JPH0726905B2 - Surface area measuring device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an apparatus for measuring the surface area of powder, porous material, and the like, and more specifically, surface area measurement using a flow method based on the principle of the gas adsorption method. Regarding the device.

<Prior Art> In a surface area measuring apparatus using a flow method, a mixed gas formed by mixing an adsorbing gas and a carrier gas at a predetermined mixing ratio is flown over a sample, and the temperature of the sample is lowered in that state. As a result, the adsorbed gas molecules are adsorbed on the sample surface. Further, from that state, the temperature of the sample is raised (returned to about room temperature) to desorb the adsorbed gas molecules. During this adsorption / desorption, the change in the mixing ratio of the mixed gas before and after the adsorption or desorption is measured by the thermal conductivity detector (hereinafter referred to as TCD) on the upstream and downstream of the sample on the mixed gas flow path, and the upstream side is the primary side. , The downstream side is referred to as the secondary side), and the surface area of the sample is calculated from the amount of adsorption or desorption based on the change in the thermal conductivity of the mixed gas. The desorption amount is actually used for the calculation of the surface area.

In this type of surface area measuring device,
The length of the mixed gas flow path between the TCD and the secondary side of the TCD can be switched according to the sample to be measured.
Of the flow path (short path) and a second flow path (long path) longer than this, and a selector valve for selecting these is provided.

The short path is a generally used flow path, and when this short path is used, the measurement time becomes extremely short. Since the advantage of the flow method lies in the quick measurement, this short path is usually used except when there is a need for switching as described later. The long pass is used for the following reasons only when the surface area of the sample is relatively large or when measuring the sample in which adsorption / desorption proceeds slowly, but the measurement time becomes long.

The reason why the long path is configured to be selectively used is as follows. Generally, at the time of adsorption or desorption, even though a mixed gas of a constant flow rate is flown,
Immediately after the start of adsorption or immediately after the start of desorption, the flow rate of the mixed gas flowing on the secondary side of the TCD changes greatly. That is, in the initial stage of adsorption, the flow rate once drops to almost zero, and in the initial stage of desorption, a large amount of mixed gas flows on the contrary. This is mainly due to a transient phenomenon in which a large amount of adsorbed gas molecules are adsorbed on or desorbed from a sample in a short time at the beginning of adsorption / desorption. The flow rate gradually returns to an appropriate value as the adsorption or desorption progresses.

The TCD output is calibrated under a predetermined proper gas flow rate, so if the mixed gas whose mixing ratio changes due to adsorption or desorption when the flow rate changes as described above reaches the secondary side of the TCD, a measurement error will occur. Will occur. After the flow rate returns to the proper value, the mixed gas with such a changed mixing ratio is
It is a normal measurement sequence to reach the D2 secondary side, its output exceeds the threshold value, and detection is started.

The flow path between the sample arrangement position and the TCD secondary side in the mixed gas flow path has a buffering function for preventing the mixed gas having a changed mixing ratio from reaching the TCD secondary side during the above-described transition period. In ordinary measurement of a sample, this buffering effect is sufficiently exhibited even if a short path is used. However, when measuring a sample with a large surface area that adsorbs / desorbs a relatively large amount of gas, using a short path causes the output on the secondary side of the TCD to exceed the threshold value before the flow rate returns to an appropriate value. However, the above-mentioned measurement error occurs. The same applies to the measurement of a sample whose adsorption or desorption is slow. Therefore, a long path is provided selectively so that even in the measurement of such a sample, the mixed gas whose mixing ratio changes during the transition period is delayed from reaching the secondary side of the TCD.

Conventionally, the flow rate of the mixed gas is configured to be monitored by disposing a flow meter on the mixed gas flow path downstream of the secondary side of the TCD. As this flow meter, an area flow meter (rotameter), which knows the flow rate by visually confirming the position of the float in the glass tube, is exclusively used.

Then, in the selector valve switching described above, the operator visually confirmed and judged the indication value of this flow meter and the display based on the TCD output during the adsorption measurement, and manually operated the selector valve before the desorption measurement. .

<Problems to be Solved by the Invention> According to the conventional device as described above, it is necessary for the operator to determine whether or not to switch the selector valve based on the result of visual reading of the indication value of the flow meter or the like. In fact, it is impossible to monitor the detected value by TCD and the indicated value of the flowmeter at the same time, and it is necessary to switch the selector valve if any of these values is reached. There is no clear standard, and it is undeniable that there is a large individual difference due to the human error in reading. If the selection of switching of the selector valve is not appropriate, it may cause an error in the desorption measurement result used for the actual surface area calculation, or the measurement may take an unnecessarily long time. . Further, there is a problem that the operator cannot leave the device during the adsorption measurement.

<Means for Solving Problems> The present invention has been made to solve the above problems, and is characterized in that during the adsorption measurement using the short path, the following desorption measurement is performed. At the time of execution, it has a judgment means for judging whether switching of the selector valve is necessary and outputting the result, and this judgment means compares the preset reference value and the sample surface area value obtained at the end of the adsorption measurement. In comparison, if this sample surface area value exceeds the reference value, or the detected thermal conductivity value exceeds the preset threshold value when performing adsorption measurement, the thermal conductivity detection position on the downstream side If the output signal of the gas flow meter provided further downstream than that does not fall within the preset proper range, the determination result indicating that switching of the selector valve is required is output.

<Operation> In the desorption measurement, the present invention provides the results of the adsorption measurement performed prior to the desorption measurement used for the actual surface area calculation,
An attempt to solve various problems in the conventional measuring device of this type by automatically reading at a predetermined timing and automatically determining a path to be used for desorption measurement based on the reading result. Is.

That is, when adsorption measurement is performed using a short path, if the sample surface area value obtained at the end of the measurement exceeds the reference value, it means that the surface area of the sample is larger than a certain reference value. In addition, when the detected value of thermal conductivity exceeds the threshold value during adsorption measurement, that is, when the gas flow rate on the TCD downstream side during the transition to the adsorption transient state is not within the appropriate range, , Means that a large flow rate change occurred in the adsorption transient state. Therefore, by using any one of these methods as a method for determining the necessity of switching the selector valve, the necessity of switching the selector valve is always determined based on a fixed standard without causing human error or individual difference. It

<Example> An example of the present invention will be described below with reference to the drawings.

FIG. 1 is an overall configuration diagram of an embodiment of the present invention.

For example, a mixed gas obtained by mixing nitrogen gas as an adsorption gas and helium gas as a carrier gas at a ratio of 3: 7 is
Introduced into the flow path 2 from the gas inlet 1, the ON / OFF valve 3,
It passes through the pressure regulating valve 4 and the flow rate control valve 5. During this period, the mixed gas flow rate is maintained at a predetermined constant value, for example, 20 cc / min.

The mixed gas having a constant flow rate then reaches the cold trap unit 6. The flow path here is a glass tube and is constantly cooled by liquid nitrogen, which is a refrigerant, and impurities in the mixed gas, particularly water vapor and the like that adversely affects TCD7, are condensed and removed at this portion.

The cleaned mixed gas reaches the measurement unit 9 through the primary side 7a of the TCD 7. A septum 10 for injecting a reference gas for surface area calibration from the outside is arranged in the middle of this.

A sample cell 12 is connected to the gas channel of the measuring unit 9 via a cell holder 11 which can be easily attached and detached. By immersing the sample cell 12 containing the sample in liquid nitrogen, the nitrogen gas in the mixed gas is physically adsorbed on the sample surface. When liquid nitrogen is removed after reaching the adsorption equilibrium, the sample naturally warms to room temperature, and the adsorbed nitrogen molecules are desorbed.

The mixed gas that has passed through the sample cell 12 passes through the sample scattering prevention filter 13 and reaches the selector valve 14. With this selector valve 14, the length of the mixed gas flow path up to the secondary side 7b of the next TCD 7 can be selected. That is,
A short path 15 and a long path 16 are provided, and either of these can be selected by operating the selector valve 14.

The selector valve 14 is connected to the valve drive device 31 and can automatically operate according to a drive command. Further, the sensing of the switching position of the selector valve 14 is also configured to be performed in this valve driving device.

The mixed gas reaches the secondary side 7b of the TCD 7 through the selector valve 14. On the primary side 7a, the gas always flows at a constant mixing ratio, whereas on the secondary side 7b, the mixing ratio of the gas flowing during adsorption / desorption changes. This change in the mixing ratio is detected by TCD7.

The detection signal of TCD7 is amplified by the amplification circuit 18 and then integrated circuit
Integral at 19. Before measurement, for example, inject 1 cc of nitrogen gas from the septum 10 for calibration to obtain a calibration peak, and set the surface area value corresponding to that calibration peak by calibration beforehand, adsorption at the time of actual measurement The integrated value of the peak / desorption peak can be converted into a surface area and displayed. The TCD 7 is always kept at a constant temperature by the temperature controller 8.

The mixed gas passing through the secondary side 7b of the TCD 7 passes through the area flow meter 20 and then the flow meter 32 capable of generating an output signal according to the gas flow rate. The output signal of the flow meter 32 is used as a determination circuit 33 described later.
Has been incorporated into.

The mixed gas passing through the flow meter 32 reaches the degassing section 21. In the degassing section 21, the sample is pretreated (degassing process). In this degassing section 21, the same cell holder 11 and sample cell 12 as those used in the measuring section 9 are used. The degassing treatment is a pretreatment for cleaning the sample surface to obtain a correct adsorption / desorption amount. By heating the unnecessary gas mixture that has passed through the measuring section 9 into the sample cell 12 and heating it by the mantle heater 22. Done. The temperature of the mantle heater 22 is feedback-controlled by the temperature adjusting circuit 23.

The mixed gas used in the degassing process is discharged from the gas outlet 25 to the outside of the device through the backflow prevention unit 24.

Now, the determination circuit 33 that takes in the output signal of the flow meter 32 described above also takes in the output signal of the TCD 7 amplified by the amplifier circuit 18. In addition, this determination circuit 33,
The valve drive device 31 is connected, and based on the result of the determination described below, a switching drive command for the selector valve 14 is given to this valve drive device 31, and the switching position of the selector valve 14 is determined by the valve drive device 31. It is supplied to the circuit 33. Further, a display device 34 is connected to the determination circuit 33, and the current position of the selector valve 14 (whether short path 15 or long path 16 is selected),
And the judgment result can be displayed. This display device
34 is composed of, for example, two LEDs for displaying the valve position and one LED for indicating whether or not the valve should be switched by turning it on or off.

The determination circuit 33 determines whether or not the output signal of the flowmeter 32 and the output signal of the TCD 7 maintain a predetermined relationship in the transition period in the initial stage of the adsorption measurement, and thus the selector valve 14 is set before the desorption measurement. A circuit that determines whether or not it is necessary to switch, for example, a microcomputer and AD
It is composed of its peripherals such as converters.

Next, a method of determining the necessity of valve switching by the determination circuit 33 will be described together with the measurement operation of the device. Second
The figure is a flow chart showing the measurement operation procedure of the embodiment of the present invention, in which the sample cell 12 containing the sample for which the pretreatment is completed is attached to the measuring section 9 and the mixed gas is flowed, at the time of starting the adsorption measurement. It shows from.

The determination circuit 33 has, in advance, the minimum input signal value at which the integration circuit 19 can perform integration, that is, the threshold value x ₁ of the output signal of TCD7, and the output of the flowmeter 32 at the flow rate calibrated TCD7, that is, the appropriate flow rate value. The value x ₂ and its allowable range Δx are set.

When the suction measurement is instructed, first, the position of the selector valve 14 is confirmed based on the sensing by the valve drive device 31, and it is judged whether or not the short path 15 is selected, so that the short path 15 is always performed before the suction measurement. Is used,
Set the position of the selector valve 14 automatically.

In this state, the sample cell 12 of the measuring section 9 is cooled with liquid nitrogen and the adsorption measurement is started. At this time, the output X _T of the TCD 7 after amplification and the output X _{F of the} flowmeter 32 are sampled moment by moment. And
X _T and x ₁ are compared, and when X _T exceeds the threshold value x ₁ , it is determined whether or not X _F is within the allowable range Δx of the proper flow rate value x ₂ . If it does not fit, set a flag.

When the adsorption measurement is completed, the desorption measurement is performed next, but before this, only when the flag is set, the selector valve 14
Is automatically switched and the long pass 16 is selected. Then, in this state, the liquid nitrogen in the measuring unit 9 is removed and the desorption measurement is started.

By the above operation, when the mixed gas with the change of the mixing ratio reaches the secondary side of TCD7 before the mixed gas flow rate returns to the proper amount in the transition period at the beginning of the adsorption measurement, it is automatically performed immediately before the desorption measurement. As a result of the selector valve 14 being switched and the long path 16 being selected, desorption measurement without error is performed and an accurate surface area value is obtained. Similarly, if the gas with a change in the mixing ratio reaches the secondary side 7b of the TCD7 after the mixed gas flow rate returns to the proper range, keep the selector valve 14 as it is and use the short path 15 for quick desorption. The measurement is taken. In other words, from the behavior of TCD7 output and flowmeter 32 output during the transition period of adsorption measurement, the size of the sample surface area and / or the slowness of adsorption / desorption progress is determined, and the flow path length to be used during desorption measurement is automatically determined. To be selected.

Incidentally, the following method can be adopted for the determination of necessity of valve switching instead of the above embodiment. That is, the output value of the integrator circuit 19 after the completion of the adsorption measurement is compared with a preset reference value, and when the output value exceeds the reference value, the selector valve 14 is switched to perform a long pass.
Determines that 16 should be used. That is, if the sample surface area value obtained by the adsorption measurement is not less than the predetermined value, the long path 16 is selected in the desorption measurement. According to this method, the short path 15 is selected for the sample with slow progress of adsorption / desorption, but in reality, such a sample is extremely rare, and it is almost the same as the effect in the above-mentioned embodiment. The same effect can be obtained. in this case,
The flow meter 32 becomes unnecessary.

Further, in any of the embodiments, it is not always necessary to automatically switch the selector valve 14 based on the determination result, and the operator is notified of that by displaying the determination result or sounding an alarm buzzer. Only by itself, the measurement error is eliminated.

<Effects of the Invention> As described above, according to the present invention, the adsorption measurement is configured to be performed by a short path, and at the time of the adsorption measurement, the sample surface area value obtained at the end of the adsorption measurement is a reference value. Desorption measurement by judging whether the gas flow rate is within the proper range at the time when the thermal conductivity exceeds the threshold value and transitions to the transient state during adsorption measurement. Occasionally configured to automatically determine which path to use, the operator no longer has to be involved in the determination of this selector valve,
As in the prior art, the influence of the reading error of the visually indicated value or the like and the influence of the erroneous judgment are eliminated, and the optimum (accurate and quick) measurement can always be performed according to the sample to be measured. As a result, the problem that the desorption measurement result used for the actual surface area calculation contains an error or the desorption measurement time becomes unnecessarily long is solved, and it is possible to develop the apparatus to be fully automated.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of an embodiment of the present invention, and FIG. 2 is a flow chart showing the measuring operation procedure. 1 …… Gas inlet, 2 …… Flow path 7 …… TCD, 7a …… Primary side 7b …… Secondary side, 9 …… Measuring section 12 …… Sample cell, 14 …… Selector valve 15 …… Short path , 16 …… Long pass 18 …… Amplifier circuit, 19 …… Integrator circuit 25 …… Gas outlet, 31 …… Valve drive device 32 …… Flowmeter, 33 …… Judgment circuit 34 …… Display device

Claims (1)

[Claims] 1. A sample is placed on a flow path of a mixed gas obtained by mixing an adsorbed gas and a carrier gas at a predetermined mixing ratio, and the adsorbed gas molecules are adsorbed on the sample surface by cooling the sample, and The temperature of the sample is raised from the state to desorb the molecules, and at the time of this adsorption and desorption, the thermal conductivity of the mixed gas is detected upstream and downstream of the arrangement position of the sample on the mixed gas channel, and based on the detected value. Is a device for determining the sample surface area, and in order to exclude the influence of the change of the mixed gas in the transient state of the adsorption / desorption phenomenon from the detected value, the thermal conductivity detection at the sample arrangement position and the downstream side is performed. The first channel and the first channel are arranged between the position and the position so as to switch the length of the mixed gas channel between these positions according to the sample.
In a device having a second flow path longer than the first flow path and provided with a selector valve for selecting the second flow path, the following desorption is performed during the adsorption measurement using the first flow path. When performing the measurement, there is a determination means for determining whether or not the selector valve should be switched and outputting the result. The determination means is a reference value set in advance and the sample obtained at the end of the adsorption measurement. If the sample surface area value exceeds the reference value by comparing with the surface area value, or when the detected value of the thermal conductivity exceeds a preset threshold value when the adsorption measurement is performed, If the output signal of the gas flow meter provided downstream of the thermal conductivity detection position on the downstream side of the mixed gas flow channel is not within the preset proper range, switch the selector valve. A surface area measuring device, characterized in that it outputs a determination result indicating that it is necessary.