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Reaction and diffusion phenomena in Ag-doped Mg2Si
Journal of Alloys and Compounds 657 (2016) 755e764
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Reaction and diffusion phenomena in Ag-doped Mg2Si Elzbieta M. Godlewska a, *, Krzysztof Mars a, Pawel Drozdz b, Adam Tchorz c, Marianna Ksiazek c a
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Al. A. Mickiewicza 30, 30-059 Krakow, Poland AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Al. A. Mickiewicza 30, 30-059 Krakow, Poland c Foundry Research Institute, ul. Zakopianska 73, 30-418 Krakow, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 August 2015 Received in revised form 17 October 2015 Accepted 19 October 2015 Available online 22 October 2015
The Ag-doped Mg2Siebased materials are known for unstable thermoelectric behavior; however, the reasons are not sufficiently understood. In this work a range of experiments, analytical tools and calculations was used to unravel some questions related to the reactivity and diffusion properties of the Ag eMgeSi system, which might contribute to the variations of thermopower. Among these, the most informative appeared quenching and diffusion experiments, calculations of phase evolution by Fact Sage and x-ray tomography of Ag-doped materials produced from powders. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ag-doped Mg2Si Powder metallurgy Diffusion Microstructure Thermochemistry
1. Introduction Magnesium silicide derivatives, in particular those alloyed with germanium or tin are under investigation as components of thermoelectric modules for energy harvesting [1e3]. It has been demonstrated that their electrical properties can be enhanced by doping with group 13 (p-type) or 15 (n-type) elements. In the former case the dopant should substitute for silicon, while in the latter the site occupancy is of no importance. The advantageous effects of antimony and bismuth as n-type dopants in Mg2Si and Mg2Si alloyed with tin or/and germanium are well documented [4,5] and the highest reported figure of merit ZT is about 1.4 [6]. Among the p-type dopants, the most promising are gallium [1] and silver [7] but in both cases maximum ZT does not exceed 0.4 at moderate temperatures. Thermoelectric properties of the silver doped Mg2Si-based materials, and thermopower in particular, are ambiguous and apparently hardly reproducible upon thermal cycling [7,8]. Attempts are made therefore to get more insight into the behavior of p-type dopants and to improve their performance. The objective of this work was to collect additional experimental data, which might shed light on the known instability problem of
* Corresponding author. E-mail address: [email protected] (E.M. Godlewska). http://dx.doi.org/10.1016/j.jallcom.2015.10.174 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Ag-doped Mg2Si, these encompassing assessments of the reaction and diffusion phenomena in the MgeSieAg system, thermodynamic calculations focused on the stability limits of the constituent phases and kinetic effects associated with the variations of the Seebeck coefficient with temperature. 2. Experimental Starting materials were Alpha Aesar elemental powders: silicon (99.999%, 100 þ 325 mesh), magnesium (99.8%, 20 þ 100 mesh) and silver (99.999%, þ325 mesh) or silver oxalate, Ag2C2O4, mixed in the proportions corresponding to the nominal compositions: Mg2Si0.98Ag0.02 and Mg2Si0.95Ag0.05. Silver oxalate was selected as silver source because it decomposes into nanosized metallic powder and carbon dioxide. It was synthesized in the laboratory from POCH S.A. analytically pure AgNO3 and C2H2O4 $ 2 H2O. The phase composition of dry precipitate was confirmed by x-ray diffraction (XRD). Freshly prepared powder was used for in situ and ex situ doping of magnesium silicide. The in situ doping procedure was similar to the earlier reported manufacturing of Mg2Si [9,10]. The powders were homogenized and cold pressed under a load of 5 T. The exothermic reaction was initiated at about 873 K (600 C) under a dynamic vacuum of 10 Pa. The products were annealed at 873 K (600 C) for 1 h, passed through a sieve and hot pressed in a graphite die under an argon atmosphere (Thermal Technology Hot
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Press). The maximum applied pressure was 25 MPa, temperature 1223 K (950 C), time e 1 h and the heating rate - 10 C/min. In the ex situ doping procedure, silver oxalate was added to the Mg2Si powder before densification. Reactants and products after each manufacturing step were analyzed for phase composition, chemical composition and microstructure by x-ray diffraction (XRD) (Panalytical X'Pert), scanning electron microscopy (SEM) (Nova Nano SEM 200 FEI Europe Company) and energy dispersive x-ray spectroscopy (EDS) (EDS/EDAX). Characterization of the final product involved measurements of density (Micrometrics Helium Pycnometer Accu-Pyc 1330), thermal diffusivity, specific heat (Netzsch instruments: STA 449 F1 Jupiter, DIL 402 C, LFA 427) and Seebeck coefficient. The latter were conducted under helium in a calibrated home-made device from room temperature (RT) to 873 K (600 C). The whole heating/cooling cycle lasted 24 h. The effects of doping procedure on the formation/disappearance of minor phases upon thermal cycling were assessed through thermochemical calculations using Fact Sage Thermochemical Software and Databases. Two limiting cases were considered: silver introduced at the synthesis step (2 Mg þ 0.098Si þ 0.02 Ag) and silver introduced at the hot pressing step (Mg2Si þ 0.02 Ag). Diffusion couples Ag/Mg2Si were prepared to identify transport processes and reactions in the ex situ doped materials. Silver was deposited onto Mg2Si substrate by magnetron sputtering. After vacuum annealing at 550 C for 1 h, a cross-section of the sample was examined by SEM/EDS and TEM/ EDS (Philips CM-20). Spatial distribution of components in the in situ doped Mg2Si0.95Ag0.05 was revealed by using Phoenix X-ray CT
nanotom with a voxel size of 1 mm. In the quenching experiment, an in situ doped sample with nominal composition Mg2Si0.98Ag0.02, was heated at 873 K (600 C) for 1 h in an evacuated quartz tube and next rapidly cooled in liquid nitrogen to preserve the hightemperature structure and properties. After quenching the procedure of Seebeck coefficient measurement was repeated. Duration of the overall cycle from RT to 873 K (600 C) and back to RT was 24 h. 3. Results 3.1. Microstructure, composition and density As earlier reported [11], the reaction products were nanostructured, consisting of agglomerates of nanosized crystallites. In general, morphology and the particle sizes of the solid products were related to those of silicon powder used in the synthesis [9,10]. Backscattered electron images (SEM-BEI) of the hot pressed materials are presented in Figs. 1 and 2. The materials are multiphase with bright Ag-rich precipitates confirmed by EDS. At a lower magnification (Fig. 1), samples with the same nominal composition (Fig. 1a,c and b,d) received in the synthesis with metallic silver (Fig. 1a,b) and silver oxalate (Fig. 1c,d), have similar microstructures. However, at a higher magnification (Fig. 2) the grains are noticeably smaller when silver is introduced as a compound. Average compositions of the sintered materials, as determined by EDS are presented in Table 1. The average silver concentration is about 0.8 at% and 1.35 at% for the nominal compositions
Fig. 1. Cross-sections of samples with nominal composition: Mg2Si0.98Ag0.02 (a, c), Mg2Si0.95Ag0.05 (b, d) reacted with metallic silver (a, b) and silver oxalate (c, d); backscattered electron images.
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Fig. 2. Cross-sections of samples with nominal composition: Mg2Si0.98Ag0.02 (a, c), Mg2Si0.95Ag0.05 (b, d) reacted with metallic silver (a, b) and silver oxalate (c, d); backscattered electron images at a higher magnification.
Table 1 Average composition of Ag-doped Mg2Si samples (EDS). Sample nominal composition (Ag source)
Mg2Si0.98Ag0.02 Mg2Si0.95Ag0.05 Mg2Si0.98Ag0.02 Mg2Si0.95Ag0.05 Mg2Si0.98Ag0.02
(Ag metallic) (Ag metallic) (Ag2C2O4) (Ag2C2O4) (Ag2C2O4 ex situ)
Concentration, at%
Mg/Si
Mg
Si
Ag
65.15 66.50 65.27 64.98 64.94
34.05 32.16 33.96 33.65 34.29
0.80 1.34 0.77 1.36 0.77
1.91 2.07 1.92 1.93 1.89
Mg2Si0.98Ag0.02 and Mg2Si0.95Ag0.05, respectively. The Mg:Si atomic ratio is about 2:1. As can be seen in Table 2, independently of the doping procedure, the densities determined by helium pycnometer are similar for the same nominal composition, about 2.01 and 2.15 g/cm3 for Mg2Si0.98Ag0.02 and Mg2Si0.95Ag0.05, respectively. According to local EDS analyses, concentration of silver in the Mg2Si
Table 2 Nominal composition and density of investigated materials. Sample composition
Density, g/cm3
Mg2Si Mg2Si0.98Ag0.02 Mg2Si0.98Ag0.02 Mg2Si0.95Ag0.05 Mg2Si0.95Ag0.05
1.99 2.01 2.04 2.15 2.15
(Ag metallic) (Ag2C2O4) (Ag metallic) (Ag2C2O4)
matrix ranged from 0.24 to 0.33 at%. Powder XRD (PXRD) patterns of samples with nominal composition Mg2Si0.98Ag0.02 received according to the in situ and ex situ doping procedure revealed Mg2Si along with AgMg and minor impurities: MgO and Si [11]. No carbides were detected in the reaction products when silver oxalate was used as silver source. The amount of AgMg increased with the concentration of this element in the initial powder mixture. 3.2. Seebeck coefficient Temperature dependence of the Seebeck coefficient for the in situ doped material, Mg2Si0.98Ag0.02, as received after hot pressing (HP) and after an additional 5-h vacuum annealing at 773 K (500 C) is shown in Fig. 3. For the sample after HP the values were positive over the entire temperature range from RT to about 873 K (600 C) with a maximum of about 290 mV/K at 423 K (150 C) and minimum of about 130 mV/K between 573 and 623 K (300 and 350 C). For the sample annealed before the measurements, the Seebeck coefficient was negative at up to 373 K (100 C) and again positive at temperatures higher than 573 K (300 C); the value of þ200 mV/K being attained at about 873 K (600 C). Variations of the Seebeck coefficient upon heating and cooling of the samples with the same nominal composition, Mg2Si0.98Ag0.02, received according to the ex situ doping procedure with silver oxalate added to Mg2Si before hot pressing, are presented in Fig. 4. The changeover from negative to positive values of the Seebeck coefficient occurred repeatedly during four consecutive cycles at about 573 K (300 C).
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The maximum positive value reached þ200 mV/K. The effect of annealing and quenching on the variations of the Seebeck coefficient upon heating and cooling is presented in Fig. 5. The in situ doped sample annealed at 873 K (600 C) for 1 h and quenched had high positive thermopower at room temperature, which remained positive upon heating. Upon cooling the variations of thermopower were practically the same as those described for other samples. 3.3. Diffusion experiment
Fig. 3. Variations of the Seebeck coefficient upon heating and cooling of Mg2Si0.98Ag0.02; ‘after HP’ e as received without any additional heat treatment, 1st run and 2nd run e samples subjected to 5-h vacuum annealing at 773 K (500 C) before the measurements; Tup, Tdwn e heating and cooling, respectively; duration of each heating/cooling cycle e 24 h.
Analysis of the near surface layers after 1-h annealing at 823 K (550 C) is shown in Fig. 6. The initial layer of silver (about 6e7 mm thick) deposited by magnetron sputtering on the surface of undoped magnesium silicide is totally transformed into AgMg. Underneath, there is an interdiffusion zone, with an almost equimolar average composition, as determined by EDS. Two constituent phases largely differ in Si concentration and Mg:Ag atomic ratio. For the bright phase with visible nanometric dispersions, the respective numbers are 19 at% and about 1:1, whereas for the dark single phase 50 at% and 2:1. TEM images and EDS mapping of elements in the interdiffusion zone are presented in Fig. 7. According to local EDS analysis, the two phases are pure silicon and AgMg. Twinned silicon crystals are visible in Fig. 7e (point 1). Very small dispersions within the AgMg matrix (area 2 in Fig. 7b) are beyond the detection limit. 3.4. Thermochemical calculations The results of thermochemical calculations for the two limiting cases, in situ doping (2 Mg þ 0.98Si þ 0.02 Ag) and ex situ doping (Mg2Si þ 0.02 Ag), are shown in Fig. 8. In particular, the graphs reveal the evolution of minor phases over the temperature range from 293 to 1073 K (20e800 C). In case 1 (in situ doping), the AgMg (BCC1) phase is rich in magnesium and its amount decreases upon cooling. This is accompanied by reversal of the deviation from stoichiometry toward silver excess brought about by the formation of AgMg3 at about 773 K (500 C), which is then stable down to the room temperature. The Mg-rich MgeAgeSi liquid (LIQU) appears already at about 873 K (600 C) and coexists with one solid phase, Mg2Si. In case 2 (ex situ doping), the AgMg (BCC1) phase is rich in
Fig. 4. Variations of the Seebeck coefficient upon heating and cooling for Mg2Si0.98Ag0.02 received according to the ex situ doping procedure with silver oxalate added before densification; Tup, Tdwn e heating and cooling, respectively; duration of each heating/cooling cycle e 24 h.
Fig. 5. Variations of the Seebeck coefficient upon heating and cooling for Mg2Si0.98Ag0.02 received according to the in situ doping procedure; HP eas hot pressed; HP þ HT þ Qch e hot pressed, heat treated at 873 K (600 C) for 1 h and quenched before the measurements; Tup, Tdwn e heating and cooling, respectively; duration of each heating/cooling cycle e 24 h.
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Fig. 6. SEM micrographs of a diffusion couple (a) before annealing e Ag layer on Mg2Si substrate; (b) after annealing (823 K/550 C, 1 h) - visible interdiffusion zone; (c) two-phase region e frame in (b) and its average composition; (d) EDS analysis in points 1e5 depicted in (b).
silver and its amount only slightly decreases upon cooling. The Ag concentration in this phase gradually increases. At about 523 K (250 C), however, the Ag-rich AgMg decomposes. This is accompanied by the formation of Ag3Mg and decrease in Si content. The Mg-rich liquid (LIQU) appears at about 973 K (700 C). Silicon as a solid phase is present over the whole temperature range under consideration.
of the Ag-doped Mg2Si. Figs. 9 b,c show locations of two minor components in this element and Fig. 9d e distribution of silicon in undoped Mg2Si. Similarity of pictures in Fig. 9c and d allows for the statement that both correspond to the unreacted or excess silicon. The 2-D distribution of Ag-rich phase (SEM image in Fig. 1b) seems to coincide with the diffused cellular structure in Fig. 9b. MgO is apparently not distinguishable by this technique.
3.5. X-ray tomography
3.6. Thermal properties
X-ray tomography was used to assess 3-D distribution of different components in the multiphase material with nominal composition Mg2Si0.95Ag0.05 received by in situ doping with metallic silver and in undoped Mg2Si. Fig. 9a shows a cube-shaped element, with the edge length of 250 mm, selected for the analysis
In the temperature range from RT to about 873 K (600 C) thermal properties of Mg2Si samples modified with silver were similar, independently of whether it was introduced as elemental powder or silver oxalate. Average thermal conductivity of the in situ doped samples decreased from about 9.5 to 4.5 W/m-K, as can be
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Fig. 7. STEM image (a) and area maps of elements from the two-phase region of the diffusion couple after annealing; (b) AgeK; (c) MgeK; (d) SieK; BF TEM image, showing twinned crystals of silicon (1) and AgMg matrix (2) at a higher magnification (e).
seen in Fig. 10. Slightly higher values were noted for the samples with the nominal composition Mg2Si0.95Ag0.05. 4. Discussion Mg2Si and its derivatives can be produced from elemental powders by different technological routes with or without preliminary mechanical activation by milling. The reaction in a powder mixture is initiated by resistance/induction heating or microwaves. Due to high volatility of magnesium, the vaporesolid reaction starts
at temperatures much lower than the melting point of this element and the products are either nanostructured powders or loosely bonded porous sinters. The additives and dopants are generally introduced to the starting powder mixture but their behavior in the reaction and subsequent heat treatment largely depends on their physical and chemical properties as well as diffusivity and solubility in the Mg2Si matrix. Low melting elements may enter the structure of Mg2Si already during the synthesis acting at the same time as sintering aids, while high melting elements may form dispersions in Mg2Si and diffuse into the matrix upon subsequent heat
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Fig. 8. Evolution of minor phases over temperature calculated by Fact Sage for two cases: (a) in situ doping: 2 Mg þ 0.98Si þ 0.02 Ag; (b) ex situ doping: Mg2Si þ 0.02 Ag.
treatment and/or sintering. Additives may also react with magnesium or silicon. Depending on thermodynamics and kinetics of such reactions, the products may consist of Mg2Si as the major phase and other compounds formed by magnesium or silicon with the additives. The side reactions with the additives may produce excess of magnesium or silicon in the final product. Indeed, according to XRD measurements, the products of self-propagating reaction in the MgeSieAg mixture contained Mg2Si, AgMg as well as small amounts of Si and/or MgO, sometimes not detectable by XRD. Unexpectedly, the samples doped at the hot pressing stage contained also AgMg, which means that Mg2Si can react with Ag. Based on the data in Table 3, AgMg is only slightly less stable than Mg2Si. At 773 K (500 C), the respective standard Gibbs free energy of formation per magnesium mol-atom is approximately 34.2 kJ/mol and 36.3 kJ/mol [12,13] and the melting point of AgMg is 820 C [14] versus 1081 C for Mg2Si [15]. The appearance of AgMg in the final product, independently of doping procedure, indicates that magnesium reactions with silicon and silver are competitive. In the case of in situ doping, in a short time interval, typical of selfpropagating reactions, magnesium vapors react with all components of the powder mixture, including oxygen adsorbed on the surface of powder particles or oxygen impurities in argon. Based on the standard Gibbs free energies of formation per oxygen mol-atom (DfG (298) in Table 3), stability of oxides in the discussed system increases in the following order: Ag2O < CO2 < SiO2 < MgO.
761
Magnesium can reduce all other oxides, including CO2, which is produced upon decomposition of silver oxalate taking place already at 140 C (Ag2C2O4 / 2Ag þ 2CO2). Carbon released in this hypothetical reaction might combine with silicon to form SiC or with magnesium to form MgC2 or Mg2C3. However, neither of these compounds has ever been detected in the reaction products even by high resolution techniques such as synchrotron x-ray diffraction [16]. Thermodynamic stability of different compounds, based on DfG (298) per magnesium or silicon mol-atom, respectively, increases in the following order: MgC2 < Mg2C3 < Mg2Si < MgO and SiC < Mg2Si < SiO2 and this does not change at 773 K (500 C). Neither of carbide phases is therefore stable in the reaction mixture. On the other hand, carbon dioxide could be lost before the onset of any reaction, when the compacted powder was heated under dynamic vacuum. Based on thermodynamic data for the stoichiometric compounds, upon subsequent processing, the AgMg intermetallic should react with silicon (2AgMg þ Si /Mg2Si þ 2 Ag) and make silver available for doping the Mg2Si matrix, as claimed in Ref. [16]. However, AgMg may deviate from stoichiometry [14], its phase field at elevated temperatures extends from 39 to 64 at% Mg at 765 K (492 C) but is much narrower at lower temperatures, e.g. from 40 to 50 at% Mg at about 573 K (300 C), which indicates that some transformations may take place on thermal cycling between 873 K (600 C) and room temperature. In particular, these may involve appearance of additional phases, such as AgMg3 on the Mg-rich side and AgeMg solid solution or Ag3Mg on the Ag-rich side of the AgMg phase field. SEM/EDS examination of the hot-pressed samples confirmed the results of XRD measurements on the as-synthesized powder products. Ag-rich precipitates in the Mg2Si matrix were clearly visible on the crosssections (Fig. 1). Their distribution can be matched with the 3-D image obtained by x-ray tomography (Fig. 9a). The diffused foamy/ cellular structure would indicate segregation of silver-rich phases to the grain boundaries of Mg2Si and maybe some penetration of this element into the grains accompanying phase transformations upon heating/cooling cycles. The diffusion experiment simulating ex situ doping brings evidence of a replacement reaction: Mg2Si þ Ag / AgMg þ Si, high mobility of magnesium and silver and extremely low solubility of silver in Mg2Si. This is not consistent with the results of similar experiments reported by other authors [17] performed with an objective to fabricate a pen junction by thermal diffusion, who indicated remarkable solubility of silver in Mg2Si. According to the Fact Sage thermochemical calculations (Fig. 8b), the Ag-rich AgMg totally transforms into Ag3Mg at about 523 K (250 C). This must be accompanied by some release of magnesium (AgMg / Ag3Mg þ Mg) and, as the amount of Si simultaneously decreases, the excess magnesium is apparently consumed in the reaction with Mg to produce more Mg2Si. Upon subsequent heating, decomposition of Ag3Mg and formation of AgMg will be accompanied by local release of silver and its further reaction with Mg2Si leading to AgMg and Si excess (Ag3Mg / AgMg þ Ag). In the case of in situ doping, the calculations show (Fig. 8a) that the AgMg phase is rich in magnesium and it does not decompose entirely upon cooling but gradually becomes enriched in silver as a result of the formation of AgMg3 at about 723 K (450 C) (AgMg / AgMg3 þ Ag). Reversal of the deviation from stoichiometry toward silver excess occurs slightly below 472 K (200 C). Upon heating, the amount of AgMg3 progressively decreases and magnesium excess is apparently ‘absorbed’ by AgMg. Total amount of AgMg in the material is greater in the case of in situ doping compared with the ex situ doping. The Ag3Mg and AgMg3 intermetallics were not detected in the investigated materials either because of very small amounts or because of kinetic barriers for the corresponding transformations.
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Fig. 9. X-ray tomography of Mg2Si0.95Ag0.5, in situ doped with metallic silver (a,b,c) and undoped Mg2Si (d): (a) cube-shaped element (edge length 250 mm) selected for the construction of 3-D maps; (b) minor component 1 e Ag-rich phases; (c) minor component 2 e silicon; (d) minor component e silicon e in undoped Mg2Si.
There is one evidence, however, of silver appearance in the in situ doped material at about 573 K (300 C) provided by high resolution synchrotron x-ray diffraction patterns [16] taken during one heating (2 C/min) and cooling (4 C/min) cycle. Overall duration of the experiment was about 7 h. It is worthwhile to note that some unidentified peaks were seen temporarily in these diffraction patterns, possibly related to the formation/disappearance of the intermetallic phases, which are unstable at elevated temperatures. The solubility of silver in Mg2Si, deduced from Rietveld refinements in Ref. [16], is in the range of fractions of atomic percent. The same can be derived from SEM/EDS. Local EDS analyses of the Mg2Si matrix in materials with different nominal compositions indicated maximum Ag concentration of 0.30 at%. This small amount of silver is distributed mostly along the grain boundaries of the Mg2Si matrix. It has been reported in the previous paper [11] that at room temperature the carrier concentration in Ag-doped Mg2Si determined from Hall effect measurements was 2.22 , 1017 or 1.72 , 1018 cm3, depending on doping procedure, and the sign of the Hall coefficient was negative. The same is clearly seen in repeated measurements of the Seebeck coefficient in Figs. 3 and 4. The drop of the Seebeck coefficient observed at about 573 K (300 C) might be related to the release of silver from Mg2Si, as suggested in Ref. [16]. After several thermal cycles (Fig. 4) the values of the
Fig. 10. Temperature dependence of thermal conductivity for Mg2Si in situ doped with metallic silver or silver oxalate at two different levels: Mg2Si0.98Ag0.02 and Mg2Si0.95Ag0.05.
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Table 3 Thermodynamic properties of selected phases in the MgeSieAgeOeC system [12e15]. Formula
Melting point [ C]
DfH (298) [kJ/mol-atom]
DfG (298) [kJ/mol]
DfG (773) [kJ/mol]
Mg2Si AgMg MgO SiO2 Ag2O CO2 SiC MgC2 Mg2C3
1081 820 2830 1710 280a
25,9 19.47 300.6 303.6 31.0
2730a N/A N/A
36.61 29.29 15.90
76.8 N/A 568.9 856.4 11.2 394.36 70.85 84.81 74.18
72.5 34.2 517.8 770.3 e 395.48 66.99
a
Decomposition.
Seebeck coefficient are reproducible and negative up to about 373 K (100 C) then close to zero within the temperature range 373e573 K (100e300 C) and positive at temperatures exceeding 573 K (300 C). The relatively high value of þ200 mV/K is attained at 773e873 K (500e600 C). In the experiment with silver oxalate introduced in the densification step, variations of the Seebeck coefficient with temperature are different. Measurements during the first heating cycle (Fig. 4a) showed a step-like change of the Seebeck coefficient from negative to positive values at about 573 K (300 C). After several heating and cooling cycles (Fig. 4a,b) the dependence became reproducible and similar to that in Fig. 3. It is interesting to note that the behavior of Ag-doped Mg2Si can be influenced by thermal history of the material. This remark is drawn from the quenching experiment. The rapidly cooled material with frozen-in point defects appeared to have high positive value of the Seebeck coefficient at room temperature (þ300 mV/K). During the subsequent measurement procedure involving slow heating to 873 K (600 C) followed by slow cooling (overall duration of 24 h), the Seebeck coefficient slightly increased reaching about þ400 mV/ K at 473 K (200 C) and then dropped to þ200 mV/K at 873 K (600 C). Next, upon cooling, it gradually decreased and finally changed the sign from positive to negative at about 473 K (200 C). The origin of these transitions can be related to the limited thermodynamic stability of different intermetallic phases (AgMg, AgMg3, and Ag3Mg), local restructuring and release of excess components (e.g. Mg from AgMg or AgMg3, Ag from AgMg or Ag3Mg), temperature-dependent solubilities (e.g. Ag in Mg2Si), concentration and mobility of predominant point defects in different phases. These basic questions regarding interrelations between microstructure and properties, thermodynamic and kinetic effects, require further consideration and supporting experimental data. 5. Concluding remarks 1. Self-propagating reaction in the mixture of elemental powders corresponding to the nominal compositions Mg2Si0.98Ag0.02, Mg2Si0.95Ag0.05 yields multiphase products with Mg2Si as the major component and AgMg, Si, MgO as the minor ones. Silver oxalate used instead of elemental silver brings about reduction of grain size of the final product and more uniform distribution of silver in the bulk. 2. Independently of the doping procedure e in situ (2 Mg þ 0.98Si þ 0.02 Ag) or ex situ (Mg2Si þ 0.02 Ag) - silver readily reacts with magnesium to form an intermetallic phase, AgMg. Not only AgMg but also other intermetallics, AgMg3 and Ag3Mg, which are stable only up to about 523 and 723 K (250 and 450 C), respectively, may serve as dopant reservoirs. 3. According to SEM/EDS and x-ray tomography, silver is mostly located along the grain boundaries of Mg2Si, which suggests significance of the composition and architecture of these
regions in the overall behavior of the Ag-doped material. Small grain size of the matrix and uniform distribution of silver are seen as the means to improve the performance of the final product. This can be achieved by replacing elemental silver with silver oxalate in the synthesis. 4. The diffusion experiment in the Ag/Mg2Si system, proves that both the affinity of silver for magnesium and mobility of these two elements are high, resulting in the formation of an AgMg layer on the Ag side and a relatively thick AgMg/Si layer on the Mg2Si side of the couple. According to SEM/EDS and TEM/EDS, the solubility of silver in Mg2Si is extremely low (fractions of at.%). 5. The erratic variations of the Seebeck coefficient upon heating and cooling can be accounted for by repeated insertion and extraction of silver and its migration between different intermetallics (Mg2Si, AgMg, Ag3Mg and AgMg3) brought about by temperature-dependent solubility limits, thermodynamics and kinetics of phase transformations and diffusion in the MgeSieAg system. Evidence is provided by thermochemical calculations and the quenching experiment, which shows that the sign and magnitude of the Seebeck coefficient depend on thermal history of the material. 6. The results of this work reveal complexity of the reaction and diffusion processes in the MgeSieAg system over the temperature range of interest for the application of silver-doped magnesium silicide in thermoelectric modules. The desirable p-type properties can be expected at temperature exceeding 573 K (300 C) and possibly at lower temperature, if only cooling is sufficiently fast. Acknowledgments The authors wish to acknowledge financial support from the ThermoMag Project, which is co-funded by the European Commission in the 7th Framework Programme (contract NMP4- SL2011-263207), by the European Space Agency and individual partner organizations and partly by the Ministry of Science and Higher Education in Poland (contract 6099/B/T02/2010/38 18.18.160.910). References [1] Y. Noda, H. Kon, Y. Furukawa, N. Otsuka, I.A. Nishida, K. Masumoto, Meter. Trans. JIM 33 (9) (1992) 845e850. [2] R. Song, Y. Liu, T. Aizawa, J. Mater. Sci. Technol. 21 5 (2005) 618e622. [3] V.K. Zaitsev, M.I. Fedorov, E.A. Gurieva, I.S. Eremin, P.P. Konstantinov, A. Yu Samunin, M.V. Vedernikov, Phys. Rev. B 74 (2006), 045207 1e5. [4] J. Tani, H. Kido, Phys. B 364 (2005) 218e224. [5] M. Fedorov, V. Zaitsev, G. Isachenko, Solid State Phenom. 170 (2011) 286e292. [6] A.U. Khan, E. Pavlidou, Th Kyratsi, in: ICT/ECT Joint Conference 2012, July 9-12, 2012 (Aalborg, Denmark). [7] K. Mars, H. Ihou-Mouko, G. Pont, J. Tobola, H. Scherrer, J. Electron. Mater 38 7 (2009) 1360e1364. [8] M. Akasaka, T. Iida, A. Matsumoto, K. Yamanaka, Y. Takanashi, T. Imai,
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N. Hamada, J. Appl. Phys. 104 (2008), 013703 1e8. [9] E. Godlewska, K. Mars, R. Mania, S. Zimowski, Intermetallics 19 (2011) 1983e1988. [10] E. Godlewska, K. Mars, K. Zawadzka, J. Solid State Chem. 193 05 (2012) 109e113. [11] G.S. Polymeris, A. Theodorakakos, K. Mars, E. Godlewska, Ch B. Lioutas, E. Hatzikraniotis, K.M. Paraskevopoulos, J. Electron. Mater 43 (10) (2014) 3876e3883. [12] W.F. Gale, T.C. Totemeier (Eds.), Smithells Metals Reference Book, eighth ed.,
Elsevier Ltd., 2004. [13] I. Barin, Thermochemical Data of Pure Substances, third ed., VCH Verlagsgesellschaft mbH, 1995. D-69451 Weinheim (Federal Republic of Germany). [14] A.A. Nayeb Hashemi, J.B. Clark, Bull. Alloy Phase Diagrams 5 (4) (1984) 348. [15] H. Okamoto, J. Phase Equilib. Diff 28 2 (2007) 229e230. [16] A. Prytuliak, E. Godlewska, K. Mars, D. Berthebaud, J. Electron. Mater 43 10 (2014) 3746e3752. [17] H. Udono, Y. Yamanaka, M. Uchikoshi, M. Isshiki, J. Phys. Chem. Solids 74 (2013) 311e314.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Reaction and diffusion phenomena in Ag-doped Mg2Si Elzbieta M. Godlewska a, *, Krzysztof Mars a, Pawel Drozdz b, Adam Tchorz c, Marianna Ksiazek c a
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Al. A. Mickiewicza 30, 30-059 Krakow, Poland AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Al. A. Mickiewicza 30, 30-059 Krakow, Poland c Foundry Research Institute, ul. Zakopianska 73, 30-418 Krakow, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 August 2015 Received in revised form 17 October 2015 Accepted 19 October 2015 Available online 22 October 2015
The Ag-doped Mg2Siebased materials are known for unstable thermoelectric behavior; however, the reasons are not sufficiently understood. In this work a range of experiments, analytical tools and calculations was used to unravel some questions related to the reactivity and diffusion properties of the Ag eMgeSi system, which might contribute to the variations of thermopower. Among these, the most informative appeared quenching and diffusion experiments, calculations of phase evolution by Fact Sage and x-ray tomography of Ag-doped materials produced from powders. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ag-doped Mg2Si Powder metallurgy Diffusion Microstructure Thermochemistry
1. Introduction Magnesium silicide derivatives, in particular those alloyed with germanium or tin are under investigation as components of thermoelectric modules for energy harvesting [1e3]. It has been demonstrated that their electrical properties can be enhanced by doping with group 13 (p-type) or 15 (n-type) elements. In the former case the dopant should substitute for silicon, while in the latter the site occupancy is of no importance. The advantageous effects of antimony and bismuth as n-type dopants in Mg2Si and Mg2Si alloyed with tin or/and germanium are well documented [4,5] and the highest reported figure of merit ZT is about 1.4 [6]. Among the p-type dopants, the most promising are gallium [1] and silver [7] but in both cases maximum ZT does not exceed 0.4 at moderate temperatures. Thermoelectric properties of the silver doped Mg2Si-based materials, and thermopower in particular, are ambiguous and apparently hardly reproducible upon thermal cycling [7,8]. Attempts are made therefore to get more insight into the behavior of p-type dopants and to improve their performance. The objective of this work was to collect additional experimental data, which might shed light on the known instability problem of
* Corresponding author. E-mail address: [email protected] (E.M. Godlewska). http://dx.doi.org/10.1016/j.jallcom.2015.10.174 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Ag-doped Mg2Si, these encompassing assessments of the reaction and diffusion phenomena in the MgeSieAg system, thermodynamic calculations focused on the stability limits of the constituent phases and kinetic effects associated with the variations of the Seebeck coefficient with temperature. 2. Experimental Starting materials were Alpha Aesar elemental powders: silicon (99.999%, 100 þ 325 mesh), magnesium (99.8%, 20 þ 100 mesh) and silver (99.999%, þ325 mesh) or silver oxalate, Ag2C2O4, mixed in the proportions corresponding to the nominal compositions: Mg2Si0.98Ag0.02 and Mg2Si0.95Ag0.05. Silver oxalate was selected as silver source because it decomposes into nanosized metallic powder and carbon dioxide. It was synthesized in the laboratory from POCH S.A. analytically pure AgNO3 and C2H2O4 $ 2 H2O. The phase composition of dry precipitate was confirmed by x-ray diffraction (XRD). Freshly prepared powder was used for in situ and ex situ doping of magnesium silicide. The in situ doping procedure was similar to the earlier reported manufacturing of Mg2Si [9,10]. The powders were homogenized and cold pressed under a load of 5 T. The exothermic reaction was initiated at about 873 K (600 C) under a dynamic vacuum of 10 Pa. The products were annealed at 873 K (600 C) for 1 h, passed through a sieve and hot pressed in a graphite die under an argon atmosphere (Thermal Technology Hot
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Press). The maximum applied pressure was 25 MPa, temperature 1223 K (950 C), time e 1 h and the heating rate - 10 C/min. In the ex situ doping procedure, silver oxalate was added to the Mg2Si powder before densification. Reactants and products after each manufacturing step were analyzed for phase composition, chemical composition and microstructure by x-ray diffraction (XRD) (Panalytical X'Pert), scanning electron microscopy (SEM) (Nova Nano SEM 200 FEI Europe Company) and energy dispersive x-ray spectroscopy (EDS) (EDS/EDAX). Characterization of the final product involved measurements of density (Micrometrics Helium Pycnometer Accu-Pyc 1330), thermal diffusivity, specific heat (Netzsch instruments: STA 449 F1 Jupiter, DIL 402 C, LFA 427) and Seebeck coefficient. The latter were conducted under helium in a calibrated home-made device from room temperature (RT) to 873 K (600 C). The whole heating/cooling cycle lasted 24 h. The effects of doping procedure on the formation/disappearance of minor phases upon thermal cycling were assessed through thermochemical calculations using Fact Sage Thermochemical Software and Databases. Two limiting cases were considered: silver introduced at the synthesis step (2 Mg þ 0.098Si þ 0.02 Ag) and silver introduced at the hot pressing step (Mg2Si þ 0.02 Ag). Diffusion couples Ag/Mg2Si were prepared to identify transport processes and reactions in the ex situ doped materials. Silver was deposited onto Mg2Si substrate by magnetron sputtering. After vacuum annealing at 550 C for 1 h, a cross-section of the sample was examined by SEM/EDS and TEM/ EDS (Philips CM-20). Spatial distribution of components in the in situ doped Mg2Si0.95Ag0.05 was revealed by using Phoenix X-ray CT
nanotom with a voxel size of 1 mm. In the quenching experiment, an in situ doped sample with nominal composition Mg2Si0.98Ag0.02, was heated at 873 K (600 C) for 1 h in an evacuated quartz tube and next rapidly cooled in liquid nitrogen to preserve the hightemperature structure and properties. After quenching the procedure of Seebeck coefficient measurement was repeated. Duration of the overall cycle from RT to 873 K (600 C) and back to RT was 24 h. 3. Results 3.1. Microstructure, composition and density As earlier reported [11], the reaction products were nanostructured, consisting of agglomerates of nanosized crystallites. In general, morphology and the particle sizes of the solid products were related to those of silicon powder used in the synthesis [9,10]. Backscattered electron images (SEM-BEI) of the hot pressed materials are presented in Figs. 1 and 2. The materials are multiphase with bright Ag-rich precipitates confirmed by EDS. At a lower magnification (Fig. 1), samples with the same nominal composition (Fig. 1a,c and b,d) received in the synthesis with metallic silver (Fig. 1a,b) and silver oxalate (Fig. 1c,d), have similar microstructures. However, at a higher magnification (Fig. 2) the grains are noticeably smaller when silver is introduced as a compound. Average compositions of the sintered materials, as determined by EDS are presented in Table 1. The average silver concentration is about 0.8 at% and 1.35 at% for the nominal compositions
Fig. 1. Cross-sections of samples with nominal composition: Mg2Si0.98Ag0.02 (a, c), Mg2Si0.95Ag0.05 (b, d) reacted with metallic silver (a, b) and silver oxalate (c, d); backscattered electron images.
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Fig. 2. Cross-sections of samples with nominal composition: Mg2Si0.98Ag0.02 (a, c), Mg2Si0.95Ag0.05 (b, d) reacted with metallic silver (a, b) and silver oxalate (c, d); backscattered electron images at a higher magnification.
Table 1 Average composition of Ag-doped Mg2Si samples (EDS). Sample nominal composition (Ag source)
Mg2Si0.98Ag0.02 Mg2Si0.95Ag0.05 Mg2Si0.98Ag0.02 Mg2Si0.95Ag0.05 Mg2Si0.98Ag0.02
(Ag metallic) (Ag metallic) (Ag2C2O4) (Ag2C2O4) (Ag2C2O4 ex situ)
Concentration, at%
Mg/Si
Mg
Si
Ag
65.15 66.50 65.27 64.98 64.94
34.05 32.16 33.96 33.65 34.29
0.80 1.34 0.77 1.36 0.77
1.91 2.07 1.92 1.93 1.89
Mg2Si0.98Ag0.02 and Mg2Si0.95Ag0.05, respectively. The Mg:Si atomic ratio is about 2:1. As can be seen in Table 2, independently of the doping procedure, the densities determined by helium pycnometer are similar for the same nominal composition, about 2.01 and 2.15 g/cm3 for Mg2Si0.98Ag0.02 and Mg2Si0.95Ag0.05, respectively. According to local EDS analyses, concentration of silver in the Mg2Si
Table 2 Nominal composition and density of investigated materials. Sample composition
Density, g/cm3
Mg2Si Mg2Si0.98Ag0.02 Mg2Si0.98Ag0.02 Mg2Si0.95Ag0.05 Mg2Si0.95Ag0.05
1.99 2.01 2.04 2.15 2.15
(Ag metallic) (Ag2C2O4) (Ag metallic) (Ag2C2O4)
matrix ranged from 0.24 to 0.33 at%. Powder XRD (PXRD) patterns of samples with nominal composition Mg2Si0.98Ag0.02 received according to the in situ and ex situ doping procedure revealed Mg2Si along with AgMg and minor impurities: MgO and Si [11]. No carbides were detected in the reaction products when silver oxalate was used as silver source. The amount of AgMg increased with the concentration of this element in the initial powder mixture. 3.2. Seebeck coefficient Temperature dependence of the Seebeck coefficient for the in situ doped material, Mg2Si0.98Ag0.02, as received after hot pressing (HP) and after an additional 5-h vacuum annealing at 773 K (500 C) is shown in Fig. 3. For the sample after HP the values were positive over the entire temperature range from RT to about 873 K (600 C) with a maximum of about 290 mV/K at 423 K (150 C) and minimum of about 130 mV/K between 573 and 623 K (300 and 350 C). For the sample annealed before the measurements, the Seebeck coefficient was negative at up to 373 K (100 C) and again positive at temperatures higher than 573 K (300 C); the value of þ200 mV/K being attained at about 873 K (600 C). Variations of the Seebeck coefficient upon heating and cooling of the samples with the same nominal composition, Mg2Si0.98Ag0.02, received according to the ex situ doping procedure with silver oxalate added to Mg2Si before hot pressing, are presented in Fig. 4. The changeover from negative to positive values of the Seebeck coefficient occurred repeatedly during four consecutive cycles at about 573 K (300 C).
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The maximum positive value reached þ200 mV/K. The effect of annealing and quenching on the variations of the Seebeck coefficient upon heating and cooling is presented in Fig. 5. The in situ doped sample annealed at 873 K (600 C) for 1 h and quenched had high positive thermopower at room temperature, which remained positive upon heating. Upon cooling the variations of thermopower were practically the same as those described for other samples. 3.3. Diffusion experiment
Fig. 3. Variations of the Seebeck coefficient upon heating and cooling of Mg2Si0.98Ag0.02; ‘after HP’ e as received without any additional heat treatment, 1st run and 2nd run e samples subjected to 5-h vacuum annealing at 773 K (500 C) before the measurements; Tup, Tdwn e heating and cooling, respectively; duration of each heating/cooling cycle e 24 h.
Analysis of the near surface layers after 1-h annealing at 823 K (550 C) is shown in Fig. 6. The initial layer of silver (about 6e7 mm thick) deposited by magnetron sputtering on the surface of undoped magnesium silicide is totally transformed into AgMg. Underneath, there is an interdiffusion zone, with an almost equimolar average composition, as determined by EDS. Two constituent phases largely differ in Si concentration and Mg:Ag atomic ratio. For the bright phase with visible nanometric dispersions, the respective numbers are 19 at% and about 1:1, whereas for the dark single phase 50 at% and 2:1. TEM images and EDS mapping of elements in the interdiffusion zone are presented in Fig. 7. According to local EDS analysis, the two phases are pure silicon and AgMg. Twinned silicon crystals are visible in Fig. 7e (point 1). Very small dispersions within the AgMg matrix (area 2 in Fig. 7b) are beyond the detection limit. 3.4. Thermochemical calculations The results of thermochemical calculations for the two limiting cases, in situ doping (2 Mg þ 0.98Si þ 0.02 Ag) and ex situ doping (Mg2Si þ 0.02 Ag), are shown in Fig. 8. In particular, the graphs reveal the evolution of minor phases over the temperature range from 293 to 1073 K (20e800 C). In case 1 (in situ doping), the AgMg (BCC1) phase is rich in magnesium and its amount decreases upon cooling. This is accompanied by reversal of the deviation from stoichiometry toward silver excess brought about by the formation of AgMg3 at about 773 K (500 C), which is then stable down to the room temperature. The Mg-rich MgeAgeSi liquid (LIQU) appears already at about 873 K (600 C) and coexists with one solid phase, Mg2Si. In case 2 (ex situ doping), the AgMg (BCC1) phase is rich in
Fig. 4. Variations of the Seebeck coefficient upon heating and cooling for Mg2Si0.98Ag0.02 received according to the ex situ doping procedure with silver oxalate added before densification; Tup, Tdwn e heating and cooling, respectively; duration of each heating/cooling cycle e 24 h.
Fig. 5. Variations of the Seebeck coefficient upon heating and cooling for Mg2Si0.98Ag0.02 received according to the in situ doping procedure; HP eas hot pressed; HP þ HT þ Qch e hot pressed, heat treated at 873 K (600 C) for 1 h and quenched before the measurements; Tup, Tdwn e heating and cooling, respectively; duration of each heating/cooling cycle e 24 h.
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Fig. 6. SEM micrographs of a diffusion couple (a) before annealing e Ag layer on Mg2Si substrate; (b) after annealing (823 K/550 C, 1 h) - visible interdiffusion zone; (c) two-phase region e frame in (b) and its average composition; (d) EDS analysis in points 1e5 depicted in (b).
silver and its amount only slightly decreases upon cooling. The Ag concentration in this phase gradually increases. At about 523 K (250 C), however, the Ag-rich AgMg decomposes. This is accompanied by the formation of Ag3Mg and decrease in Si content. The Mg-rich liquid (LIQU) appears at about 973 K (700 C). Silicon as a solid phase is present over the whole temperature range under consideration.
of the Ag-doped Mg2Si. Figs. 9 b,c show locations of two minor components in this element and Fig. 9d e distribution of silicon in undoped Mg2Si. Similarity of pictures in Fig. 9c and d allows for the statement that both correspond to the unreacted or excess silicon. The 2-D distribution of Ag-rich phase (SEM image in Fig. 1b) seems to coincide with the diffused cellular structure in Fig. 9b. MgO is apparently not distinguishable by this technique.
3.5. X-ray tomography
3.6. Thermal properties
X-ray tomography was used to assess 3-D distribution of different components in the multiphase material with nominal composition Mg2Si0.95Ag0.05 received by in situ doping with metallic silver and in undoped Mg2Si. Fig. 9a shows a cube-shaped element, with the edge length of 250 mm, selected for the analysis
In the temperature range from RT to about 873 K (600 C) thermal properties of Mg2Si samples modified with silver were similar, independently of whether it was introduced as elemental powder or silver oxalate. Average thermal conductivity of the in situ doped samples decreased from about 9.5 to 4.5 W/m-K, as can be
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Fig. 7. STEM image (a) and area maps of elements from the two-phase region of the diffusion couple after annealing; (b) AgeK; (c) MgeK; (d) SieK; BF TEM image, showing twinned crystals of silicon (1) and AgMg matrix (2) at a higher magnification (e).
seen in Fig. 10. Slightly higher values were noted for the samples with the nominal composition Mg2Si0.95Ag0.05. 4. Discussion Mg2Si and its derivatives can be produced from elemental powders by different technological routes with or without preliminary mechanical activation by milling. The reaction in a powder mixture is initiated by resistance/induction heating or microwaves. Due to high volatility of magnesium, the vaporesolid reaction starts
at temperatures much lower than the melting point of this element and the products are either nanostructured powders or loosely bonded porous sinters. The additives and dopants are generally introduced to the starting powder mixture but their behavior in the reaction and subsequent heat treatment largely depends on their physical and chemical properties as well as diffusivity and solubility in the Mg2Si matrix. Low melting elements may enter the structure of Mg2Si already during the synthesis acting at the same time as sintering aids, while high melting elements may form dispersions in Mg2Si and diffuse into the matrix upon subsequent heat
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Fig. 8. Evolution of minor phases over temperature calculated by Fact Sage for two cases: (a) in situ doping: 2 Mg þ 0.98Si þ 0.02 Ag; (b) ex situ doping: Mg2Si þ 0.02 Ag.
treatment and/or sintering. Additives may also react with magnesium or silicon. Depending on thermodynamics and kinetics of such reactions, the products may consist of Mg2Si as the major phase and other compounds formed by magnesium or silicon with the additives. The side reactions with the additives may produce excess of magnesium or silicon in the final product. Indeed, according to XRD measurements, the products of self-propagating reaction in the MgeSieAg mixture contained Mg2Si, AgMg as well as small amounts of Si and/or MgO, sometimes not detectable by XRD. Unexpectedly, the samples doped at the hot pressing stage contained also AgMg, which means that Mg2Si can react with Ag. Based on the data in Table 3, AgMg is only slightly less stable than Mg2Si. At 773 K (500 C), the respective standard Gibbs free energy of formation per magnesium mol-atom is approximately 34.2 kJ/mol and 36.3 kJ/mol [12,13] and the melting point of AgMg is 820 C [14] versus 1081 C for Mg2Si [15]. The appearance of AgMg in the final product, independently of doping procedure, indicates that magnesium reactions with silicon and silver are competitive. In the case of in situ doping, in a short time interval, typical of selfpropagating reactions, magnesium vapors react with all components of the powder mixture, including oxygen adsorbed on the surface of powder particles or oxygen impurities in argon. Based on the standard Gibbs free energies of formation per oxygen mol-atom (DfG (298) in Table 3), stability of oxides in the discussed system increases in the following order: Ag2O < CO2 < SiO2 < MgO.
761
Magnesium can reduce all other oxides, including CO2, which is produced upon decomposition of silver oxalate taking place already at 140 C (Ag2C2O4 / 2Ag þ 2CO2). Carbon released in this hypothetical reaction might combine with silicon to form SiC or with magnesium to form MgC2 or Mg2C3. However, neither of these compounds has ever been detected in the reaction products even by high resolution techniques such as synchrotron x-ray diffraction [16]. Thermodynamic stability of different compounds, based on DfG (298) per magnesium or silicon mol-atom, respectively, increases in the following order: MgC2 < Mg2C3 < Mg2Si < MgO and SiC < Mg2Si < SiO2 and this does not change at 773 K (500 C). Neither of carbide phases is therefore stable in the reaction mixture. On the other hand, carbon dioxide could be lost before the onset of any reaction, when the compacted powder was heated under dynamic vacuum. Based on thermodynamic data for the stoichiometric compounds, upon subsequent processing, the AgMg intermetallic should react with silicon (2AgMg þ Si /Mg2Si þ 2 Ag) and make silver available for doping the Mg2Si matrix, as claimed in Ref. [16]. However, AgMg may deviate from stoichiometry [14], its phase field at elevated temperatures extends from 39 to 64 at% Mg at 765 K (492 C) but is much narrower at lower temperatures, e.g. from 40 to 50 at% Mg at about 573 K (300 C), which indicates that some transformations may take place on thermal cycling between 873 K (600 C) and room temperature. In particular, these may involve appearance of additional phases, such as AgMg3 on the Mg-rich side and AgeMg solid solution or Ag3Mg on the Ag-rich side of the AgMg phase field. SEM/EDS examination of the hot-pressed samples confirmed the results of XRD measurements on the as-synthesized powder products. Ag-rich precipitates in the Mg2Si matrix were clearly visible on the crosssections (Fig. 1). Their distribution can be matched with the 3-D image obtained by x-ray tomography (Fig. 9a). The diffused foamy/ cellular structure would indicate segregation of silver-rich phases to the grain boundaries of Mg2Si and maybe some penetration of this element into the grains accompanying phase transformations upon heating/cooling cycles. The diffusion experiment simulating ex situ doping brings evidence of a replacement reaction: Mg2Si þ Ag / AgMg þ Si, high mobility of magnesium and silver and extremely low solubility of silver in Mg2Si. This is not consistent with the results of similar experiments reported by other authors [17] performed with an objective to fabricate a pen junction by thermal diffusion, who indicated remarkable solubility of silver in Mg2Si. According to the Fact Sage thermochemical calculations (Fig. 8b), the Ag-rich AgMg totally transforms into Ag3Mg at about 523 K (250 C). This must be accompanied by some release of magnesium (AgMg / Ag3Mg þ Mg) and, as the amount of Si simultaneously decreases, the excess magnesium is apparently consumed in the reaction with Mg to produce more Mg2Si. Upon subsequent heating, decomposition of Ag3Mg and formation of AgMg will be accompanied by local release of silver and its further reaction with Mg2Si leading to AgMg and Si excess (Ag3Mg / AgMg þ Ag). In the case of in situ doping, the calculations show (Fig. 8a) that the AgMg phase is rich in magnesium and it does not decompose entirely upon cooling but gradually becomes enriched in silver as a result of the formation of AgMg3 at about 723 K (450 C) (AgMg / AgMg3 þ Ag). Reversal of the deviation from stoichiometry toward silver excess occurs slightly below 472 K (200 C). Upon heating, the amount of AgMg3 progressively decreases and magnesium excess is apparently ‘absorbed’ by AgMg. Total amount of AgMg in the material is greater in the case of in situ doping compared with the ex situ doping. The Ag3Mg and AgMg3 intermetallics were not detected in the investigated materials either because of very small amounts or because of kinetic barriers for the corresponding transformations.
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Fig. 9. X-ray tomography of Mg2Si0.95Ag0.5, in situ doped with metallic silver (a,b,c) and undoped Mg2Si (d): (a) cube-shaped element (edge length 250 mm) selected for the construction of 3-D maps; (b) minor component 1 e Ag-rich phases; (c) minor component 2 e silicon; (d) minor component e silicon e in undoped Mg2Si.
There is one evidence, however, of silver appearance in the in situ doped material at about 573 K (300 C) provided by high resolution synchrotron x-ray diffraction patterns [16] taken during one heating (2 C/min) and cooling (4 C/min) cycle. Overall duration of the experiment was about 7 h. It is worthwhile to note that some unidentified peaks were seen temporarily in these diffraction patterns, possibly related to the formation/disappearance of the intermetallic phases, which are unstable at elevated temperatures. The solubility of silver in Mg2Si, deduced from Rietveld refinements in Ref. [16], is in the range of fractions of atomic percent. The same can be derived from SEM/EDS. Local EDS analyses of the Mg2Si matrix in materials with different nominal compositions indicated maximum Ag concentration of 0.30 at%. This small amount of silver is distributed mostly along the grain boundaries of the Mg2Si matrix. It has been reported in the previous paper [11] that at room temperature the carrier concentration in Ag-doped Mg2Si determined from Hall effect measurements was 2.22 , 1017 or 1.72 , 1018 cm3, depending on doping procedure, and the sign of the Hall coefficient was negative. The same is clearly seen in repeated measurements of the Seebeck coefficient in Figs. 3 and 4. The drop of the Seebeck coefficient observed at about 573 K (300 C) might be related to the release of silver from Mg2Si, as suggested in Ref. [16]. After several thermal cycles (Fig. 4) the values of the
Fig. 10. Temperature dependence of thermal conductivity for Mg2Si in situ doped with metallic silver or silver oxalate at two different levels: Mg2Si0.98Ag0.02 and Mg2Si0.95Ag0.05.
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Table 3 Thermodynamic properties of selected phases in the MgeSieAgeOeC system [12e15]. Formula
Melting point [ C]
DfH (298) [kJ/mol-atom]
DfG (298) [kJ/mol]
DfG (773) [kJ/mol]
Mg2Si AgMg MgO SiO2 Ag2O CO2 SiC MgC2 Mg2C3
1081 820 2830 1710 280a
25,9 19.47 300.6 303.6 31.0
2730a N/A N/A
36.61 29.29 15.90
76.8 N/A 568.9 856.4 11.2 394.36 70.85 84.81 74.18
72.5 34.2 517.8 770.3 e 395.48 66.99
a
Decomposition.
Seebeck coefficient are reproducible and negative up to about 373 K (100 C) then close to zero within the temperature range 373e573 K (100e300 C) and positive at temperatures exceeding 573 K (300 C). The relatively high value of þ200 mV/K is attained at 773e873 K (500e600 C). In the experiment with silver oxalate introduced in the densification step, variations of the Seebeck coefficient with temperature are different. Measurements during the first heating cycle (Fig. 4a) showed a step-like change of the Seebeck coefficient from negative to positive values at about 573 K (300 C). After several heating and cooling cycles (Fig. 4a,b) the dependence became reproducible and similar to that in Fig. 3. It is interesting to note that the behavior of Ag-doped Mg2Si can be influenced by thermal history of the material. This remark is drawn from the quenching experiment. The rapidly cooled material with frozen-in point defects appeared to have high positive value of the Seebeck coefficient at room temperature (þ300 mV/K). During the subsequent measurement procedure involving slow heating to 873 K (600 C) followed by slow cooling (overall duration of 24 h), the Seebeck coefficient slightly increased reaching about þ400 mV/ K at 473 K (200 C) and then dropped to þ200 mV/K at 873 K (600 C). Next, upon cooling, it gradually decreased and finally changed the sign from positive to negative at about 473 K (200 C). The origin of these transitions can be related to the limited thermodynamic stability of different intermetallic phases (AgMg, AgMg3, and Ag3Mg), local restructuring and release of excess components (e.g. Mg from AgMg or AgMg3, Ag from AgMg or Ag3Mg), temperature-dependent solubilities (e.g. Ag in Mg2Si), concentration and mobility of predominant point defects in different phases. These basic questions regarding interrelations between microstructure and properties, thermodynamic and kinetic effects, require further consideration and supporting experimental data. 5. Concluding remarks 1. Self-propagating reaction in the mixture of elemental powders corresponding to the nominal compositions Mg2Si0.98Ag0.02, Mg2Si0.95Ag0.05 yields multiphase products with Mg2Si as the major component and AgMg, Si, MgO as the minor ones. Silver oxalate used instead of elemental silver brings about reduction of grain size of the final product and more uniform distribution of silver in the bulk. 2. Independently of the doping procedure e in situ (2 Mg þ 0.98Si þ 0.02 Ag) or ex situ (Mg2Si þ 0.02 Ag) - silver readily reacts with magnesium to form an intermetallic phase, AgMg. Not only AgMg but also other intermetallics, AgMg3 and Ag3Mg, which are stable only up to about 523 and 723 K (250 and 450 C), respectively, may serve as dopant reservoirs. 3. According to SEM/EDS and x-ray tomography, silver is mostly located along the grain boundaries of Mg2Si, which suggests significance of the composition and architecture of these
regions in the overall behavior of the Ag-doped material. Small grain size of the matrix and uniform distribution of silver are seen as the means to improve the performance of the final product. This can be achieved by replacing elemental silver with silver oxalate in the synthesis. 4. The diffusion experiment in the Ag/Mg2Si system, proves that both the affinity of silver for magnesium and mobility of these two elements are high, resulting in the formation of an AgMg layer on the Ag side and a relatively thick AgMg/Si layer on the Mg2Si side of the couple. According to SEM/EDS and TEM/EDS, the solubility of silver in Mg2Si is extremely low (fractions of at.%). 5. The erratic variations of the Seebeck coefficient upon heating and cooling can be accounted for by repeated insertion and extraction of silver and its migration between different intermetallics (Mg2Si, AgMg, Ag3Mg and AgMg3) brought about by temperature-dependent solubility limits, thermodynamics and kinetics of phase transformations and diffusion in the MgeSieAg system. Evidence is provided by thermochemical calculations and the quenching experiment, which shows that the sign and magnitude of the Seebeck coefficient depend on thermal history of the material. 6. The results of this work reveal complexity of the reaction and diffusion processes in the MgeSieAg system over the temperature range of interest for the application of silver-doped magnesium silicide in thermoelectric modules. The desirable p-type properties can be expected at temperature exceeding 573 K (300 C) and possibly at lower temperature, if only cooling is sufficiently fast. Acknowledgments The authors wish to acknowledge financial support from the ThermoMag Project, which is co-funded by the European Commission in the 7th Framework Programme (contract NMP4- SL2011-263207), by the European Space Agency and individual partner organizations and partly by the Ministry of Science and Higher Education in Poland (contract 6099/B/T02/2010/38 18.18.160.910). References [1] Y. Noda, H. Kon, Y. Furukawa, N. Otsuka, I.A. Nishida, K. Masumoto, Meter. Trans. JIM 33 (9) (1992) 845e850. [2] R. Song, Y. Liu, T. Aizawa, J. Mater. Sci. Technol. 21 5 (2005) 618e622. [3] V.K. Zaitsev, M.I. Fedorov, E.A. Gurieva, I.S. Eremin, P.P. Konstantinov, A. Yu Samunin, M.V. Vedernikov, Phys. Rev. B 74 (2006), 045207 1e5. [4] J. Tani, H. Kido, Phys. B 364 (2005) 218e224. [5] M. Fedorov, V. Zaitsev, G. Isachenko, Solid State Phenom. 170 (2011) 286e292. [6] A.U. Khan, E. Pavlidou, Th Kyratsi, in: ICT/ECT Joint Conference 2012, July 9-12, 2012 (Aalborg, Denmark). [7] K. Mars, H. Ihou-Mouko, G. Pont, J. Tobola, H. Scherrer, J. Electron. Mater 38 7 (2009) 1360e1364. [8] M. Akasaka, T. Iida, A. Matsumoto, K. Yamanaka, Y. Takanashi, T. Imai,
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N. Hamada, J. Appl. Phys. 104 (2008), 013703 1e8. [9] E. Godlewska, K. Mars, R. Mania, S. Zimowski, Intermetallics 19 (2011) 1983e1988. [10] E. Godlewska, K. Mars, K. Zawadzka, J. Solid State Chem. 193 05 (2012) 109e113. [11] G.S. Polymeris, A. Theodorakakos, K. Mars, E. Godlewska, Ch B. Lioutas, E. Hatzikraniotis, K.M. Paraskevopoulos, J. Electron. Mater 43 (10) (2014) 3876e3883. [12] W.F. Gale, T.C. Totemeier (Eds.), Smithells Metals Reference Book, eighth ed.,
Elsevier Ltd., 2004. [13] I. Barin, Thermochemical Data of Pure Substances, third ed., VCH Verlagsgesellschaft mbH, 1995. D-69451 Weinheim (Federal Republic of Germany). [14] A.A. Nayeb Hashemi, J.B. Clark, Bull. Alloy Phase Diagrams 5 (4) (1984) 348. [15] H. Okamoto, J. Phase Equilib. Diff 28 2 (2007) 229e230. [16] A. Prytuliak, E. Godlewska, K. Mars, D. Berthebaud, J. Electron. Mater 43 10 (2014) 3746e3752. [17] H. Udono, Y. Yamanaka, M. Uchikoshi, M. Isshiki, J. Phys. Chem. Solids 74 (2013) 311e314.