JP2000206022A - Method for heating sample for temperature-rise desorption method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heating method in which the measuring error of a temperature is removed and in which the measuring accuracy of the temperature is enhanced by a method wherein a sample is heated up to a prescribed maximum temperature and the polarity of an applied voltage is inverted in a point of time when the temperature of the sample is stabilized. SOLUTION: When the arithmetic mean of outputs of two thermocouples is found, an error voltage is offset, only the electromotive force of the thermocouples is left, and the correct temperature of a sample 4 is indicated. Data on a temperature equilibrium region can be obtained in such a way that, e.g. the temperature of the sample 4 is raised in a current direction 1, that a current is changed over to an opposite direction 2 by using a switch 15 when the temperature reaches the temperature equilibrium region, that the outputs of the thermocouples 5 at this time are recorded for a short time and that the difference between the outputs of the thermocouples in the two current directions in the temperature equilibrium region is found. As a result of an experiment, it is found out that the temperature, of the sample, which is found by a value obtained by subtracting ηE from the outputs of the thermocouples in the current direction 1 gives a correct value and that the same result can be obtained even when the temperature of the sample is found on the basis of a value in which the ηE is added to the outputs of the thermocouples in the current direction 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for heating a conductive sample by applying a direct current at a uniform heating rate in a vacuum.
The present invention relates to a sample temperature measuring method for measuring the correct sample temperature by removing the influence of the sample heating current on the output of a thermocouple for temperature measurement attached to the sample.

[0002]

2. Description of the Related Art Thermal desorption is described in Vacuum, Vol.
(2 years) As described on page 203 et seq. (Referred to as Reference 1), a sample in a vacuum is heated at a uniform heating rate, and the time change of the pressure rise due to gas released from the sample is measured. It is used to study the state of gas adsorption on a solid surface. The gas adsorbed on the sample surface desorbs into the vacuum as the temperature of the sample rises. The desorption rate (the amount of gas molecules desorbed from the unit area of the sample per unit time) is plotted against the sample temperature. Then it becomes a middle and high mountain shape. Temperature corresponding to the top of the mountain (peak temperature)
Is determined by the activation energy of desorption according to the strength of adsorption of the gas and the order of the desorption reaction.

[0003] The shape of the peak also depends on the order of the elimination reaction. Therefore, by examining the relationship between the heating rate and the peak temperature, the shape of the mountain, the total amount of desorbed gas, etc., it is possible to know various physical quantities related to gas adsorption and desorption including the activation energy for desorption. Can be. In order to obtain highly reliable data in this thermal desorption method, it is necessary to accurately measure the sample temperature.

When the sample of the thermal desorption method has conductivity, the temperature can be easily raised by applying a direct current as a strip-shaped thin plate or a linear form. A thermocouple attached to the sample is used for measuring the temperature of the sample. As described in Document 1, the control of the sample temperature is performed by feeding back the measured value of the sample temperature to the power supply and the control device. When a current for heating is applied to the sample, a potential gradient is generated in the sample, and a potential difference may be generated between the two wires depending on how the thermocouple is attached.

[0005] This potential difference is measured by being superimposed on the output of the thermocouple, and becomes an error with respect to the sample temperature. A thermocouple wire with a diameter of 0.1 mm or less is used and attached with great care to the sample surface. However, the mounting position of the two wires is not affected by the sample heating current. As described above, it is difficult to control precisely, and it is unavoidable that a slight shift in the direction of the sample heating current is inevitable. Since the output of the thermocouple is as small as several mV to several tens mV, the potential difference caused by the slight displacement of the mounting position affects the temperature measurement. Since it is difficult to strictly control the mounting position, the deviation slightly changes for each sample, and the temperature measurement error also slightly changes. This means that the measured value of the temperature changes for each sample.

When the temperature of the sample is controlled by feeding back the output of the thermocouple, the output to the sample is adjusted so that the output of the thermocouple including the effect of the sample heating current changes for a predetermined time. The recording of the output of the pair always shows the same time change, and the effect of the sample heating current cannot be taken out. Therefore, the measured temperature always includes the influence of the sample heating current as an error.

As described above, in the prior art, the measured value of the sample temperature includes the error of the sample heating current as an error.
Since the magnitude of the error differs for each sample, there is a disadvantage that accuracy and reproducibility of the temperature measurement of the sample are impaired.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for heating a strip-shaped thin plate or a strip-shaped conductive sample at a uniform heating rate in a vacuum. An object of the present invention is to provide a sample temperature measuring method for a temperature programmed desorption method, which can remove an influence of a sample heating current included in the method and obtain an accurate sample temperature.

[0009]

A DC power supply is provided as a power supply for heating a sample, and a program defining a time change of an output from the DC power supply is stored in advance, and a DC power supply for heating the sample is stored in accordance with the storage. A sample heating device having a controller capable of transmitting a signal for controlling an output to a power supply, wherein the temperature of the sample is raised by an output of a DC power supply controlled by a signal from the controller; A switch is provided between the DC power supply and the sample to invert the polarity of the current, and the sample is heated to the predetermined maximum temperature in the temperature programmed desorption test and the polarity of the applied voltage is inverted when the sample temperature stabilizes. Thus, the above problem can be solved.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. The vacuum vessel 1 made of stainless steel has an ultrahigh vacuum by an exhaust device 11 composed of an ultrahigh vacuum pump 2 capable of withstanding a large gas load such as a turbo molecular pump and a roughing pump 3 such as a rotary pump. Evacuated to vacuum. The vacuum vessel 1 has its surface polished to almost a mirror surface by buff polishing and electric field polishing to reduce gas emission.
The dimensions are approximately 200 x 400.

The pumping speed of the ultra-high vacuum pump is about 0.18 m ³
At s ⁻¹ , baking (heating degassing) the vessel at 200 ° C. for 12 hours, to achieve a pressure of the order of 10 ⁻⁸ Pa. An elongated strip-shaped or linear metal sample 4 is attached to a current introduction terminal 12 and
Is supplied with power for heating via the switch 15, and the polarity of the applied voltage can be arbitrarily switched by the switch 15.

The sample output controller 7 comprises a storage device, an arithmetic device and a DC voltage generator, and controls a voltage supplied from the DC power supply 6 to the sample 4. The DC power supply 6 includes a control device and a power output device.
As a control signal, a voltage corresponding to the control DC voltage can be continuously supplied from the power output device to the sample 4 while the control DC voltage is being supplied.

The thermocouple 5 attached to the sample has a current introduction terminal 1
3 is connected to a recorder 10 to record the time change of the sample temperature. A vacuum gauge 8 and a quadrupole mass spectrometer 9 are attached to measure the pressure and gas components inside the vacuum vessel, and the measurement results are recorded on a recorder 10.

First, a sample heating method will be described. When a DC voltage of 0 to 10 V is applied from the outside, the DC power supply 6 has a maximum output voltage E in proportion to the applied voltage.
0% to 100% of _max can be output. The output controller 7 can output a DC voltage of 0 to 10 V as a control DC voltage, and its output update period Δt is a predetermined value (constant). The input and output modes of the output controller 7 will be described with reference to FIG.

During a certain time interval t _i , the DC voltage output of the output controller 7 is increased from e _i−1 V (0 ≦ e _i−1 <10) by an output voltage increment de _i V to e _{i− 1} + de _i V (0 ≦ e
_i-1 + de _i ≤ 10). First, the output controller 7 stores e _i−1 , t _i, and de _i , and t _i is set so that only an integral multiple of the output update period Δt is accepted.

The arithmetic unit built in the output controller 7 determines that the output changes linearly (along a thick solid line) from point a to point e during t _i , and the voltage corresponding to the output update period Δt Calculate the increment Δe _i V and use the output set value e _i−1 V at the point a of the output start to calculate Δt at the time interval t _i .
Output voltage e _i-1 from each of points a, b, and c to points d and e
+ Δe _i , e _i−1 + 2Δe _i ,..., E _i (= e _i−1 + de _i )
Is calculated by the DC voltage generator, starting from point a, Δt
The DC voltage is output up to the point e every time.

The time change of the DC voltage output from the output controller 7 to the power supply 6 in this manner is as shown by a broken line in FIG. Illustration between point c and point d on the way is omitted. By detecting the DC voltage from the output controller 7,
The DC power source 6 receives E from the power output device during the time interval t _i.
i−1 (= E _max × {e _i-1 + Δe _i } / 10) to E _i (= E _max
× e _i / 10) is output to the sample 4.

FIG. 3 shows a temporal change of the DC voltage output from the power supply 6 to the sample 4 (thick solid line). Uppercase letters A through E
Corresponds to lowercase letters a through e in FIG. Illustration between point C and point D on the way is omitted. When a gradually increasing input is given to the sample in this manner, the sample temperature rises at a heating rate determined by the balance between the calorific value, the heat capacity of the sample, the heat conduction to the introduction terminal 12 and the radiant heat transfer to the vacuum vessel 1. Therefore, by appropriately selecting the heat generation amount, that is, the input voltage to the sample, it is possible to control the sample heating rate.

Further, the same temperature rising rate can be realized by changing the same input voltage over time for samples of the same material and dimensions. In the thermal desorption experiment, the sample
_{When the} temperature is increased from _sta to T _end at the temperature increasing rate β, the output controller 7 _informs the output controller 7 of the output voltage e ₀ at the output start time t _sta and a set of I (t _i , de _i ) (i = 1, 2). ... I) is stored. Time t required for heating from T _sta to T _end
H is the sum of t _i , which is (T _end −T _sta ) / β.

Next, when the output controller 7 and the power supply 6 are operated, the control DC voltage is updated and output from the output controller 7 to the power supply 6 sequentially from e ₀ every output update period Δt. voltage for heating the sample 4 are sequentially outputted from the _{E 0 (= E max × e} 0/10) from the power supply 6 in response to. As a result, the sample temperature changes from T _sta to T _end during the time t _H.
Until the temperature rise rate β.

An example of a sample temperature rise obtained based on this embodiment will be described with reference to FIGS. The output controller 7 used for this measurement is commercially available and can be easily obtained. The output update period Δt of the output controller is 0.5 seconds,
Outputs a DC voltage of 0V to 10V. The set time interval of t _i is 1 second, and (t _i , de _i ) can be easily input according to the display screen of the output controller 7.

The power supply 6 is a DC power supply having a maximum output voltage _Emax of 10 V, and supplies a DC voltage of 0 V to 10 V to the sample 4 corresponding to the DC voltage of 0 V to 10 V from the output controller 7. Can be. The sample was a thin plate of stainless steel SUS304 having a thickness of 0.15 mm, which was cut into a strip of 4 mm × 300 mm, and had a resistance of about 0.5Ω.

FIG. 4 shows the set values of the time interval and the output voltage stored in the output controller, and the voltage output from the power supply 6 to the sample 4 by white squares (□) and black circles (●), respectively. The set values of the output controller connect 29 sets of (t _i , de _i ) in the order of output, and are displayed as a time change of the output voltage set value. Two adjacent white squares (□) are a in FIG.
The point and the point e correspond to each other, and the difference between the time and the voltage at the two points corresponds to (t _i , de _i ). In response to the signal from the output controller 7, the voltage (black circle) output from the power supply 6 to the sample 4 is stable at 91% of the open square, indicating the characteristics of the power supply.

Although the sample is heated from room temperature to about 850 ° C., the output voltage at the start of heating is not 0, and the maximum value is 40.
% Is necessary to realize a predetermined heating rate from the start of heating. Although the output voltage increases almost in the form of a quadratic function of time, slight corrections have been made based on actual measurements in order to keep the heating rate constant.

By electrically connecting the output controller 7, the power source 6 and the sample 4, and providing the output controller 7 with an execution start signal, the entire process of heating the sample is automatically executed. FIG. 5 shows the temperature change of Sample 4. It rises from room temperature to 850 ° C. at a constant heating rate (30.0 K · s ⁻¹ ). The fluctuation is very stable at ± 5% per 30 K of temperature rise. 9
The time required to raise the temperature to 00K is about 30 seconds.

Next, an example of measuring the thermocouple output will be described with reference to FIGS. The thermocouple used was the wire diameter φ
0.1 mm alumel-chromel spot welded to the stainless steel surface of the sample. FIG. 6 shows the time change of the output ε (mV) of the thermocouple when the DC voltage having the time change shown in FIG. 4 is supplied to the sample while changing the application direction. The sample current direction 1 and the sample current direction 2 are opposite directions. There is a clear difference in the output of the thermocouple depending on the direction of the sample current. The polarity of the applied voltage that gives a large thermocouple output is direction 1 and the polarity of the applied voltage that gives a small thermocouple output is direction 2.

In the time range from 34 seconds to 37 seconds, the applied voltage is constant at 7.66 V. The amount of heat generation and the amount of heat radiation are balanced regardless of the direction of the sample current, and the output ε of the thermocouple is saturated. ,
This range is called a temperature equilibrium region. FIG. 7 is a graph in which the difference Δε between the outputs of the thermocouples in the sample current directions 1 and 2 is plotted against the DC voltage applied to the sample.

It is clear that the difference in thermocouple output is distributed along the dashed line described for comparison and is proportional to the DC voltage applied to the sample. That is, an error caused by a potential gradient generated in the sample due to a direct current flowing through the sample is superimposed on the thermocouple output in FIG. 6 because the position of the thermocouple wire is slightly shifted in the current direction. It is understood that there is. The sign of the error voltage due to the potential gradient of the sample is equal because the sign is inverted depending on the direction of the DC current flowing through the sample.

Therefore, when the arithmetic average of the two thermocouple outputs in FIG. 6 is taken, the error voltage is canceled out, and only the electromotive force of the thermocouple remains, indicating the correct sample temperature. The large circle arranged at the position of the applied voltage of 7.66 V in FIG. 7 is the data of the temperature equilibrium region in FIG. 6, and this data is, for example, when the sample is heated in the current direction 1 and reaches the temperature equilibrium region. The current can be switched to the direction 2 using the switch 15, the thermocouple output at that time is recorded for a short time, and the difference between the thermocouple outputs in the two current directions in the temperature equilibrium region can be obtained.

The dashed line is a straight line connecting this circle with the sample applied voltage 0 and the difference between the thermocouple outputs 0, and is expressed by the following equation.

[0031]

Δ１ = 2 · η · E (Equation 1) In FIG. 8, η · E was subtracted from the sample temperature obtained from the average value of the thermocouple outputs in different current directions and the thermocouple output in the current direction 1. The sample temperature obtained from the value is indicated by a white square and a black triangle, respectively. The two types of points almost completely overlap, indicating that the sample temperature obtained from the value obtained by subtracting η · E from the thermocouple output in the current direction 1 gives a correct value. The same result can be obtained by obtaining the sample temperature from the value obtained by adding η · E to the thermocouple output in the current direction 2.

As described above, according to the present invention, the temperature measurement caused by the potential gradient of the sample created by the heating current due to the slight displacement of the mounting position of the two thermocouple wires to the sample. Errors can be eliminated to obtain the correct sample temperature.

In the above embodiment, an alumel-chromel is used as a thermocouple. However, the same method can be applied to other thermocouples.

[0034]

As described in detail in the description of the embodiments, according to the present invention, the potential of the sample generated by the heating current is caused by a slight displacement of the mounting position of the two thermocouple wires to the sample. The correct sample temperature can be obtained by eliminating the temperature measurement error caused by the gradient. Also,
It is possible to eliminate the influence of the attachment position of the thermocouple wires slightly different for each sample to the sample.

Further, since it is possible to quantitatively evaluate the temperature measurement error, the method of mounting the thermocouple is improved and the error is reduced by comparing and contrasting the thermocouple mounting method with the temperature measurement error. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a block diagram of a thermal desorption apparatus showing one embodiment of the present invention.

FIG. 2 is a graph schematically showing a time change of an output voltage from an output controller 7;

FIG. 3 is a graph schematically showing a time change of an output voltage from a power supply 6;

FIG. 4 is a graph showing a set value of an output voltage for an output controller 7 and a measured value of a time change of an output voltage from a sample heating power supply 6 in the embodiment.

FIG. 5 is a graph showing a measured value of a time change of a sample temperature in an example.

FIG. 6 is a graph showing a thermocouple output when the direction of a sample heating current is changed in the example.

FIG. 7 is a graph showing the relationship between the difference in thermocouple output and the sample heating voltage when the direction of the sample heating current is changed in the example.

FIG. 8 is a graph showing a relationship between a sample temperature and a sample temperature obtained from a value obtained by subtracting η · E from the thermocouple output in the direction 1 of the sample heating current.

[Explanation of symbols]

1 ... vacuum vessel, 2 ... ultra-high vacuum pump, 3 ... roughing pump,
4 sample, 5 thermocouple, 6 sample heating power supply, 7 output controller, 8 hot cathode ionization gauge, 9 quadrupole mass spectrometer,
Reference numeral 10: recorder, 11: exhaust device, 12, 13: current introduction terminal, 14: heating device, 15: switch.

Claims (1)

[Claims] In a thermal desorption method, a current is applied to a sample placed in a vacuum vessel to heat and raise the temperature by the heat generated by the sample itself, and a time change of a pressure rise due to gas released from the sample is measured. A signal that has a DC power supply as a power supply for heating the sample, stores in advance a program that determines the time change of the output from the DC power supply, and controls the output to the DC power supply for heating the sample according to the storage. A sample heating device that raises the temperature of the sample by the output of a DC power supply controlled by a signal from the controller, and inverts the polarity of the current between the DC power supply and the sample. When the sample temperature is stabilized by heating the sample to the predetermined maximum temperature in the thermal desorption test, the polarity of the applied voltage is reversed and at least the time during which the thermocouple output can be recorded Sample heating method for the inverted current Atsushi Nobori spectroscopy, characterized in that to hold the.