JP2005069779A - Complex dielectric constant measuring probe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a measuring probe suitable for complex dielectric constant measurement by a time region reflection method, which measures up to a high temperature region of about 250°C, by solving the problems wherein a change, deformation or the like of an electrical characteristic of a probe occurs by heat because polyimide, Teflon (R) or the like is filled as a dielectric between an internal conductor and an external conductor in the conventional probe, and thereby high temperature measurement can not be performed. <P>SOLUTION: This complex dielectric constant measuring probe has a constitution wherein SiO<SB>2</SB>powder 26 which is an inorganic dielectric is filled between the internal conductor 23 and the external conductor 24 of a coaxial cable 21, and a connecter 22 to be connected to a measuring device is provided on one end of the coaxial cable 21, and a sample is brought into contact with the coaxial cable end on the opposite side to the connecter 22. Hereby, measurement is performed without occurrence of such a change, deformation or the like of the electrical characteristics in a temperature zone from the room temperature to 250°C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a measurement probe used for measurement of complex permittivity, particularly, complex permittivity measurement using a time domain reflection method.

In chemical reaction studies, to increase the reaction yield or shorten the reaction time, the sample is heated by irradiating microwaves with a frequency of 300 MHz to 30 GHz (eg 2.45 GHz for water). Has been done. Here, in order to control the chemical reaction temperature, it is necessary to control the intensity of the irradiated microwave. As a parameter for determining the relationship between the microwave intensity and the sample temperature, there is a complex dielectric constant of the sample. The complex permittivity ε ^* is expressed by ε ^* = ε ′ + jε ″ (j is an imaginary unit), ε ′ which is a real component is called a dielectric constant, and ε ″ which is an imaginary component is called a dielectric loss coefficient. The greater the dielectric loss coefficient ε ″, the greater the energy lost when the sample is irradiated with microwaves. Correspondingly, the temperature rise of the sample due to absorption of microwave energy increases. Therefore, in order to control the temperature of the sample, it is necessary to measure the complex dielectric constant of the sample in advance. Furthermore, since the complex dielectric constant changes depending on the temperature of the sample, it is necessary to measure the complex dielectric constant over the entire temperature range in which heating for chemical reaction is performed.

  There are the following two methods for measuring the complex dielectric constant. One method is called Frequency Domain Reflectometry (hereinafter referred to as “FDR method”). A high frequency electric field is applied to a sample by a vector network analyzer, and the measured reflection coefficient and phase are complex. Obtain the dielectric constant. In this method, the frequency characteristic of the complex dielectric constant is measured by sweeping the frequency of the applied high-frequency electric field. Another method is called Time Domain Reflectometry (hereinafter referred to as “TDR method”), which measures the waveform of the reflected wave reflected by the sample by applying a pulsed electric field to the sample. The frequency characteristics of the complex dielectric constant are obtained by subjecting the obtained reflected wave to Fourier transform or other predetermined signal processing. In the former method, since the frequency sweep of the high-frequency electric field is performed, it takes time to measure, whereas in the TDR method, the frequency characteristic of the complex dielectric constant can be obtained in a short time.

  The measurement by the TDR method is performed, for example, by a measurement system as shown in the schematic configuration diagram of FIG. The measurement apparatus 11 is provided with a transmission / reception unit 12 that transmits a predetermined electrical signal to the sample via the measurement probe 13 and receives the electrical signal reflected by the sample. The measurement probe 13 is formed of a coaxial cable and is connected to the transmission / reception unit 12 by a connector 14 provided at one end thereof. The end of the measurement probe 13 opposite to the connector 14 (sample-side end) is a flat open end, and this end is in contact with the sample 15. The sample 15 is put in the container 16 and is arranged at a position where it contacts the sample side end. The sample 15 is heated by a heater provided in the container 16 and its temperature is controlled. The measuring device 11 is connected to a personal computer (PC) 17 through an interface such as GPIB, and control related to measurement is performed by the PC 17.

  At the time of measurement, the measurement device 11 transmits a pulse electric field from the transmission / reception unit 12. This pulse electric field propagates from the connector 14 connected to the transmission / reception unit 12 through the coaxial cable of the measurement probe 13, reaches the sample side end, and is reflected by the sample 15. The reflected electric field (reflected wave) returns through the coaxial cable of the measurement probe 13, is received by the transmission / reception unit 12, and is measured by the measurement device 11. The complex dielectric constant of the sample 15 is obtained by Fourier transforming this reflected wave.

As described in Non-Patent Documents 1 and 2, the complex dielectric constant of the sample is obtained as follows from the waveform of the reflected wave. Waveform signal R _x (t) of the reflected wave reflected by the sample to be measured and a reflected wave measured in advance using a standard sample (for example, air) having a known frequency spectrum ε _s ^* (ω) From the waveform signal R _s (t), the frequency spectrum ε _x ^* (ω) of the complex dielectric constant of the sample to be measured can be obtained using the following equation.

Where d is the length of the probe, γd is the electrical length (electric length) of the probe including the distance of the electric field extending from the sample side end of the probe into the sample, c is the speed of light in vacuum, and ω is It is the angular frequency of the microwave. V _x (ω) and V _s (ω) are Fourier transforms of R _x (t) and R _s (t), respectively. By solving Equation (1) for ε _x ^* (ω), the frequency spectrum of the complex dielectric constant of the sample to be measured can be obtained.

RH Cole et al., The Journal of Physical Chemistry, (USA), American Chemical Society, 1980, 84, 786-793 (RH Cole et al., "Evaluation of Dielectric Behavior by Time Domain Spectroscopy. 3. Precision Difference Methods ", The Journal of Physical Chemistry, (US), American Chemical Society, 1980, vol. 84, pp. 786-793) S. Mashimo et al., The Journal of Chemical Physics, (USA), American Institute of Physics, 1989, Vol. 90, pages 3292-3294 (S. Mashimo et al., "The dielectric relaxation of mixtures of water and primary alcohol ", The Journal of chemical physics, (US), American Institute of Physics, 1989, vol. 90, pp. 3292-3294)

  In a conventional measurement probe, a dielectric such as Teflon (registered trademark), Daiflon, or polyimide is filled between an outer conductor and an inner conductor. Therefore, when the temperature of the sample is higher than about 60 ° C., the electrical characteristics of the measurement probe change, and furthermore, the measurement probe may be deformed due to the difference in thermal expansion coefficient between the metal and the dielectric. . For this reason, the conventional measurement probe cannot accurately measure the complex dielectric constant at a high temperature.

  On the other hand, a coaxial cable type probe using quartz glass as a dielectric and a measurement probe that can be used even at high temperature is commercially available, but this is for the FDR method. Even if this measurement probe is used in the TDR method, the length is only a few centimeters, so the reflected wave is reflected at the connection between the probe and the transmitter / receiver and reflected back to the sample again. The reflected wave reaches the transmission / reception unit in a short time. Therefore, the multiple reflected wave overlaps a part of the waveform of the reflected wave having time spread, and measurement cannot be performed with sufficient accuracy. In particular, the lower the frequency region to be measured, the longer the reflected waveform in the time region is required, so the influence of multiple reflected waves increases. Even if a coaxial cable is inserted between the measuring probe and the transmitter / receiver of the measuring device to increase the pulse propagation path, reflection occurs at the connection between the measuring probe and the coaxial cable. Measurement cannot be performed with. Furthermore, if the high temperature measurement is performed for a long time, the temperature of the connected coaxial cable may increase and deform.

  As described above, there has been no measurement probe that can measure the complex dielectric constant using the TDR method in a temperature range of about 60 ° C. or higher.

  The problem to be solved by the present invention is to provide a measurement probe suitable for complex permittivity measurement by the time domain reflection method capable of measuring up to a high temperature region of about 250 ° C.

The complex dielectric constant measuring probe according to the present invention, which has been made to solve the above-mentioned problems, has one end connected to a pulse electric field transmitting / receiving device and the other end in contact with a sample so that the pulse electric field is between them. Is a probe for measuring complex permittivity by time domain reflection method,
a) a coaxial cable having a length of 0.3 m to 5 m, filled with an inorganic dielectric between the outer conductor and the inner conductor;
b) a connector connected to one end of the coaxial cable for connecting to the pulsed electric field transmitting / receiving device;
It is characterized by providing.

Embodiments and effects of the invention

  The complex dielectric constant measurement probe according to the present invention is mainly a coaxial cable in which an inorganic dielectric is filled between an outer conductor and an inner conductor. The connector of the measurement probe according to the present invention is connected to the pulse electric field transmission / reception device (or the pulse electric field transmission / reception unit of the complex dielectric constant measurement device), and the other end of the measurement probe is brought into contact with the sample. In this state, when a pulse electric field is transmitted from the pulse electric field transmitter / receiver (or pulse electric field transmitter / receiver) to the measurement probe, the pulse electric field propagates through the coaxial cable and reaches the sample, and is reflected at the interface with the sample. At that time, the waveform of the reflected wave changes according to the physical properties of the sample. The reflected wave propagates again through the coaxial cable and returns to the pulse electric field transmitting / receiving device (or pulse electric field transmitting / receiving unit), and the complex dielectric constant measuring device analyzes the waveform to calculate the complex dielectric constant of the sample.

  In order to avoid the influence of multiple reflected waves that are slightly generated when reflected waves are reflected at the connection between the measurement probe and the pulse electric field transmitting / receiving device (or the pulse electric field transmitting / receiving unit of the complex dielectric constant measuring device) and reflected again on the sample. In addition, the length of the measurement probe is set to a predetermined value or more. Thereby, it is possible to prevent the signal of the multiple reflected wave from reaching the transmitting / receiving unit in a short time and overlapping the waveform of the reflected wave, and to obtain sufficient measurement accuracy. In order to perform measurement without being affected by such multiple reflected wave signals in the microwave frequency range (300 MHz to 30 GHz) used to control the chemical reaction temperature, the length of the measurement probe is 30 cm. It is desirable to set it above. On the other hand, if the measurement probe is too long, the measurement accuracy (S / N ratio) decreases due to propagation loss in the coaxial cable. Therefore, it is desirable that the length is at most 5 m.

As the inorganic dielectric, a material that hardly deforms or changes in dielectric constant due to a temperature change within a temperature range in which the probe is used or that is small enough to be ignored in practical use is used. As such an inorganic dielectric, SiO ₂ or Al ₂ O ₃ can be suitably used. When these materials are used, measurement can be performed with a practically sufficient accuracy in a temperature range from room temperature to 250 ° C.

  It is desirable to use a powdery inorganic dielectric. When the powdery inorganic dielectric is used in this way, the probe has flexibility, so that the positional relationship between the dielectric constant measuring device body and the sample can be set relatively freely. Of course, even if a bulk inorganic dielectric is used, the effect of the present invention is not changed.

  It is desirable that the particle diameter of the powder be as fine as possible in order to prevent the dielectric distribution from becoming dense and dense inside the cable. When using a powdered inorganic dielectric, the inorganic dielectric at the end of the sample side is used to prevent the inorganic dielectric from leaking out of the probe or conversely entering the space filled with the inorganic dielectric during measurement. The exposed part of the body is sealed with a sealing material. As the sealing material, ceramic glass, solid of the same material as the powder dielectric, solid of other inorganic dielectric, or the like can be used. Or you may seal by melt-solidifying the inorganic dielectric powder of an edge part.

  When sealing is performed as described above, in the cable at the end of the cable provided with the sealing material and the cable at the position of the inorganic dielectric, the diameters of the inner conductor and the outer conductor are set to appropriate values, respectively. It is desirable to make the characteristic impedances equal. In this case, the diameters of the inner conductor and the outer conductor are different at the cable end and other positions, and therefore there is a step at a position where the diameter is changed.

  Furthermore, it is desirable to provide a region having a high characteristic impedance between the position of the cable end portion where the sealing material is provided and the position of the inorganic dielectric. By providing such a high impedance region, signal reflection can be prevented from occurring at the boundary between the region provided with the sealing material and the region filled with the inorganic dielectric. By shifting the position in the longitudinal direction of the cable where the step is provided between the step of the inner conductor and the step of the outer conductor, the region between these two steps can be made a high impedance region. The magnitude of the difference between the two is preferably about 1/8 of the diameter of the outer conductor which is thicker.

In the complex dielectric constant measurement probe of the present invention, since an inorganic dielectric is used between the inner conductor and the outer conductor, it can be used at a higher temperature than a conventional measurement probe using Teflon, Daiflon, polyimide, or the like. . For example, when SiO ₂ or Al ₂ O ₃ is used, it can be used in the temperature range from room temperature to 250 ° C without causing problems of changes in electrical characteristics and deformation due to heat. The complex dielectric constant of the sample at can be measured.

  The measurement probe according to the present invention is used for suitably performing the measurement by the TDR method, but can also be used for the measurement by the FDR method.

  An embodiment of the complex permittivity measuring probe according to the present invention is shown in FIG. (a) is an external view of this probe. An SMA male connector 22 that can be connected to a PC3.5 female connector provided in a transmission / reception unit of the measuring apparatus is provided at one end of a coaxial cable 21 having an internal structure as described below. Instead of the SMA male connector 22, other high frequency standard connectors such as a PC3.5 male connector may be used. The opposite end of the SMA male connector 22 is the sample side end. The length of the coaxial cable 21 is 1.06 m (3.5 feet) in this embodiment.

FIG. 2B shows a cross-sectional view including the central axis of the coaxial cable 21, in which the vicinity of the end portion on the sample side is enlarged. Over the entire length between the inner conductor 23 and the outer conductor 24, the SiO ₂ powder 26, which is an inorganic dielectric, is filled. In order to prevent the SiO ₂ powder 26 from leaking to the outside or the sample from entering the SiO ₂ powder 26 at the time of measurement, a seal 27 made of ceramic glass is provided at the portion of the SiO ₂ powder 26 at the sample side end. . The inner conductor 23 is made of Cu, and the outer conductor 24 is made of a Cu film deposited on the inner surface of a stainless steel (SUS) jacket 25.

In addition, a complex dielectric constant measurement probe having a sample side end portion as shown in FIG. In this probe, the diameters of the inner conductor 23 and the outer conductor 24 are set for each region so that the characteristic impedances of the region 31 provided with the seal and the region 33 filled with SiO ₂ powder (excluding the region 32 described later) are equal. Set. Therefore, steps 28 and 29 occur in the inner conductor 23 and the outer conductor 24, respectively. Further, by shifting the positions of the steps 28 and 29 in the cable longitudinal direction, a high impedance region 32 formed by the geometric shape is provided between the region 31 and the region 33. By providing the high impedance region 32 in this way, it is possible to prevent signal reflection between the region where the seal is provided and the region where the SiO ₂ powder is filled. The length of the high impedance region 32 in the cable longitudinal direction is set to about 1/8 of the diameter r of the outer conductor 24 on the region 33 side. A dielectric 30 is filled between the inner conductor 23 and the outer conductor 24 in the high impedance region 32. Since the impedance of the high impedance region 32 varies depending on the dielectric constant of the filled dielectric 30, the impedance of this region can be set by appropriately selecting the filled dielectric. The dielectric 30 may be made of SiO ₂ or the seal 27.

  As shown in FIG. 1, the measurement probe 20 of this embodiment is attached to the measurement apparatus 11 to constitute a measurement system. In this measurement system, the SMA male connector 22 of the measurement probe 20 is connected to the PC3.5 female connector provided in the transmission / reception unit 12 of the measurement apparatus 11. At the time of measurement, a pulse electric field is transmitted from the transmission / reception unit 12 of the measurement apparatus 11, and this pulse electric field is introduced into the measurement probe 20 via the PC3.5 female connector and the SMA male connector 22. The pulse electric field propagates through the coaxial cable 21 to reach the sample 15 and is reflected at the interface between the end face of the coaxial cable and the sample 15. The reflected wave propagates again through the coaxial cable 21 and is received by the transmission / reception unit 12. From the waveform of the reflected wave thus received, the complex dielectric constant of the sample 15 is obtained using the above equation (1).

  The results of measuring 1,5-pentanediol are shown below as an example of the measurement results. The measurement was performed in a temperature range from room temperature (21 ° C.) to 231 ° C. using a pulse electric field having a voltage of 200 mV and a rise time of 35 picoseconds. In addition, the result of measurement with respect to air under the same conditions as described above was used as data of the standard sample.

  The measurement results are shown in FIGS. FIG. 3 is a graph showing a signal obtained by subtracting the signal of the standard sample from the signal of the measurement target sample at each measurement temperature. The horizontal axis of this graph indicates the time from when the pulse electric field is transmitted from the measuring device 11 until it is reflected by the sample and received by the measuring device 11. In the figure, the temperature without brackets is the temperature of the sample to be measured, and the temperature with brackets is the measurement temperature of the standard sample. Since the relaxation time is long at room temperature, the value decrease with time elapses gradually, but as the temperature rises, the relaxation time becomes shorter and the value decrease with time elapses faster. Although not shown in the figure, at the temperature of 150 ° C. or higher, almost the same waveform was shown at any of the measured temperatures.

  FIG. 4 shows the results of obtaining the frequency spectra ε ′ (f) and ε ″ (f) of the dielectric constant and dielectric loss coefficient from the measurement results of FIG. 3 using the above equations (1) to (3). Here, f is a frequency (f = ω / 2π). Also, the horizontal axis of FIG. 4 is the logarithm of the frequency f. Among these, the data connected by the solid line is the dielectric loss coefficient ε ″ (f). If the temperature change of the dielectric loss coefficient ε '' (f) at the corresponding heating frequency (e.g. 2.45 GHz in the case of an aqueous solution) is determined in this way, depending on the sample that is the purpose of the experiment, depending on the temperature By controlling the intensity of the microwave, the temperature of the chemical reaction of the sample can be accurately controlled.

  In order to investigate the influence of changes in the electrical characteristics of the measurement probe and deformation due to heat, the signal reflected by the air is measured at room temperature, then the probe is heated to 200 ° C and then returned to room temperature and reflected again by air. Signal was measured. The difference between the signals obtained by these two measurements is shown in FIG. There is little difference between the two signals, except for a few peaks seen at the rising edge of the pulse. This means that there is almost no change or deformation of the electrical characteristics of the measurement probe due to heat before and after heating.

  On the other hand, when a similar experiment was performed using a conventional measurement probe using Teflon as a dielectric, the cable was deformed by heat, and measurement was not possible. In addition, the measurement probe using polyimide as a dielectric did not show any deformation, but the electrical characteristics of the tip of the cable changed due to heat, and the signal was distorted before and after the rise of the pulse. could not. As described above, according to the present invention, the complex permittivity can be measured up to a high temperature that could not be measured by the conventional probe.

The schematic block diagram of the measurement system used for the time domain difference method. The schematic diagram and sectional drawing which show one Example of the complex-dielectric-constant measuring probe which concerns on this invention. The graph which shows the difference of the reflected signal of 1, 5-pentanediol and the reflected signal of air measured using the complex-dielectric-constant measuring probe of this invention. FIG. 4 is a graph of frequency spectra ε ′ (f) and ε ″ (f) of dielectric constant and dielectric loss coefficient of 1,5-pentanediol determined from the measurement results of FIG. 3. The graph which shows the difference of the reflected signal of the air measured at room temperature before and after heating the complex-dielectric-constant measuring probe of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Measuring apparatus 12 ... Transmission / reception part 13, 20 ... Measurement probe 14 ... Connector 15 ... Sample 16 ... Container 17 ... PC
21 ... coaxial cable 22 ... SMA male connector 23 ... inner conductor 24 ... outer conductor 25 ... jacket 26 ... SiO ₂ powder 27 ... seal

Claims (5)

This probe connects one end to a pulsed electric field transmitter / receiver and contacts the other end to the sample to propagate the pulsed electric field between them, and measures the complex dielectric constant by the time domain reflection method. And
a) a coaxial cable having a length of 0.3 m to 5 m, filled with an inorganic dielectric between the outer conductor and the inner conductor;
b) a connector connected to one end of the coaxial cable for connecting to the pulsed electric field transmitting / receiving device;
A complex dielectric constant measuring probe comprising: The complex dielectric constant measuring probe according to claim 1, wherein the inorganic dielectric is made of SiO ₂ or Al ₂ O ₃ .   The complex dielectric constant measuring probe according to claim 1, wherein the inorganic dielectric is powdery.   The exposed surface of the inorganic dielectric at the end of the coaxial cable on the side in contact with the sample is sealed with a sealing material, and the diameter of the inner conductor and the outer conductor at the position of the cable end provided with the sealing material and the position of the inorganic dielectric is determined. 4. The complex permittivity measuring probe according to claim 1, wherein characteristic impedances at both positions are equal.   5. The complex dielectric constant measuring probe according to claim 4, wherein a region having a high characteristic impedance is provided between a position of an end of the cable provided with the sealing material and a position of the inorganic dielectric.

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