KR20120125125A - Thermoelectric composition of Magnesium Silicide and the synthesis method of the same - Google Patents

Abstract

PURPOSE: A thermoelectric composition of magnesium silicide and a synthesizing method of the same are provided to control the pollution of the thermoelectric composition due to impurities from atmospheric gas. CONSTITUTION: A thermoelectric composition of magnesium silicide includes the following steps: magnesium and silicon are respectively weighed and mixed to prepare a mixture; and the mixture is sealed under a vacuum condition and is thermally treated and synthesized under the sealed state. The thermally treating and synthesizing process is arranges the mixture in a crucible and introduces the crucible in a quartz container to be sealed.

Description

Thermoelectric composition of Magnesium Silicide and the synthesis method of the same}

The present invention relates to a magnesium silicate thermoelectric composition and a method for synthesizing the same, and more particularly, to prepare a mixture for preparing a magnesium silicate thermoelectric composition by weighing and mixing Mg and Si, respectively; And sealing the mixture in a vacuum-maintained state, and thermally treating the mixture in a sealed state to provide a method of synthesizing the magnesium silicate thermoelectric composition, and magnesium silicate synthesized therefrom.

According to the present invention as described above, a high figure of merit can be derived even through a simplified process, and thus, an effect of simply preparing a thermoelectric composition having highly improved thermoelectric properties is expected. That is, the present invention derives a process that can be industrialized, but by greatly simplifying such a process, it is possible to improve process economics, and nevertheless, it is possible to greatly improve thermoelectric performance as compared to a thermocomposite manufactured by a conventional method.

Silicide thermoelectric materials have the advantage that they are abundant on earth and that there is no toxicity in the by-products. Among them, Mg 2 Si thermoelectric material is attracting attention because Mg 2 Si is evaluated as a very efficient thermoelectric power generator. This is because it has higher electrical conductivity and lower thermal conductivity than other silicide compositions (for example, FeSi2). Mg2Si has a similar working temperature range compared to thermoelectric materials such as PbTe and CoSb3, and figure of merit of Mg2Si is known to be inferior to PbTe and CoSb3. Nevertheless, since it is evaluated as a very promising thermoelectric generator, related research is very active. In addition, Mg2Si is environmentally friendly, so devices built using it can be safely operated and handled.

The efficiency of a thermoelectric material is determined by a dimensionless thermoelectric figure of merit, which is expressed as ZT = S2σT / κ, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, and total, respectively. Thermal conductivity means absolute temperature. Therefore, low lattice thermal conductivity and high carrier mobility are desirable for an improved figure of merit.

Vining et al. Reported that the factor A '= (T / 300) (m * / me) 3 / 2μ / κph is relatively larger for Mg2Si systems than for SiGe or β-FeSi2 systems. Where m * is the effective mass of the carrier, μ is the mobility with units of cm2 / (Vs), and κph is the lattice thermal conductivity with units of mW / (cmK). In general, when the effective mass m * is large, the Seebeck coefficient is large, and a high voltage is obtained. When the μ is large, the electrical conductivity is high. In addition, the smaller the κph, the larger the A 'value and the less the heat loss, the higher the efficiency. Therefore, the Mg2Si system can be expected to have higher ZT value in the future through the R & D process.

For this reason, a lot of research has been conducted regarding the original electrical properties of Mg 2 Si. For example, Morris et al. Reported the electrical properties of single crystals with bandgap energy measured up to 0.78. Tamura et al. Also reported that Mg 2 Si crystals have very narrow donor levels (˜9 meV). They also found that impurities play a significant role in determining the electrical properties of crystals. That is, impurities act as donors or acceptors to change the electrical properties of the crystal.

As an example, the electrical properties of the crystals changed significantly when the purity of the Mg starting material changed from 4N to 6N. Therefore, maintaining purity of Mg starting material is a very important task.

On the other hand, the conventional manufacturing method of Mg2Si is based on the process of precipitating Mg, Si, and the like containing a single body at a high temperature, and then precipitates Mg2Si. However, the precipitation of Mg 2 Si is a problem that the secondary phase is mixed, or the high volatility of Mg is a problem, the loss of Mg is inevitable when melting at a temperature above the melting point of Mg, Mg 2 Si to match the stoichiometric composition There was a difficulty in manufacturing.

The present invention has been made to solve the problems of the prior art as described above, it is an object of the present invention to improve the thermoelectric properties according to the synthesis by pure Mg2Si by performing a melting process while maintaining a vacuum state.

In the present invention, in order to suppress the volatilization of Mg as much as possible, and to control contamination by impurities introduced from the atmosphere gas, in the present invention, the crucible was charged into a quartz ampoule in a vacuum and then sealed. Thus attempting to synthesize Mg 2 Si in a sealed state in vacuum is a key element of the present invention.

In addition, the present invention is sintered using a spark plasma sintering method, similarly to the vacuum melting method described above, (a) solid phase reaction between Mg and Si, (b) compaction in a short time at a relatively low temperature by implementing a low melting point Mg and plate Another object is to effectively control the thermoelectric properties of the thermoelectric composition by suppressing volatilization of the heat.

That is, the vacuum melting method and the SPS method are important elements in synthesizing the stoichiometric Mg 2 Si according to the present invention, and the optimum Mg 2 Si can be synthesized by organic bonding between the two elements.

In order to achieve the above object, the present invention comprises the steps of preparing a mixture for preparing a magnesium silicide thermoelectric composition by weighing and mixing Mg and Si, respectively; And sealing the mixture in a state where vacuum is maintained, and synthesizing the mixture by heat treatment in the sealed state.

Sealing the mixture in a vacuum maintained state, and heat-synthesizing it in the sealed state; the sealing is preferably to accommodate the mixture in a crucible and to charge the crucible in a quartz container under vacuum. .

In addition, the present invention provides a magnesium silicate thermoelectric composition prepared by the above method to improve the figure of merit.

In another aspect, the present invention provides a magnesium silicate thermoelectric is prepared by sintering the thermoelectric composition prepared by the above method at a temperature range of 950 ~ 1150K.

In the present invention, the sintering is preferably based on the spark plasma sintering method.

According to the present invention as described above, a high figure of merit can be derived even through a simplified process, and thus, an effect of simply preparing a thermoelectric composition having highly improved thermoelectric properties is expected.

That is, the present invention derives a process that can be industrialized, but by greatly simplifying such a process, it is possible to improve process economics, and nevertheless, it is possible to greatly improve thermoelectric performance as compared to a thermocomposite manufactured by a conventional method.

In addition, the present invention can synthesize Mg 2 Si by stoichiometric synthesis of Mg 2 Si to prevent volatilization of Mg and to prevent the incorporation of impurities by atmospheric gases during the synthesis process, and thus reproducibly synthesize Mg 2 Si having excellent physical properties. have.

In addition, the present invention is sintered using a spark plasma sintering method, according to the present invention (a) solid phase reaction between Mg and Si, (b) compaction in a short time at a relatively low temperature by volatilization of low melting point Mg and dopant By suppressing the thermoelectric properties of the thermoelectric composition can be effectively controlled.

1 is a graph showing X-ray analysis of Mg 2 Si prepared by changing the composition ratio of Mg: Si according to an embodiment of the present invention, respectively.
FIG. 2 shows (a) electrical conductivity and (b) Seebeck coefficient for Mg 2 Si samples prepared by varying the sintering temperature according to an embodiment of the present invention.
FIG. 3 shows (a) thermal conductivity and (b) dimensionless figure of merit for Mg 2 Si samples prepared by varying the sintering temperature according to one embodiment of the present invention.
Figure 4 is a graph showing the ZT value of the Mg2Si sample prepared by one embodiment of the present invention compared with the ZT value of Mg2Si prepared by other methods previously reported.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and embodiments.

<Production Example>

The Mg2Si phase is the only stoichiometric silicide in the phase equilibrium diagram and has a eutectic point of 1358K. Since the boiling point of pure Mg is 1363K, eutectic crystallization by stoichiometric melting of Mg2Si must be handled with great care during the synthesis, since Mg evaporates to a significant level. In the present invention, polycrystalline Mg 2 Si was synthesized from a non-stoichiometric melt state by controlling the Mg / Si ratio from 67:33 to 70:30 atomic%.

For this purpose, high purity Mg (> 99.8%) and Si (99.999%) were used as starting materials. Mg and Si powder were accommodated in an alumina crucible, charged into a quartz ampoule while maintaining a vacuum, and then sealed. The crucible was then heat treated at a temperature of about 1358 K for 30 minutes in an electric furnace.

From this, a polycrystalline ingot was obtained, which was pulverized, sieved in a sieve having a scale of 45 μm, and then pressured at 40 MPa for 10 minutes at a temperature of 973 to 1123 K on a graphite die by a SPS (Spark Plasma Sintering) method. Sintered by. The sintered specimens were then cut and polished. Therefore, the sintering temperature according to the present invention can be set in the range of 950 ~ 1150K, which is significant because the temperature is much lower than the melting point of Mg2Si, considering the optimum temperature range considering sintering and economic feasibility.

&Lt; Evaluation example &

The specimens thus processed were analyzed by X-ray. In addition, the surface of the pellet was observed by the field emission electron microscope (JSM-6700F, JEOL). In addition, thermoelectric properties including Seebeck coefficient and electrical conductivity were measured using a measuring apparatus manufactured by Ozawa Science (RZ2001i) in the temperature range from room temperature to 800K. In addition, the thermal diffusion coefficient and specific heat were measured using a laser flash apparatus (Netzsch LFA 457).

Thermal conductivity k was calculated from k = DaCp, where a is the coefficient of thermal diffusion, Cp is the heat capacity per unit volume, and D is the density of the material.

X-ray analysis was performed by adjusting the ratio of Mg: Si at the ratio of 67:33, 68:32, 69:31, and 70:30 on an atomic ratio basis, and the results are shown in FIG. 1. For all samples, the main peak was found to be Mg 2 Si, but for 67:33 samples Si peak was detected as secondary phase. And in the case of 69:31 and 70:30 ratio, Mg was detected as a secondary phase. All of the above atomic ratios were set for the case where a small amount of Mg was added, of which the most stoichiometric Mg 2 Si was synthesized at a ratio of 68:32. In other words, in order for the stoichiometric Mg 2 Si to be synthesized, only a small amount of Mg is added. This is because, in the present invention, but vacuum-sealed in the quartz ampoule, Mg may be consumed while reacting in a small amount with the quartz ampoule.

Although the morphology of the sintered samples is not shown, the densified specimens were obtained for the sintered specimen at 1123K by SPS method, and the highest density and homogeneous solid solution was possible. In addition, as a result of sintering the Mg 2 Si sample synthesized by the present synthesis process, no noticeable pores were observed. However, the change in density and porosity was very large even by the small sintering temperature of about 50K.

The temperature dependence of the electrical conductivity of Mg 2 Si is shown in Figure 2 (a). As shown, semiconductivity was shown for all samples of sintered Mg2Si. Electrical conductivity increased monotonically in the range of 300K to 650K.

Mg2Si semiconductors have a narrow donor level and their conductivity increases with increasing SPS temperature. This is thought to be due to the increase in density and the decrease in porosity. As a result, Mg 2 Si sintered by the SPS method at 1123 K showed higher electrical conductivity than the sample sintered at other temperatures.

The temperature dependence of the Seebeck coefficient of Mg 2 Si is shown in FIG. 2 (b). The Seebeck coefficient was negative for all samples, and hence the n-type behavior. The absolute value of Seebeck coefficient for all specimens tended to increase with decreasing sintering temperature by SPS method. The highest Seebeck coefficient was found to be 279 μVK-1 when the Mg 2 Si sintered at 588 K was sintered at a temperature of 1123 K by SPS. Usually, Seebeck coefficient and electrical conductivity are inversely related. Derived together with the formula is as follows.

Seebeck coefficient can be expressed by the following equation (2).

(2)

(2)

Here, ± represents the contribution of holes and electrons. Also, ζ = (EF / kT) represents a reduced Fermi energy, k represents Boltzmann constant, T represents absolute temperature, and s represents scattering index. The reduced Fermi energy in equation (2) is related to the carrier concentration n, where n can be expressed as in the following equation (3).

(3)

(3)

Where h is the Frank's constant.

From Equations (2) and (3), it can be seen that the Seebeck coefficient is inversely proportional to the carrier concentration. On the other hand, electrical conductivity and carrier concentration are directly proportional. Therefore, the inverse relationship between the electrical conductivity and the Seebeck coefficient is established. Therefore, the high electrical conductivity imparted by sintering at 1123 K by the SPS method causes the low Seebeck coefficient.

The temperature dependence of the thermal conductivity κ of Mg 2 Si is shown in FIG. 3 (a). The measured kappa decreased monotonously with increasing temperature, indicating an increase in phonon scattering. For samples sintered at 973K by the SPS method, that is, samples sintered at the lowest temperature, a minimum thermal conductivity of 0.03 W / cmK was shown at an operating temperature of 773K. In general, both thermal conductivity and electrical conductivity are proportional to each other because they depend on porosity and density. Therefore, the highest electrical conductivity due to the high density is the cause of the highest thermal conductivity.

In addition, the dimensionless figure of merit ZT is measured in the temperature range of 800K from room temperature is shown in Figure 3 (b). ZT value increased with increasing temperature. The maximum value of ZT was 0.28 at the working temperature of 773 K for the sample heat-treated at 1123 K by the SPS method.

The ZT values of these pure Mg 2 Si samples are higher than the data reported to date. 4 shows a comparison of the data reported by the present invention and the data reported in the prior art. Here, ■ is ZT of the sample according to the present invention in combination with vacuum melting and SPS, ○ is Jun-ichi Tani et al. ZT, Δ of the SPS sintered samples by Masayasu Akasaka et al. ZT of samples sintered using the Vertical Bridgeman method by ▽ is Masayasu Akasaka et al. ZT of the samples sintered using the Vertical Bridgeman method and the SPS method, respectively. Although the ZT value according to the present invention was lower than any of the reported values in some cases, the example showing the high ZT value was obtained by an advanced method called a crystal growth method, and therefore, such an advanced method was not used. The value of the present invention, which derives a high ZT value, is that it implements an advantageous process for commercialization. That is, the value from which the higher ZT is derived from the reported value than the present invention is disadvantageous compared to the present invention in terms of commercialization. (It should be reinforced in conjunction with the conventional synthesis process. This will be based on what is reinforced in the background art.)

According to the present invention, in order to synthesize accurate stoichiometric Mg 2 Si, the vacuum melting method is used, considering the volatilization of Mg, the generation of secondary phases by preventing the incorporation of impurities, and the change of electrical properties. In addition, it is meaningful that the spark plasma sintering process was further added.

Therefore, while the vacuum melting method itself forms a feature of the present invention, the vacuum melting method and the spark plasma sintering method are those in which they are organically combined to enable stable homogeneity of Mg 2 Si.

As described above, the present invention has been described based on the preferred embodiments, but the present invention should not be construed as being limited to the above embodiments, but the contents described in the claims and the detailed description of the invention for the same. The range will have to be determined.

Claims (5)

Preparing a mixture for preparing a magnesium silicide thermoelectric composition by weighing and mixing Mg and Si, respectively; And
Sealing the mixture in a vacuum state and heat-treating it in the sealed state to synthesize it;
Method of synthesizing the magnesium silicide thermoelectric composition comprising a. The method of claim 1,
Sealing the mixture in a vacuum maintained state, and heat treating the sealed state in a sealed state;
The sealing is a method for synthesizing the magnesium silicide thermoelectric composition, characterized in that the mixture is accommodated in the crucible, and the crucible is charged into a quartz container under vacuum. Magnesium silicide thermoelectric composition prepared by the method of claim 1, characterized in that the improved figure of merit. Magnesium silicide thermoelectric, characterized in that produced by sintering the thermoelectric composition prepared by the method of claim 1 at a temperature range of 950 ~ 1150K. The method of claim 4, wherein
Magnesium silicide thermoelectric, characterized in that by the spark plasma sintering method.

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