity [11]. Also, dry milling leads to the decrease of the quantity of MnSi secondary phase, which reduces the thermoelectric proprieties. In the milling experiments, using different process control agents (PCA) it is possible to control the MnSi phase. Hexane conducts to the formation of 38.8% of MnSi phase, acetone to 8.7% and ethanol to 5.3%. Milling without any PCA leads to the formation of 49.5% MnSi/HMS phase [10]. In order to obtain the proper HMS, the conditions can be summarized to be small milling time and high rotation speed according to [6, 13]. Prolonged milling conducts to the decomposition of HMS compound in MnSi phase as a result of the excess energy which is generated by collisions [14]. The increase in the thermoelectric properties can be achieved by doping. Adding Yb, the carrier concentration increases, and MnSi phase quantity decreases [14]. By doping with Co a homogenous microstructure is obtained and the ZT increases proportionally to the concentration of Co [12]. Other chemical elements that are studied for increasing the thermoelectric proprieties are Cr, Ti, Fe, Al, and Ge. The doping increases the thermoelectric proprieties only if the concentration of elements does not exceed the limit of solubility because the doped elements are located at Mn sites [15-18]. The present paper is focused on the synthesis of HMS with the chemical composition MnSi1.75. The formation of this compound by mechanical milling is studied as a function of the milling time. The paper presents the evolution of the powder morphology and the distribution of the chemical elements in the samples after milling. The thermal stability of powders is also presented and discussed. Experimental The thermoelectric material has been obtained starting from elemental powders of manganese with purity of 99.3% (-325 mesh, Alfa Aesar) and silicon with purity of 99.9% (-100 mesh, Alfa Aesar), in a stoichiometric ratio corresponding to MnSi1.75 compound formula. The powder mixture was loaded into the vial with grinding media after the prior homogenisation of the elemental powders. The mechanical milling was made in a planetary ball mill Fritsch Pulverisette 6 using a ball to powder mass ratio (BPR) of 10:1, and a 400 rpm rotational speed of vial. The vial and balls are of stainless steel with a diameter of 14 mm. For the protection of powders which are subjected to milling process from oxidation, the milling was done under argon atmosphere. The milling was conducted up to 18 hours, and sampling was done after the following milling times: 0, 1, 2, 4, 6, 8, 10, 14 and 18 hours. The structural study and phases composition of the samples were investigated by X-ray diffraction using an INEL 3000 Equinox diffractometer using Kα radiation of Co (λ = 1.79026 Å). To study the morphology of the powders and the local chemical homogeneity a JEOLJSM 5600 LV electron microscope equipped with EDX spectrometer (Oxford Instruments, INCA 2000 soft) was used. The thermal stability of the samples was investigated by differential scanning calorimetry (DSC), using a LabSys-Setaram apparatus. The DSC investigations were performed in an argon atmosphere, up to 1000 °C, with heating/cooling rate of 10 °C/min using alumina as a reference sample. The presence of oxide inside the probe was investigated by the Fourier Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 82 Transform Infrared (FTIR) technique using Spectrum BX II apparatus. The experiment was realised by embedding the Mn-Si powder into the potassium bromide pellet. Results and discussion X-ray diffraction patterns of the mechanically milled powders are presented in Fig. 1. In diffraction pattern of the starting sample are identified the peaks corresponding to the used elemental powders. In the diffraction patterns corresponding to the sample milled for one hour is observed a reduction of the peaks intensity and a pronounced broadening. This is assigned to the reduction of crystallite size and an increase of the internal stresses [19]. The sample milled for 2 hours presents similar behavior. A new MnSi phase appears after 4 hours of milling. The formation of the HMS compound begins after 6 hours of mechanical milling. The complete reaction of the elements is observed after 18 hours. Fig. 1. X-ray diffraction of elemental powder mixture corresponding to chemical composition MnSi1.75 at different milling times. The evolution of morphology and the distribution map of chemical elements was analyzed and is presented in Fig. 2. The SEM image presented in Fig. 2a is recorded on starting powders mixture. The particles present polyhedral irregular shapes. The distribution map for starting mixture reveals a good homogenisation of particles before loading in the vial for the milling process. A good homogenisation of powders is necessary to reduce as much as possible the silicon deposit on the balls, being more ductile than manganese, this can lead to the increase of the mechanical alloying duration as has been already reported in [20]. The image of powder milled for 4 hours (Figure 2 b), presents agglomeration of powder particles. After 4 hours of milling, manganese is more homogenously distributed as compared with the starting sample, due to initiation of the alloying process by milling. Powder milled 18 hours presents an irregular shape, Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 83 the dimension of the particles is small than 50 μm and much smaller as compared to particles of the starting sample. The sample milled for 18 hours shows particles with a size of less than 1 μm and particles with a size of a few micrometers that are composed of fine particles that are welded together. After 18 hours of mechanical milling, the distributions maps of manganese and silicon are uniform. Fig. 2. SEM images for probe milled a) 0h; b) 4h; 18h. Map distribution of manganese is presented in d, e, f, and for silicon in g, h, i for initial mixture; and 4 hours; and respectively 18 hours.â Fig. 3. DSC analysis of the sample milled for 4 hours. Heating was made up to 1000 C with a heating rate of 10 C / min. Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 84 DSC analyses of the sample milled for 4 hours is shown in Fig. 3. The DSC curve on heating up to 1000 C present 5 distinct phenomena but the curve on cooling does not present any phenomena. To identify the phase transition after each event, X-ray d\",\"PeriodicalId\":20390,\"journal\":{\"name\":\"Powder Metallurgy and Advanced Materials\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":0.0000,\"publicationDate\":\"2018-11-05\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Powder Metallurgy and Advanced Materials\",\"FirstCategoryId\":\"1085\",\"ListUrlMain\":\"https://doi.org/10.21741/9781945291999-9\",\"RegionNum\":0,\"RegionCategory\":null,\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"\",\"JCRName\":\"\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Powder Metallurgy and Advanced Materials","FirstCategoryId":"1085","ListUrlMain":"https://doi.org/10.21741/9781945291999-9","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"","JCRName":"","Score":null,"Total":0}
引用次数: 0

摘要

在行星球磨机上对锰粉和硅粉进行了18 h的机械研磨。在铣削18小时后记录的x射线衍射图中检测到MnSi相和Mn15Si26化合物。用扫描电镜观察了元素完全反应后粉末的团聚现象。在1000℃下加热,未反应的样品,研磨4小时,发现有完成元素反应的效果,但形成氧化物。在取样过程中,在没有保护气氛的情况下处理粉末,发现会形成氧化物。FTIR分析证实了样品的氧化作用。现代社会有增加转化为能源的碳氢化合物数量的趋势,这对环境有负面影响。为了减少这种影响,寻找替代方案。热电材料是通过减少燃烧产物气体来改善环境质量的一种解决方案。这些材料能够将热能直接转化为电能,反之亦然。热电材料的质量可以用性能曲线ZT=SσT/k来评价,其中S为塞贝克系数,σ为电导率,T为温度,k为导热系数[1]。热电材料可以转换来自不同热源的热量,如太阳能、地热或废气[2]。从所研究的热电材料来看,硅基热电材料,特别是高硅化锰(HMS)具有良好的环境友好性,被认为是有前途的候选材料。HMS具有化学稳定性[3],相对于在相同温度范围内工作的基于Pb-Te的HMS, HMS是首选。HMS材料是无毒的,其组成化学元素[4]。HMS是具有p型导电性的热电化合物,通式MnSix, x值为1.67 ~ 1.87[5],能隙为0.77 [eV][6]。HMS体系包含四种化合物Mn4Si7、Mn11Si19、Mn15Si26和Mn27Si47,它们具有相同的电子结构[7]。HMS化合物的晶体结构属于诺沃特尼烟囱梯(nootny chimney ladder, NCL)相,其中锰位于四边形的四角,硅在其内呈螺旋状排列[8]。MnSi1.75化合物ZT值最大,MnSi1.77化合物ZT值最小。低值的优点是影响粉末冶金和先进材料- RoPM&AM 2017材料研究论坛LLC材料研究论文集8 (2018)88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 81大导热系数[6]。HMS的问题是获取方法会影响最终阶段。根据制备方法的不同,可以得到不同的化合物:真空悬浮熔融可以得到Mn15Si26,真空悬浮感应熔融和干磨可以得到Mn4Si7[9-11]。熔融制备导致组织不均匀,微观结构粗糙[12]。机械合金化法的优点是获得的晶粒尺寸小,导热系数较低[11]。干磨导致MnSi二次相数量减少,降低了热电性能。在铣削实验中,使用不同的过程控制剂(PCA)可以控制MnSi相。己烷传导形成38.8%的MnSi相、8.7%的丙酮相和5.3%的乙醇相。在没有任何PCA的情况下进行铣削,形成了49.5%的MnSi/HMS相[10]。要获得合适的HMS,根据[6,13]可以总结为铣削时间短、转速高。由于碰撞产生的多余能量,长时间的铣削导致了MnSi相HMS化合物的分解[14]。热电性能的提高可以通过掺杂来实现。添加Yb,载流子浓度增加,MnSi相数量减少[14]。通过掺杂Co得到均匀的微观结构,ZT随Co浓度成比例增加[12]。其他被研究用于提高热电性能的化学元素有Cr、Ti、Fe、Al和Ge。由于掺杂元素位于Mn位[15-18],所以只有在元素浓度不超过溶解度限制的情况下,掺杂才会增加热电性能。本文主要研究了化学成分为MnSi1.75的HMS的合成。研究了该化合物在机械铣削过程中的形成与铣削时间的关系。本文介绍了粉末经碾磨后的形貌变化及样品中化学元素的分布。并对粉体的热稳定性进行了讨论。
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Caracterisation of high manganese silicides prepared by mechanical milling
The mechanical milling of manganese and silicon powder in a planetary ball mill up to 18 h was performed. In the X-ray diffraction pattern recorded after 18 hours of milling the MnSi phase and Mn15Si26 compound are detected. The agglomeration of powders after complete reaction of the elements was observed by scanning electron microscopy. Heating up at 1000 °C, an unreacted sample, milled 4 hours, has found to have the effect of completing the reaction of elements, but forms oxides. Handling of the powder during sampling, without protective atmosphere was found to form oxides. The oxidation of the samples was evidenced by FTIR analysis. Introduction The modern society has the tendency to increase the quantity of hydrocarbons which are transformed into energy, with negative effects on the environment. To reduce this impact alternatives are searched. Thermoelectric materials represent a solution to improve the quality of the environment by reducing the combustion product gases. These materials are able to convert the thermal energy directly into electrical energy and vice versa. The quality of a thermoelectric material can be estimated by the figure of merit ZT=SσT/k where: S is the Seebeck coefficient, σ is electrical conductivity, T is temperature and k is thermal conductivity [1]. Thermoelectric materials can convert heat from a different source such as solar heat, geothermal heat or exhaust gases [2]. From the studied thermoelectric materials, those based on silicon, especially High Manganese Silicide (HMS) is friendly with the environment and considered as promising candidates. HMS is chemically stable [3] and are preferred in detriment of those based on Pb-Te which operate in the same range of temperature. The HMS materials are nontoxic as well as their constituent chemical elements [4]. HMS are thermoelectric compounds with p-type conduction, having general formula MnSix where the x value ranges from 1.67 up to 1.87 [5] and with an energy gap of 0.77 [eV] [6]. HMS system contains four compounds, Mn4Si7, Mn11Si19, Mn15Si26, and Mn27Si47, all with the same electronic structure [7]. Crystallographic structure of HMS compounds belongs to Nowotny chimney ladder (NCL) phases, where manganese is located in the corners of tetragon and silicon are arranged inside in the form of a spiral [8]. MnSi1.75 compound presents the largest ZT, while the MnSi1.77 compound has the smallest value. The low value for the figure of merit is the effect of Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 81 a large thermal conductivity [6]. Problem with HMS is that obtaining method influence the final phase. Based on the preparation method it is possible to obtain different compounds: by vacuum levitation melting Mn15Si26 is obtained, Mn4Si7 may obtain by vacuum levitation-induction melting and by dry milling [9-11]. Preparation by melting leads to an inhomogeneous structure and coarse microstructure [12]. The obtaining by mechanical alloying has the advantage of obtaining a small crystallite size which leads to lower thermal conductivity [11]. Also, dry milling leads to the decrease of the quantity of MnSi secondary phase, which reduces the thermoelectric proprieties. In the milling experiments, using different process control agents (PCA) it is possible to control the MnSi phase. Hexane conducts to the formation of 38.8% of MnSi phase, acetone to 8.7% and ethanol to 5.3%. Milling without any PCA leads to the formation of 49.5% MnSi/HMS phase [10]. In order to obtain the proper HMS, the conditions can be summarized to be small milling time and high rotation speed according to [6, 13]. Prolonged milling conducts to the decomposition of HMS compound in MnSi phase as a result of the excess energy which is generated by collisions [14]. The increase in the thermoelectric properties can be achieved by doping. Adding Yb, the carrier concentration increases, and MnSi phase quantity decreases [14]. By doping with Co a homogenous microstructure is obtained and the ZT increases proportionally to the concentration of Co [12]. Other chemical elements that are studied for increasing the thermoelectric proprieties are Cr, Ti, Fe, Al, and Ge. The doping increases the thermoelectric proprieties only if the concentration of elements does not exceed the limit of solubility because the doped elements are located at Mn sites [15-18]. The present paper is focused on the synthesis of HMS with the chemical composition MnSi1.75. The formation of this compound by mechanical milling is studied as a function of the milling time. The paper presents the evolution of the powder morphology and the distribution of the chemical elements in the samples after milling. The thermal stability of powders is also presented and discussed. Experimental The thermoelectric material has been obtained starting from elemental powders of manganese with purity of 99.3% (-325 mesh, Alfa Aesar) and silicon with purity of 99.9% (-100 mesh, Alfa Aesar), in a stoichiometric ratio corresponding to MnSi1.75 compound formula. The powder mixture was loaded into the vial with grinding media after the prior homogenisation of the elemental powders. The mechanical milling was made in a planetary ball mill Fritsch Pulverisette 6 using a ball to powder mass ratio (BPR) of 10:1, and a 400 rpm rotational speed of vial. The vial and balls are of stainless steel with a diameter of 14 mm. For the protection of powders which are subjected to milling process from oxidation, the milling was done under argon atmosphere. The milling was conducted up to 18 hours, and sampling was done after the following milling times: 0, 1, 2, 4, 6, 8, 10, 14 and 18 hours. The structural study and phases composition of the samples were investigated by X-ray diffraction using an INEL 3000 Equinox diffractometer using Kα radiation of Co (λ = 1.79026 Å). To study the morphology of the powders and the local chemical homogeneity a JEOLJSM 5600 LV electron microscope equipped with EDX spectrometer (Oxford Instruments, INCA 2000 soft) was used. The thermal stability of the samples was investigated by differential scanning calorimetry (DSC), using a LabSys-Setaram apparatus. The DSC investigations were performed in an argon atmosphere, up to 1000 °C, with heating/cooling rate of 10 °C/min using alumina as a reference sample. The presence of oxide inside the probe was investigated by the Fourier Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 82 Transform Infrared (FTIR) technique using Spectrum BX II apparatus. The experiment was realised by embedding the Mn-Si powder into the potassium bromide pellet. Results and discussion X-ray diffraction patterns of the mechanically milled powders are presented in Fig. 1. In diffraction pattern of the starting sample are identified the peaks corresponding to the used elemental powders. In the diffraction patterns corresponding to the sample milled for one hour is observed a reduction of the peaks intensity and a pronounced broadening. This is assigned to the reduction of crystallite size and an increase of the internal stresses [19]. The sample milled for 2 hours presents similar behavior. A new MnSi phase appears after 4 hours of milling. The formation of the HMS compound begins after 6 hours of mechanical milling. The complete reaction of the elements is observed after 18 hours. Fig. 1. X-ray diffraction of elemental powder mixture corresponding to chemical composition MnSi1.75 at different milling times. The evolution of morphology and the distribution map of chemical elements was analyzed and is presented in Fig. 2. The SEM image presented in Fig. 2a is recorded on starting powders mixture. The particles present polyhedral irregular shapes. The distribution map for starting mixture reveals a good homogenisation of particles before loading in the vial for the milling process. A good homogenisation of powders is necessary to reduce as much as possible the silicon deposit on the balls, being more ductile than manganese, this can lead to the increase of the mechanical alloying duration as has been already reported in [20]. The image of powder milled for 4 hours (Figure 2 b), presents agglomeration of powder particles. After 4 hours of milling, manganese is more homogenously distributed as compared with the starting sample, due to initiation of the alloying process by milling. Powder milled 18 hours presents an irregular shape, Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 83 the dimension of the particles is small than 50 μm and much smaller as compared to particles of the starting sample. The sample milled for 18 hours shows particles with a size of less than 1 μm and particles with a size of a few micrometers that are composed of fine particles that are welded together. After 18 hours of mechanical milling, the distributions maps of manganese and silicon are uniform. Fig. 2. SEM images for probe milled a) 0h; b) 4h; 18h. Map distribution of manganese is presented in d, e, f, and for silicon in g, h, i for initial mixture; and 4 hours; and respectively 18 hours.â Fig. 3. DSC analysis of the sample milled for 4 hours. Heating was made up to 1000 C with a heating rate of 10 C / min. Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 88-88 doi: http://dx.doi.org/10.21741/9781945291999-9 84 DSC analyses of the sample milled for 4 hours is shown in Fig. 3. The DSC curve on heating up to 1000 C present 5 distinct phenomena but the curve on cooling does not present any phenomena. To identify the phase transition after each event, X-ray d
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