JP3094670B2 - Surface area measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring a specific surface area of a granular material or a porous material based on a gas adsorption method.

[0002]

2. Description of the Related Art In a surface area measuring apparatus based on a gas adsorption method, an adsorption isotherm is generally obtained by measuring the amount of an adsorbed gas under a plurality of adsorption equilibrium pressures. The data is a data string of (P / P ₀ , V), where P is the pressure at the time of adsorption equilibrium, P _{0 is the} saturated vapor pressure, and V is the amount of gas actually adsorbed. Then, the specific surface area of the solid sample can be obtained by performing an operation using the BET method or the Langmuir method on this data string.

In calculations using the BET method and the Langmuir method, a plurality of points of data at relatively low pressure (0.05 <P / P ₀ <0.35) are used except for a simple BET one-point method. Then, by performing variable conversion on the data of the plurality of points, a linear relationship including two unknowns, ie, the adsorption amount Vm of the single molecule and the adsorption parameter C is obtained. Vm and C can be obtained from the intercept and gradient when this linear relationship is plotted, and the surface area can be calculated.

[0004]

By the way, the above B
In both the ET method and the Langmuir method,
Depending on which range of relative pressure P / P ₀ data is extracted from the measurement data sequence (P / P ₀ , V) and used for calculation,
The obtained surface area value may fluctuate greatly. In other words, even for the same measurement data sequence, for example, 0.05 <P / P ₀
<When data in the range of 0.20 is used, 0.10 <
When using data in the range of P / P ₀ <0.30,
The resulting surface area values will be different. Here, it is not always clear which range of data to use is closer to the true value depending on the material of the sample to be measured, etc., especially when performing measurement on a new material, etc. There is no criterion to decide, and it is left to the discretion of the operator of the device, and the surface area value will be different accordingly.

[0005] The appropriate range of data to be used can be determined to some extent from the shape of the adsorption isotherm. However, in the measurement of a new material as described above, the adsorption isotherm is once determined. The data range cannot be defined until after it is obtained. In order to obtain an adsorption isotherm, it usually takes several hours or several tens of hours, so that there is a problem that the operation for determining such a data range becomes extremely complicated.

In addition, many of these types of apparatuses are automated, and the obtained surface area values are often viewed without examining the analysis conditions for determining which data range was used.
If there is an error in the setting of the data range, there is also a problem that an erroneous surface area value is obtained and used unnoticed.

[0007] The present invention has been made in view of such circumstances, and even when measuring a new material or the like, without performing measurement for determining the data range in advance, and without knowing an erroneous surface area value. It is an object of the present invention to provide a surface area measuring device that does not cause any troubles such as the use of the same.

[0008]

A structure for achieving the above object will be described with reference to a basic conceptual diagram shown in FIG. 1. The surface area measuring apparatus according to the present invention employs an apparatus for adsorbing an adsorbed gas on a sample to be measured. The adsorption amount / equilibrium pressure measuring means a for measuring the relationship between the amount and the adsorption equilibrium pressure at a plurality of points, the storage means b for storing the measured data sequence, and a predetermined operation using the stored data sequence In an apparatus provided with a calculating means c for calculating a surface area of a sample to be measured, a boundary for determining a boundary point between a linear part of an adsorption isotherm and a bent part connected to the low-pressure side by calculation from a data string in a storage means b. Point determining means d and calculating means c according to the obtained boundary points
Is characterized by having a data section setting means e for automatically setting a relative pressure section of data used in.

[0009]

The adsorption isotherm pattern obtained from a plurality of adsorption equilibrium pressures and the amount of gas adsorbed at each can be classified as shown in FIG. 4 (KSWSing et al., Pure Appl. Chem.,
57, 603 (1985)). When calculating the specific surface area, generally,
At liquid nitrogen temperature, nitrogen gas is used as the adsorption gas. In that case, most cases show any of the I, II and IV types. In such a mold, first, there is a portion where the relative pressure is steep from 0 on the adsorption isotherm, and from there, gradually transitions to a gentle gradient, and a linear portion having a certain length is seen.

When the BET method is applied, it is appropriate to extract a plurality of measurement points including a point (referred to as a point B, see FIG. 5) separated from the straight line portion and to use the extracted measurement points for the calculation of the surface area. That is, the point B corresponds to a state where only one layer of the adsorbed gas molecules is adsorbed on the surface of the sample to be measured.

Accordingly, the boundary point determining means d in the present invention
Thus, the above-mentioned point B on the adsorption isotherm is calculated from the actual measurement data sequence, and the data section setting means e determines the data section to be subjected to the calculation by the calculating means c according to the calculated point B. By automatically setting an ideal relative pressure section, for example, by using data of several points before and after the point B as a center, it is not necessary to perform pre-measurement even for a new material, etc. The intended purpose can be achieved by the apparatus determining the optimal data range without waiting for

[0012]

FIG. 2 is an overall configuration diagram of an embodiment of the present invention, showing an example in which the present invention is applied to a surface area measuring device using a constant volume method based on a gas adsorption method.

A sample cell 1 for storing a sample is connected to a manifold 2 at a cell mounting portion 2a. The manifold 2 is provided with seven on-off valves 4 to 10 in addition to the on-off valve 3 for partitioning from the sample cell 1.
And 5 via a vacuum pump 11, a nitrogen gas source as an adsorbed gas via on / off valves 6 and 7, a helium gas source as a non-adsorbed gas via an on / off valve 8, and calibration via an on / off valve 9 And a tube 13 for measuring the saturated vapor pressure via the on-off valve 10. 14 and 15 are apertures.

The pressure in the manifold 2 is measured by a vacuum gauge 16
And the pressure is measured by a pressure detector 17. The measurement result is digitized by an A / D converter 18 and then taken into a computer 19.

Below the manifold 2, there is provided a Dewar bottle 20 containing liquid nitrogen as a coolant for cooling the sample cell 1, and this Dewar bottle 20 is moved up and down by an elevator mechanism 21.

The computer 19 has a CPU, ROM, RA
M, a keyboard, a display, a floppy disk drive, an input / output interface, and the like. The elevator mechanism 21, the vacuum pump 11, and the valve driver 22 for driving each of the on-off valves 3 to 10 are provided via the input / output interface. It is configured to give a control command.

The above hardware configuration is the same as that of the conventional apparatus, and the procedure for measuring the gas adsorption amount under a plurality of adsorption equilibrium pressures with the sample sealed in the sample cell 1 is the same as the conventional apparatus. It is as follows.

First, the on-off valves 3 and 4 or 3 and 5 are opened with the sample sealed in the sample cell 1, and the inside of the sample cell 1 and the manifold 2 is evacuated with the other valves closed. Then, the on-off valves 3 and 4 or 3 and 5 are closed. next,
Helical gas is introduced into the manifold 2 by opening the on-off valve 8, and after the on-off valve 8 is closed, the on-off valve 3 is opened to diffuse the helium gas into the sample cell 1. From the pressure values before and after opening and closing the on-off valve 3 and the internal volume of the manifold 2, the volume of a portion excluding the sample volume of the sample cell 1 is obtained. After that, the on-off valves 4 and 5 are opened and the inside of the sample cell 1 and the manifold 2 is evacuated again, and a part of the sample cell 1 is immersed in the Dewar bottle 20.

Next, after all the on-off valves are closed, the on-off valves 6
The nitrogen gas is introduced into the manifold 2 by opening the head 7, and the pressure is detected by the pressure detector 17, and when the predetermined pressure is reached, the on-off valve 6 or 7 is closed. After closing the on-off valve 6 or 7, the pressure detection value P _a by the pressure detector 17 at the time when the pressure in the manifold 2 is stable, stored in the memory of the computer 19.

Next, the on-off valve 3 is opened to diffuse nitrogen gas into the sample cell 1, and the nitrogen gas is physically adsorbed on the sample.
Then, store the pressure value P at the time of reaching adsorption equilibrium in memory, based on the difference between the pressure P _a stored in the P and above, to calculate the gas adsorption amount V of the sample at the pressure P.
That, P _a corresponds to the number of molecules of nitrogen gas introduced into the system, P is equivalent to the number of molecules not adsorbed to the sample of the nitrogen gas introduced. Therefore, the difference between the two becomes a value correlated to the number of molecules of the nitrogen gas adsorbed on the sample, that is, the gas adsorption amount V, and the adsorption amount V can be obtained from the value. However, at this time, a correction relating to the behavior of the gas at the temperature of the refrigerant is made.

[0021] The above operation by repeating while changing the value of P _a, each of the gas adsorption amount in a plurality of adsorption equilibrium pressure is obtained, a plurality of relative pressure P / P ₀ (P
The gas adsorption amount V at ₀ (saturated vapor pressure) is obtained, and the adsorption isotherm of the sample as illustrated in FIG. 5 can be obtained from this data sequence. When P reaches the saturated vapor pressure P ₀ , the desorption isotherm can be obtained by replacing the above-described gas introduction process with a vacuuming process and performing similar processing.

After the above measurement operation is completed, the computer 19 calculates the surface area of the sample by performing the following processing using the data string stored in the memory.

FIG. 3 is a flowchart showing the contents of a data processing program written in the RAM of the computer 19. The operation of the embodiment of the present invention will be described below with reference to FIG.

In this example, it is possible to select whether or not to execute the estimation mode prior to the measurement. Only when the estimation mode is selected, ST4 is a feature of the embodiment of the present invention. The following routine is executed, and if no selection is made, for example, 0.05 to 0.
BE using a data sequence within a fixed relative pressure range of 30
By the operation based on the T method or the Langmuir method,
The specific surface area of the sample is calculated (ST1, 2, 3). That is, the BET method has already spread as a method of relative comparison, and there is a case where analysis must be performed within a fixed relative pressure range regardless of the position of point B in the adsorption isotherm. In this case, "the estimation mode is not used" and "relative pressure range 0.05 to 0.30" are set in advance before the measurement, and the calculation is performed in the conventional manner.

In the estimation mode, first, after confirming the measurement range, it is determined whether or not the data includes data having a relative pressure of 0.35 or more (ST4, ST4).
5) If this is satisfied, the relative pressure is 0 to 0.35
Is divided into 35 equal parts, and a relative pressure data string {p (i)} (i = 1,
35, p (1) = 0.1, p (2) = 0.2,
p (35) = 0.35) is created (ST6). Next, the measured data string {P / P ₀ (j), V stored in the memory
(j) An adsorption amount W (i) corresponding to each relative pressure of the relative pressure data string {p (i)} is obtained by interpolation calculation using 補 間, for example, Lagrange interpolation calculation, and the data string {p (i), W (i)} are created (ST7).

Next, a differential coefficient α (i) at a point corresponding to each relative pressure p (i) on the adsorption isotherm illustrated in FIG. 5 is sequentially calculated (ST8). This differential coefficient α (i) is equal to the slope at each relative pressure p (i) on the adsorption isotherm.
Further, as an initial value of a coefficient β used for comparison with a reference value A shown below, for example, β =
1.05 is set (ST9).

Of the calculated differential coefficients, the relative pressure p (3
The differential coefficient α (35) in 5) is set as a reference value A (ST1).
0) Then, from p (34), it is sequentially determined whether or not the differential coefficient α (i) at each relative pressure is greater than or equal to β times the reference value A, in the direction of lower relative pressure. (ST11, ST11
12, ST13, ST14). Generally, a relative pressure of 0.35
Is considered to be included in a straight line portion on the adsorption isotherm, the point at which the slope is clearly larger than the slope at this point is detected from the side where the relative pressure is higher using the coefficient β, so that the adsorption isotherm is obtained. A point on the line close to point B is detected.

Differential coefficient α (i) having a magnitude β times the reference value A
Is found, it is determined whether or not the value p (i) is in the range of 0.05 to 0.25 (ST1).
5) If it is within this range, the actually measured data string {P /
Among P ₀ (j), V (j)}, three points P / P ₀ (j) which are larger than the value p (i) and which are closest to the value p (i), and the value p Three points P / P ₀ (j) smaller than (i) and closest to the value p (i) are extracted (ST16), and a total of 6 points are extracted.
BET using point data sequence {P / P ₀ (j), V (j)}
The specific surface area is calculated based on the method or Langmuir method (ST3).

The reason for setting the above-mentioned range in the above operation of estimating the point B is that nitrogen gas is used as the adsorbing gas and the B value on the adsorption isotherm measured at the temperature of liquid nitrogen.
Most of the points should fall within the above-mentioned range, and outside this range, the differential coefficient α of β times or more of the reference value A.
This is because even if (i) is found, it is highly likely that the point is not point B. Then, when a point having the above differential coefficient α (i) is found outside this range, β
Is increased by 0.05, and the same estimation operation is continued (ST17). This operation is repeatedly performed until β exceeds 1.30 (ST18).

By the above operation, the point B on the adsorption isotherm is automatically estimated, and the surface area is calculated by ideal data using actual measurement data centered on the point B.

When there is no differential coefficient α (i) having a value β times the reference value A in the initial value of β, and even when β exceeds 1.30, the relative pressure is 0.05 to 0. .25
If there is no such differential coefficient α (i) within the range, it is considered that the point B is not a special isotherm that can be estimated in this estimating operation, and the process proceeds to ST2 to fix the same as the conventional one. The surface area is calculated using the data group of the relative pressure range (ST12, ST17).

In the above description, the relative pressure is divided into 35 equal parts, the initial value of β and the method of increasing the β are not limited to these values, and the present invention is not limited thereto. Can be adjusted.

Further, there is a so-called t-plot method as a method of analyzing the adsorption data, and this method also derives physical property information from the degree of change in the gradient of the adsorption isotherm.
Therefore, the present invention can be applied to this analysis method.

[0034]

As described above, according to the present invention,
The point B on the adsorption isotherm is estimated by calculation from the measured data sequence of each adsorption amount under a plurality of adsorption equilibrium pressures, and the relative pressure section of the data to be used for surface area calculation is automatically set based on the estimation result. Therefore, even if the sample to be measured is a new material or the like, there is no need to perform measurement in advance to determine the relative pressure section to be used for calculation. This is particularly effective, for example, when it is necessary to continuously measure samples having different properties, and can significantly reduce the work of examining analysis conditions before measurement. Further, in an apparatus in which measurement to analysis is automated, there is no problem such as using an erroneous surface area value without knowing it due to setting of an erroneous analysis condition.

[Brief description of the drawings]

FIG. 1 is a basic conceptual diagram showing a configuration of the present invention.

FIG. 2 is an overall configuration diagram of an embodiment of the present invention.

FIG. 3 is a flowchart showing the contents of a data processing program written in a ROM of the computer 19;

FIG. 4 is an explanatory diagram of an example of a pattern of an adsorption isotherm.

FIG. 5 is an explanatory diagram of a point B in an adsorption isotherm.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sample cell 2 Manifold 3-10 On-off valve 11 Vacuum pump 16 Vacuum gauge 17 Pressure detector 18 A / D converter 19 Computer 20 Dewar bottle 21 Elevator mechanism

Claims (1)

(57) [Claims] An adsorption amount / equilibrium pressure measuring means for measuring a relationship between an adsorption amount of an adsorbed gas by a sample to be measured and an adsorption equilibrium pressure at a plurality of points; a storage means for storing a measured data string; An apparatus provided with calculating means for calculating the surface area of the sample to be measured by a predetermined calculation using the obtained data sequence, wherein a straight line portion in the adsorption isotherm and a bent portion connected to the low pressure side thereof are obtained from the data sequence in the storage means. And a data section setting means for automatically setting a relative pressure section of data used by the calculation means according to the obtained boundary point. Surface area measuring device.