JP2006040989A - Thermoelectric characteristic measuring apparatus for semiconductor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To allow measurements to be done in a temperature zone removed from the room temperature in a thermoelectric characteristic measuring apparatus which is so structured as to measure a plurality of thermoelectric characteristics including Seebeck coefficient, electric resistivity, and thermal conductivity by causing DC current to flow in a thermoelement. <P>SOLUTION: The thermoelectric characteristic measuring apparatus comprises: a heat reflector 18 which surrounds an internal space 19 wherein the thermoelement 16 is stored to which a current line 30 and a thermo-couple 36 are connected, and which reflects the radiant heat from the thermoelement 16 into the internal space 19; a means for controlling the heat reflector 18 to nearly the same temperature as that of the thermoelement 16; and a means for controlling the current line 30 and thermo-couple 36 to nearly the same temperature as that of the thermoelement 16. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an apparatus for measuring thermoelectric properties of a semiconductor element (hereinafter referred to as a thermoelectric element) used for thermoelectric conversion such as thermoelectric power generation.

As described in Non-Patent Document 1, a method of measuring thermoelectric characteristics of a plurality of items of a thermoelectric element, such as Seebeck coefficient, electrical resistivity, and thermal conductivity, by measuring the temperature of the thermoelectric element when a current is passed through the thermoelectric element As one of them, for example, a method called “transient method” is known. A measuring device using the “transient method” applies a direct current to a thermoelectric element, measures the voltage applied to both ends of the thermoelectric element, and the temperature difference between both ends of the element, and based on this, the Seebeck coefficient, electrical resistivity, and thermal resistance are measured. The thermoelectric characteristics of multiple items such as conductivity are obtained simultaneously. In this device, thermocouples and electric wires or electrodes for applying current are attached to both ends or one end of the thermoelectric element, and measurement is performed in a temperature region near room temperature.
Richard J. Buist, CRC Handbook of Thermoelectrics, Ed. Rowe, 1995, CRC press, inc. Chapter 18

  When a thermoelectric element is used for thermoelectric power generation, for example, it is necessary to analyze thermoelectric characteristics in a temperature region higher than room temperature. For example, it is required to measure thermoelectric characteristics in a temperature range from room temperature to about 300 degrees Celsius.

  However, in the conventional thermoelectric characteristic measuring apparatus configured to measure the thermoelectric characteristics of multiple items such as Seebeck coefficient, electrical resistivity, and thermal conductivity by passing a direct current through the thermoelectric element as described above, it is far from room temperature. It is difficult to measure thermoelectric properties in the temperature range. The reason is that an extra heat flow is generated between the thermoelectric element and the surrounding environment, and this heat flow interferes with the characteristic measurement. The farther the measurement temperature is from the room temperature, the greater the temperature difference between the thermoelectric element and the surrounding environment, which increases the excess heat flow and makes accurate measurement more and more difficult.

  Accordingly, an object of the present invention is to provide a thermoelectric characteristic measuring apparatus configured to measure a plurality of items of thermoelectric characteristics such as Seebeck coefficient, electrical resistivity, and thermal conductivity by passing a direct current through the thermoelectric element. It is to enable measurement of thermoelectric characteristics in a certain temperature range.

  According to the present invention, the apparatus for measuring the thermoelectric characteristics of the thermoelectric element by measuring the temperature of the thermoelectric element when a direct current is passed through the thermoelectric element measures the current line for passing the direct current and the temperature. And a thermal reflector that surrounds an inner space in which a thermoelectric element to which a thermocouple is attached is received and reflects radiant heat from the thermoelectric element to the inner space.

  In a preferred embodiment, thermal reflector temperature control means is further provided for controlling the temperature of the thermal reflector so as to be substantially equal to the temperature of the thermoelectric element.

  According to another aspect of the present invention, an apparatus for measuring the thermoelectric characteristics of the thermoelectric element by measuring the temperature of the thermoelectric element when a direct current is passed through the thermoelectric element is provided for passing a direct current through the thermoelectric element. Wire temperature control means is provided that is coupled to a thermocouple for measuring the temperature of the current line and the thermoelectric element, and controls the temperature of the current line and the thermocouple to be substantially equal to that of the thermoelectric element.

  According to the thermoelectric characteristic measuring apparatus having the thermal reflector of the present invention, the thermal reflector reflects the radiant heat from the thermoelectric element and returns it to the internal space even in a temperature range away from room temperature. The rate is reduced and the heat flow between the thermoelectric element and the surrounding environment is suppressed, improving the accuracy of the measurement.

  Further, when the heat reflector temperature control means is further provided, the heat flow between the thermoelectric element and the surrounding environment is further effectively suppressed, so that the measurement accuracy is further improved.

  According to the thermoelectric characteristic measuring apparatus having the electric wire temperature control means of the present invention, the temperature of the current line or thermocouple connected to the thermoelectric element by the electric wire temperature control means can be adjusted even in a temperature region away from room temperature. Since the temperature is controlled to be almost equal to the temperature of the current, the heat flow between the thermoelectric element flowing through the current line or thermocouple and the surrounding environment is suppressed, so that the measurement accuracy is further improved.

  FIG. 1 shows the overall structure of a thermoelectric characteristic measuring apparatus according to an embodiment of the present invention.

As shown in FIG. 1, a plate-like thermal anchor 12 is placed on the heater block 10, and a plate-like sample holder 14 is placed on the thermal anchor 12, and a central portion on the surface of the sample holder 14 is placed. A thermoelectric element sample 16 is fixed to the surface. The sample 16 is a semiconductor block having a thermoelectric conversion function such as n-type Bi ₂ Te ₃ . The heater block 10, the thermal anchor 12, and the sample holder 14 are in close contact so that there is no gap between them.

The heater block 10 is driven by a control device (not shown), and the amount of heating is controlled so that the sample holder 14 has a predetermined set temperature. The thermal anchor 12 is a part made of an electrically insulating material having good thermal conductivity such as Al ₂ O ₃ or AlN, and the temperature of the current wire 30 and the thermocouple 36 connected to the sample 16 is equal to that of the sample 16. The current line 30 and the thermocouple 36 are electrically insulated from each other, and the electrical insulation between the sample holder 14 and the current line 30 and between the sample holder 14 and the thermocouple 36 are performed. Electrical insulation is performed. The heater block 10, the thermal anchor 12 and the sample holder 14 are coupled in close contact so that there is no gap between them, and heat transfer between these components is good.

  The sample holder 14 is a metal plate having good thermal conductivity and electrical conductivity, such as copper or aluminum. The surface of the sample holder 14 is processed so as to be glossy and is plated with a metal having a good infrared reflectance such as nickel. The lower end surface of the sample 16 is joined to the nickel-plated surface of the sample holder 14 by soldering, brazing, or other metallurgical method with low electrical and thermal interface resistance. The sample holder 14 and the sample 16 are in good electrical and thermal conduction. The nickel plating layer on the surface of the sample holder 14 serves to reduce the effective radiation rate of the sample 16 by reflecting the radiant heat (infrared rays) from the sample 16 with a high reflectance, and the base material (for example, copper) of the sample holder 14 Has a function of preventing the sample 16 from diffusing into the sample 16.

  For measuring the temperature of the sample holder 14 (substantially the lower end temperature of the sample 16) and a current line 20 for passing a direct current for measurement at two locations in the vicinity of the sample 16 on the surface of the sample holder 14 Thermocouples 24 are bonded by solders 22 and 26 (or metallurgical methods with low electrical and thermal interface resistance such as brazing), respectively.

  An electrode piece 28 is joined to the upper end surface of the sample 16, and the electrode piece 28 is a small plate of copper or aluminum plated with nickel, for example, like the sample holder 14. A current line 30 for passing a measurement current and a thermocouple 36 for measuring the temperature of the electrode plate 28 (substantially the upper end temperature of the sample 16) are soldered to two locations on the upper surface of the electrode piece 28, respectively. , 38 (or a metallurgical method having low interface resistance electrically and thermally such as brazing).

A box-shaped heat reflector 18 is placed on the sample holder 14, and the sample 16 and its surrounding thermocouples, current wires 20 and 30, and thermocouples 24 and 36 are placed in an internal space 19 surrounded by the heat reflector 18. Be contained. The heat reflector 18 is a metal plate having good thermal conductivity such as copper or aluminum. The surface of the heat reflector 18 (in particular, the inner surface exposed to the internal space 19) is processed to be glossy and is plated with a material having good infrared reflectance such as nickel. Alternatively, the heat reflector 18 is made of a material having high infrared reflectance, such as stainless steel, and the inner surface thereof is processed to be glossy. The lower end surface of the thermal reflector 18 is coupled in a close contact state so that there is no gap between the upper surface of the sample holder 14, and the thermal conductivity between the two is good. The thermal reflector 18 has a function of reflecting the radiant heat (infrared ray) from the sample 16 with a high reflectance and returning it to the inner space 19, thereby reducing the effective radiation rate of the sample 16. The inner space 19 of the heat reflector 18 is brought into a vacuum state of 10 ⁻² or less by a vacuum pump (not shown). In addition, a through hole (not shown) is provided in a part of the heat reflector 18, and the current lines 20 and 30 and the thermocouples 24 and 36 in the internal space 19 are drawn out of the heat reflector 18 through the through hole. And connected to a signal processing device (not shown). The passage holes of the current lines 20 and 30 are electrically insulated, and the passage holes of the thermocouples 24 and 36 are made as small as possible.

  The current line 30 and the thermocouple 36 connected to the electrode piece 28 at the upper end of the sample 16 are a thermal anchor made of a material having a good thermal conductivity at the midpoint of the portion drawn out of the thermal reflector 18. Crimp terminals 40, 42, 44 are provided. The thermal anchor crimp terminals 40, 42, and 44 are connected to the thermal anchor 12 in close contact with, for example, screws, so that the thermal conductivity between the thermal anchor 12, the current line 30, and the thermocouple 36 is improved. It is getting better.

  In the measuring apparatus configured as described above, the thermoelectric characteristics of the sample 16 are measured by a signal processing apparatus (not shown) using a method in which a direct current is passed through the thermoelectric element (for example, a transient method) according to the following processing procedure. .

  In a state where the soaking of the sample 16 is ensured, a direct current of such a magnitude that the temperature difference before and after the current application at the upper end of the sample 16 is about several degrees is applied to the sample 16. Thereafter, the current is cut off when the temperature distribution of the sample 16 is stabilized. The temperature at the upper and lower ends of the sample 16 immediately before the current interruption and the voltage change at the upper and lower ends of the sample 16 immediately after the current interruption are observed. First, the electrical resistivity and Seebeck coefficient of the sample 16 are calculated. Thereafter, the thermal conductivity and the figure of merit are obtained from the applied current, the temperature difference between the upper and lower ends of the sample 16, the average temperature of the sample 16, and the value of the Seebeck coefficient.

  When there is no heat outflow or inflow to the external environment of the sample 16, the dimensionless figure of merit can be obtained by a well-known equation (described later) by the Herman method. However, when the temperature of the sample 16 is set to a temperature equal to or higher than room temperature, it is actually impossible to make the outflow and inflow of heat to the external environment zero. Therefore, the performance index obtained from the equation of dimensionless figure of merit, and the thermal conductivity calculated back using the electrical resistivity and Seebeck coefficient obtained in the previous calculation are used as the initial values for analytical calculation, and more accurate heat conduction Obtain the rate and performance index. Details of the calculation method will be described later. By performing the above measurement in a state where the temperature of the sample 16 is set to each set temperature, the thermoelectric performance for each temperature of the sample 16 can be evaluated.

  In the measurement described above, the thermal conductivity of the sample 16 can be obtained by dividing the sum of Peltier heat absorption, Joule heat, and heat flowing in / out from the outside by the temperature difference between the upper and lower ends of the sample 16. The heat flowing into and out of the external environment of the sample 16 can be rewritten as a function of only the temperature at the upper end of the sample 16. The main factors of heat flowing into and out of the external environment are heat radiation of the sample 16, heat conduction by the current line 30 and the thermocouple 36 attached to the upper end of the sample 16, heat conduction by the atmosphere, and heat flow by the convection of the atmosphere. . The smaller these values are, the more accurate measurement can be made.

  In this measuring apparatus, since the sample 16 is placed in a vacuum, first, heat conduction by the atmosphere and heat flow by the convection of the atmosphere can be ignored. Next, although heat conduction by the current line 30 and the thermocouple 36 can be reduced by making the current line 30 and the thermocouple 36 elongated, temperature measurement with less Joule heat and noise generated in the current line 30 and the thermocouple 36 can be performed. Therefore, there is a limit to make the current line 30 and the thermocouple 36 elongated. Therefore, in this measuring apparatus, the relay point of the current line 30 and the thermocouple 36 is coupled to a thermal anchor having good thermal conductivity inserted between the sample holder 14 and the heater block 10, and is almost equivalent to the sample 16. Heated to temperature. As a result, the inflow and outflow of heat from the current line 30 and the thermocouple 36 is suppressed as much as possible regardless of whether the set temperature is room temperature or higher. Since the thermal conductivity of the current line 30 and the thermocouple 36 can be estimated relatively accurately from the material and size thereof, measurement errors caused by the current line 30 and the thermocouple 36 can be greatly reduced.

  Finally, the largest uncertain factor is the effective emissivity of the sample 16, and it is possible to estimate the heat conductivity more accurately by keeping this value as small as possible and obtaining it accurately. Therefore, in this measuring apparatus, a heat reflector 18 that reflects radiant heat inward is disposed around the sample 16 so as to surround the sample 16, and the heat reflector 18 is in close contact with the sample holder 14 below the sample 16 and While being held at substantially the same temperature, the radiant heat from the sample 16 is reflected and returned to the sample 16 side. Thereby, the effective emissivity of the sample 16 is reduced regardless of whether the set temperature is room temperature or higher.

  Actually, when the effective emissivity of the sample 16 in this measuring apparatus was calculated, as shown in FIG. 2, even in a temperature range considerably higher than room temperature of about 100 to 300 degrees Celsius, the effective emissivity is 0. It was confirmed that it was very low of about 15.

  Therefore, according to this measuring apparatus, it is possible to evaluate the performance of the thermoelectric element with higher accuracy in a temperature range far from room temperature. Moreover, not only a single thermoelectric element but also a performance evaluation of a thermoelectric module composed of a plurality of thermoelectric elements is possible.

  In the embodiment described above, the lower end surface of the heat reflector 18 is coupled to the sample holder 14 in order to make the temperature of the heat reflector 18 substantially equal to that of the sample 16. As a modified example, in addition to this configuration, a heater may be introduced on the upper surface of the heat reflector 18 to improve the thermal uniformity of the heat reflector 18. As another modification, the thermal anchor 12 for setting the temperature of the current line 30 and the thermocouple 36 to substantially the same temperature as that of the sample 16 needs to be arranged between the heater block 10 and the sample holder 14. Not necessarily, but may be arranged in another location.

  Finally, the specific calculation of the measurement process of thermoelectric characteristics by the transient method that can be employed in this measuring apparatus will be described.

  FIG. 3 is a schematic diagram of the heat flow flowing through the sample.

In FIG. 3A, the heat absorption amount (Q _c ) at the upper part (low temperature T _c side) of the sample 16 and the heat dissipation amount (Q _h ) at the lower part (high temperature T _h side) of the sample 16 are respectively expressed by the following equations. .

Q _c = αT _c I−RI ² / 2−K (T _h −T _c ) Equation 1
Q _h = αT _h I + RI ² / 2−K (T _h −T _c ) Equation 2
Where K is the thermal conductance,
I is the current flowing through the sample 16,
From Formula 1 + Formula 2
Q _c + Q _h = α (T _c + T _h ) I−2K (T _h −T _c ) Equation 3
It becomes.

Next, considering the case where the direction of the current is reversed as shown in FIG. 3B, the endothermic amount (Q _c ') of the lower part of the sample 16 (low temperature T _c ' side) and the upper part of the sample 16 (high temperature T _h 'side). ) (Q _h ') is expressed by the following formula.

Q _c '= αT _c 'I'-RI' ² / 2−K (T _h ' -T _c ') Equation 4
Q _h '= αT _h ' I '+ RI' ² / 2−K (T _h '−T _c ') Equation 5
Where I ′ is the current flowing through the sample 16,
From Equation 4 + Equation 5
Q _c ′ + Q _h ′ = α (T _c ′ + T _h ′) I′−2K (T _h ′ −T _c ′) Equation 6
It becomes.

From Equation 3 + Equation 6
Q _c + Q _h + Q _c '+ Q _h ' = α {(T _c + T _h ) I + (T _c '+ T _h ') I '}-2K {(T _h -T _c ) + (T _h ' -T _c ')} ・ ・ ・ Formula 7
Is obtained. If the first term on the right side of Equation 7 is −Q _p and the second term is Q _κ , Equation 7 can be rewritten as follows.

−Q _p = Q _κ + (Q _c + Q _h + Q _c '+ Q _h ') Equation 8
If the cause of heat absorption and heat dissipation in parentheses in Equation 8 is radiation (Q _ri + Q _re ), atmospheric convection (Q _c ), and thermal conduction (Q _a ) by the atmosphere, the following equation is obtained.

_{-Q p = Q κ + Q ri} + Q re + Q c + Q a ··· formula 9
Here, considering the heat absorption (Q _w ) from the current line 30 and the thermocouple 36 at the upper end of the sample 16 and the heat dissipation (Q _pl ) to the sample holder 14 at the lower end of the sample 16, Equation 9 is Can be expressed as:

−Q _p = Q _pl + Q _w + (Q _κ + Q _ri + Q _re + Q _c + Q _a )
Here, _assuming that the external temperature is T _rt , the thermal conductance of the current line 30 and the thermocouple 36 is K _w , and the thermal conductance of the sample holder 14 is K _pl.
Q _pl + Q _w = K _pl {(T _h −T _rt ) + (T _rt −T _c ′)} + K _w {(T _rt −T _c ) + (T _h ′ −T _rt )}
Because
Q _pl + Q _w = K _pl (T _h −T _c ′) + K _w (T _h ′ −T _c ) Equation 10
Is obtained.

Here, the temperature of the lower end portion (T _bot ) of the sample 16 is made constant, and the temperature of the upper end portion (T _top ) is changed above and below the temperature of the lower end portion (T _bot ) by reversing the direction of the current. Waving. Therefore, T _h = T _c ′, and Q _pl = K _pl (T _h −T _c ′) = 0. At this time, Equation 9 becomes as follows.

_{-Q p = Q κ + Q w} + Q ri + Q re + Q c + Q a ··· formula 11
Substituting the first and second terms of Equation 7 into Equation 11,
α {(T _c + T _h ) I + (T _c '+ T _h ') I '} = 2K {(T _h −T _c ) + (T _h ' −T _c ')} + Q _w + Q _ri + Q _re + Q _c + Q _a
Therefore,
α / K = [2 {(T _h −T _c ) + (T _h ′ −T _c ′)} + {(Q _w + Q _ri + Q _re + Q _c + Q _a ) / K}] / {(T _c + T _h ) I + (T _c '+ T _h ') I '}
It is.

Here, D _a = (T _h −T _c ) + (T _h ′ −T _c ′), IT _a = (T _c + T _h ) I / 2 + (T _c ′ + T _h ′) I ′ / 2 , K = κ / L (L = l / s),
α / κ = [2D _a + {(Q _w + Q _ri + Q _re + Q _c + Q _a ) L / κ}] / (2LIT _a )
Therefore,
α / κ = D _a [1 + {(Q _w + Q _ri + Q _re + Q _c + Q _a ) L / (2Daκ)}] / (LIT _a )
Therefore,
α / κ = CD _a / (LIT _a ) (12)
C = [1 + {(Q _w + Q _ri + Q _re + Q _c + Q _a ) L / (2Daκ)} = 1 + Q _w L / (2Daκ) + R _adi + R _ade + C _onv + C _air
... Formula 13
Where C is a correction factor (1 if there is no ideal heat loss),
R _adi = 4T ³ R _i L / 2κ R _i = σ × ε × Internal_surface_area
^{_{R ade = 4T 3 R e L}} / 2κ R e = σ × ε × External_surface_area
C _onv = HL / 2κ H = Convection_coefficient × External_surface_area
C _onv ＝ K _a L ／ κ Ka ＝ Air_conductivity × Internal_surface_area
Where σ is the Stefan-Boltzmann constant,
ε is the emissivity,
Internal_surface_area is the surface area of the environment surrounding the sample 16,
External_surface_area is the surface area of sample 16 and electrode,
Convection_coefficient is the convection coefficient,
Air_conductivity is the thermal conductivity of air,
It becomes.

  Here, the relational expression of the potential of the sample 16 being measured is as follows.

V _o = α (T _h −T _c ) + α _w (T _h −T _c )
V _o '= α (T _h ' −T _c ') + α _w (T _h ' −T _c ')
V _i = ρLI + V _o
V _i '＝ ρLI' ＋ V _o '
α _w = See the following formula for the Seebeck coefficient α and the electrical resistivity ρ from the absolute thermoelectric power equation of the voltage wire: α = 2V _oa / D _a −α _w Equation 14
ρ = (V _ia −V _oa ) / (LI _a )...
V _oa = (V _o + V _o ') / 2
V _ia = (V _i + V _i ') / 2
I _a = (I + I ') / 2
It can be expressed as

From Equation 14 ÷ Equation 12, the thermal conductivity κ is
κ = ² LIT _a V _oa / (CD _a ² ) + LIT _a α _w / (CD _a ) Equation 16
From Equation 12 × Equation 14 ÷ Equation 15, the figure of merit Z is
_{Z = 2CI a V oa / {} IT a (V ia -V oa)} - (CI a α w / D a) / {IT a (V ia -V oa)} ··· Equation 17
It becomes.

In this measurement device, is the formula for calculating the dimensionless figure of merit by the Herman method.
ZT = V _oa / (V _ia −V _oa )
Thus, the initial values of Z and κ are obtained, and the final Z, κ and C are determined by repeatedly calculating from Equation 16.

  As mentioned above, although embodiment of this invention was described, this embodiment is only the illustration for description of this invention, and is not the meaning which limits the scope of the present invention only to this embodiment. The present invention can be implemented in various other modes without departing from the gist thereof.

The perspective view of the thermoelectric characteristic measuring apparatus concerning one Embodiment of this invention. The figure which shows the relationship between the measurement temperature and the radiation rate in the same embodiment. The schematic diagram of the heat flow which flows through a sample.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Heater block 12 Thermal anchor 14 Sample holder 16 Sample 18 Thermal reflector 20, 30 Current line 24, 36 Thermocouple 40, 42, 44 Thermal anchor crimp terminal

Claims (3)

In an apparatus for measuring the thermoelectric characteristics of the thermoelectric element (16) by measuring the temperature of the thermoelectric element (16) when a direct current is passed through the thermoelectric element (16),
Enclosing an inner space (19) in which the thermoelectric element (16) to which the current lines (20, 30) for passing the direct current and the thermocouple (24, 36) for measuring the temperature are attached is accommodated; A thermoelectric characteristic measuring apparatus comprising a heat reflector (18) for reflecting radiant heat from the thermoelectric element (16) to the inner space. The thermoelectric characteristic measuring apparatus according to claim 1, further comprising thermal reflector temperature control means for controlling the temperature of the thermal reflector (18) to be substantially equal to the temperature of the thermoelectric element (16). In an apparatus for measuring the thermoelectric characteristics of the thermoelectric element (16) by measuring the temperature of the thermoelectric element (16) when a direct current is passed through the thermoelectric element (16),
The current line (30) and the thermocouple are coupled to a current line (30) for passing a direct current through the thermoelectric element (16) and a thermocouple (36) for measuring the temperature of the thermoelectric element (16). A thermoelectric characteristic measuring apparatus comprising electric wire temperature control means for controlling the temperature of (36) so as to be substantially equal to the temperature of the thermoelectric element (16).

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