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