Space charge distribution measurement method

Abstract

Provided is a space charge distribution measurement method having high reproducibility of space charge distribution measurement.　In this space charge distribution measuring method, an object (1) being measured is disposed between a voltage application electrode (2) and a detection electrode (3), a pulsed voltage is applied to the voltage application electrode, an acoustic wave generated inside the object being measured is detected using a piezoelectric element (6) provided on the detection electrode, and the space charge distribution within the object being measured is measured on the basis of the waveform of the detected elastic wave, wherein a gap between the detecting electrode and the object being measured is filled with an electrically conductive fluid (5).

Description

Space charge distribution measurement method

[0001] The present disclosure relates to a space charge distribution measurement method.

[0002] The pulsed electro-acoustic method (hereinafter referred to as the PEA method) is known as a method for measuring the space charge distribution inside an insulator or dielectric. The principle of the PEA method is explained below. An object to be measured is placed between a voltage application electrode and a detection electrode, and a pulse voltage is applied to the voltage application electrode, causing the charges inside the object to vibrate due to Coulomb force, generating an elastic wave. This elastic wave is detected as an electrical signal by a piezoelectric element attached to the detection electrode. The waveform of the elastic wave is determined by the amount and distribution of charge inside the object, so the space charge distribution inside the object can be determined from the waveform of the elastic wave.

[0003] From a microscopic perspective, there is an air layer between the object to be measured and the detection electrode due to the unevenness of their respective surfaces. The acoustic impedance of this air layer is smaller than that of the object to be measured. As a result, some of the elastic waves generated inside the object to be measured are reflected by the air layer toward the object to be measured, reducing the proportion of elastic waves that reach the piezoelectric element installed in the detection electrode. This results in problems such as reduced measurement accuracy and sensitivity.

[0004] As a conventional PEA method that addresses this problem, a method of filling insulating grease between the object to be measured and the detection electrode has been disclosed (see, for example, Patent Document 1).

[0005] Japanese Patent Application Laid-Open No. 2001-4682

[0006] However, the conventional PEA method in which insulating grease is filled between the object to be measured and the detection electrode has a problem of low reproducibility in space charge distribution measurement.

[0007] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a method for measuring space charge distribution with high reproducibility in the PEA method.

[0008] The space charge distribution measuring method disclosed herein comprises placing an object to be measured between a voltage application electrode and a detection electrode, applying a pulse voltage to the voltage application electrode to detect elastic waves generated inside the object to be measured using a piezoelectric element provided in the detection electrode, and measuring the space charge distribution inside the object to be measured based on the waveform of the detected elastic waves, wherein a conductive fluid is filled between the detection electrode and the object to be measured.

[0009] In the space charge distribution measuring method of the present disclosure, the conductive fluid is filled between the detection electrode and the object to be measured, so that the reproducibility of the space charge distribution measurement can be improved.

[0010] FIG. 1 is a schematic diagram of a space charge distribution measuring device according to embodiment 1. FIG. 2 is a diagram for explaining a space charge distribution measuring method of comparative example 1 according to embodiment 1. FIG. 3 is a diagram for explaining a space charge distribution measuring method of comparative example 2 according to embodiment 1. FIG. 4 is a diagram for explaining an equivalent circuit in the space charge distribution measuring method of comparative example 2 according to embodiment 1. FIG. 5 is a diagram for explaining the space charge distribution measuring method according to embodiment 1. FIG. 6 is a diagram for explaining an equivalent circuit in the space charge distribution measuring method according to embodiment 1. FIG. 7 is a diagram for explaining a step of filling a gap between an object to be measured and a detection electrode with a conductive fluid according to embodiment 1. FIG. 8 is a diagram for explaining a step of filling a gap between an object to be measured and a detection electrode with a conductive fluid according to embodiment 1. FIG. 9 is a schematic diagram showing a main part of a space charge distribution measuring device according to embodiment 2. FIG. 10 is a schematic diagram showing a main part of a space charge distribution measuring device according to embodiment 2.

[0011] Hereinafter, a space charge distribution measuring device and a space charge distribution measuring method according to embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the same reference numerals in each drawing indicate the same or corresponding parts.

[0012] Embodiment 1. FIG. 1 is a schematic diagram of a space charge distribution measuring device according to Embodiment 1. The space charge distribution measuring device 100 of this embodiment is a device that measures the space charge distribution inside an object under test using the PEA method. In the space charge distribution measuring device 100 of this embodiment, a voltage application electrode 2 is arranged above the object under test 1, and a detection electrode 3 is arranged below the object under test 1. The potential of the detection electrode 3 is set to ground potential. The object under test 1 is a solid insulator or dielectric material, such as a resin, a composite material containing glass fiber, or ceramic. A semiconductive elastic body 4 is provided between the object under test 1 and the voltage application electrode 2. A conductive fluid 5 is filled between the object under test 1 and the detection electrode 3. A piezoelectric element 6 is provided on the surface of the detection electrode 3 opposite the object under test 1. A piezoelectric element electrode 7 is provided on the surface of the piezoelectric element 6 opposite the detection electrode 3.

[0013] A DC power supply 11 and a pulse power supply 12 are connected to the voltage application electrode 2. A constant bias voltage is applied to the voltage application electrode 2 from the DC power supply 11, and a pulse voltage is applied from the pulse power supply 12 superimposed on the bias voltage.

[0014] When a pulse voltage is applied to the voltage application electrode 2, electric charges present from the surface to the interior of the object to be measured 1 vibrate, generating elastic waves. These elastic waves are typically ultrasonic waves. The ultrasonic waves generated inside the object to be measured 1 propagate through the conductive fluid 5 to the detection electrode 3 and reach the piezoelectric element 6 attached to the detection electrode 3. A voltage corresponding to the ultrasonic waves is induced in the piezoelectric element 6, generating a potential difference between the detection electrode 3 and the piezoelectric element electrode 7. An amplifier 13 is connected to the piezoelectric element electrode 7, which amplifies the potential difference between the detection electrode 3 and the piezoelectric element electrode 7 and outputs it as an electrical signal. The electrical signal output from the amplifier 13 is recorded on an oscilloscope 14 synchronized with a trigger signal output from the pulse power supply 12 simultaneously with the pulse voltage, and the waveform of the electrical signal is transmitted from the oscilloscope 14 to a computer 15. The computer 15 calculates the space charge distribution inside the object to be measured 1 from the waveform of the transmitted electrical signal.

[0015] In the space charge distribution measuring method according to the present embodiment, the space between the object to be measured 1 and the detection electrode 3 is filled with the conductive fluid 5, which increases the reproducibility of the space charge distribution measurement. The reason for this will be explained below.

[0016] FIG. 2 is a diagram illustrating a space charge distribution measurement method according to Comparative Example 1 of the present embodiment. FIG. 2 is an enlarged schematic diagram of the interface between the object to be measured 1 and the detection electrode 3 in Comparative Example 1, in which nothing is filled between the object to be measured 1 and the detection electrode 3. As shown in FIG. 2 , when viewed microscopically, a gap is formed between the object to be measured 1 and the detection electrode 3 due to the unevenness of their surfaces, forming an air layer 8. The acoustic impedance of this air layer 8 is smaller than the acoustic impedance of the object to be measured 1. Therefore, the ratio of the acoustic impedance between the surface 1 a of the object to be measured 1 and the air layer 8 becomes large. As a result, most of the ultrasonic waves generated inside the object to be measured 1 are reflected at the interface between the surface 1 a of the object to be measured 1 and the air layer 8 and do not reach the detection electrode 3. This results in problems such as reduced measurement accuracy and sensitivity. To avoid this problem, it is conceivable to fill the space between the object to be measured 1 and the detection electrode 3 with a substance having an acoustic impedance close to that of the object to be measured 1.

[0017] FIG. 3 is a diagram illustrating a space charge distribution measurement method of Comparative Example 2 according to the present embodiment. FIG. 3 is an enlarged schematic diagram of the interface between the object to be measured 1 and the detection electrode 3 in Comparative Example 2, in which the gap between the object to be measured 1 and the detection electrode 3 is filled with insulating grease 9. Examples of the insulating grease include silicone grease. Filling the gap between the object to be measured 1 and the detection electrode 3 with insulating grease 9 reduces the ratio of acoustic impedance between the surface 1 a of the object to be measured 1 and the insulating grease 9, allowing most of the ultrasonic waves generated inside the object to be measured 1 to reach the detection electrode 3. As a result, the space charge distribution measurement method of Comparative Example 2 has improved measurement accuracy compared to the space charge distribution measurement method of Comparative Example 1.

[0018] However, it was found that the reproducibility of measurements was low in the space charge distribution measurement method of Comparative Example 2. After further investigation into the cause of this, we found that the cause was that voltage was applied to the electrical two-layer structure of the insulating object to be measured and the insulating grease.

[0019] 4 is a diagram showing an equivalent circuit in the space charge distribution measurement method of Comparative Example 2. As shown in FIG. 4, the components of equivalent circuit 20 in the space charge distribution measurement method of Comparative Example 2 are power supply 21, which shows DC power supply 11 and pulse power supply 12 as a single power supply, capacitance component 22 and resistance component 23 of the object under test, and capacitance component 24 and resistance component 25 of the insulating grease. In this equivalent circuit 20, attention is focused on bottom surface potential 26 of the object under test, which is the interface between the object under test and the grease and corresponds to the potential indicated by the arrow. When the voltage of power supply 21 is increased to the bias voltage in the PEA method, bottom surface potential 26 of the object under test is determined by the balance between capacitance component 22 of the object under test and capacitance component 24 of the grease, and is an intermediate voltage between the bias voltage and ground voltage.

[0020] After a certain bias voltage is reached, charges move inside the grease, which has a lower electrical resistance than the object under test. At this time, the behavior in equivalent circuit 20 is that the charges accumulated in capacitance component 24 of the grease flow to the ground side via resistance component 25 of the grease, and as a result, bottom surface potential 26 of the object under test drops to ground potential. When bottom surface potential 26 of the object under test becomes ground potential, it means that the entire voltage of power supply 21 is applied to the object under test.

[0021] The value of the resistance component 25 of the grease plays an important role in such a change in the bottom surface potential 26 of the object under test. When the value of the resistance component 25 of the grease changes, the time required for the bottom surface potential 26 of the object under test to change also changes. In reality, the resistance component of the grease changes depending on the method of applying the grease when installing the object under test and the method of managing the grease. This is the reason why the space charge distribution measurement method of Comparative Example 2 has low measurement reproducibility.

[0022] FIG. 5 is a diagram for explaining the space charge distribution measurement method of this embodiment. FIG. 5 is an enlarged schematic diagram of the interface between the object to be measured 1 and the detection electrode 3 in this embodiment, in which the gap between the object to be measured 1 and the detection electrode 3 is filled with a conductive fluid 5. FIG. 6 is a diagram showing an equivalent circuit in the space charge distribution measurement method of this embodiment. In the space charge distribution measurement method of this embodiment, the conductive fluid can be considered a conductor. Therefore, as shown in FIG. 6, the components of an equivalent circuit 20 in the space charge distribution measurement method of this embodiment are only a power source 21, in which the DC power source 11 and the pulse power source 12 are represented as a single power source, and a capacitance component 22 and a resistance component 23 of the object to be measured.

[0023] 6, in the space charge distribution measuring method of this embodiment, potential 26 of the bottom surface of the object to be measured always coincides with the ground potential, and there is no time change in potential 26 of the bottom surface of the object to be measured. As a result, the space charge distribution measuring method of this embodiment can improve the reproducibility of the space charge distribution measurement.

[0024] Next, limitations on the conductivity of the conductive fluid in the space charge distribution measurement method of this embodiment will be described. The conductivity of the conductive fluid is closely related to the measurement conditions, i.e., the time interval at which the pulse voltage is applied from the start of bias voltage application to acquire space charge distribution data. In the PEA method, the waveform of the ultrasonic wave obtained by one pulse voltage contains many noise components, so it is necessary to remove the noise by averaging the waveforms of the ultrasonic waves obtained by multiple pulse voltages. It is desirable to determine the conductivity of the conductive fluid to be used depending on such measurement conditions.

[0025] In the PEA method, we focus on the temporal change in the space charge distribution after applying a bias voltage to the object under test. At the time of bias voltage application, no space charge exists within the object under test; charge exists only at the interface between the object under test and the voltage application electrode and at the interface between the object under test and the detection electrode. The space charge distribution acquired in this state is called the initial distribution. When the space charge distribution is acquired again after a certain time has passed from this initial distribution, the behavior of the space charge distribution can be determined by the change from the initial distribution. The time when the bias voltage is fully increased to a certain voltage is defined as t = 0, and the time required to obtain the initial distribution, which is defined as the product of the pulse voltage period (reciprocal of the pulse voltage frequency) and the number of averaging processes required to acquire the ultrasonic waveform, is defined as t1. To accurately measure the space charge distribution using the PEA method, the charge transfer within the conductive fluid must be complete at t = t1, and the voltage applied to the conductive fluid must be zero. This condition can be expressed as τ<t1, using the charge relaxation time τ, which is the product of the volume resistivity and dielectric constant of the conductive fluid. More specifically, it is preferable to satisfy the following formula (1): τ<t1 / 10 (1)

[0026] The reproducibility of space charge distribution measurements can be improved by using a conductive fluid having a charge relaxation time τ that satisfies this equation (1). Next, specific values ​​under typical conditions are shown as examples.

[0027] Assume that the frequency of the pulse voltage applied from the pulsed power supply is 10 Hz and that averaging of 100 pulses is required to obtain the space charge distribution. In this case, the time required to obtain one signal is 10 seconds. That is, the first signal is obtained 10 seconds after the bias voltage application is completed, so t1 = 10 seconds. According to equation (1), τ < 1 second. Examples of conductive fluids that satisfy τ < 1 second include aqueous solutions containing dissolved conductive ions, organic solvents containing dissolved conductive ions, and ionic molten salts. Specifically, examples of conductive fluids that can be used include aqueous solutions containing 0.1 wt% sodium chloride and conductive pastes containing conductive fillers such as silver powder dispersed in an acrylic resin binder. Note that a charge relaxation time τ of a conductive fluid less than 1 second is merely an example. A conductive fluid with an appropriate charge relaxation time τ must be selected based on the actual measurement conditions. The conductive fluid materials exemplified above are merely examples and are not intended to be limiting.

[0028] The selection of a specific material for the conductive fluid may be limited not only by the charge relaxation time but also by the conditions under which the measurement is performed. For example, when measuring the space charge distribution in a high-temperature environment of 100° C., an ion-molten salt or a conductive paste having a boiling point of 100° C. or higher may be selected as the conductive fluid.

[0029] Using a conductive fluid with thixotropy, such as a conductive paste, makes it easier to fill the gap between the object to be measured and the detection electrode. A thixotropic material is a material that has high viscosity when stationary but decreases in viscosity when subjected to shear stress. Figures 7 and 8 are diagrams illustrating the process of filling the gap between the object to be measured and the detection electrode in this embodiment. First, as shown in Figure 7, a dispenser 16 is used to apply a thixotropic conductive fluid 5 to the center of the surface of the detection electrode 3. At this point, the thixotropic conductive fluid 5 does not flow to the outer periphery of the detection electrode. Next, as shown in Figure 8, the object to be measured 1 is pressed against the detection electrode 3. At this time, the viscosity of the thixotropic conductive fluid 5 decreases due to the pressure applied to the object to be measured 1, and the conductive fluid 5 spreads between the object to be measured 1 and the detection electrode 3. In this manner, the conductive fluid 5 can be filled into the gap between the object to be measured 1 and the detection electrode 3.

[0030] Embodiment 2 Fig. 9 is a schematic diagram showing the main part of a space charge distribution measuring apparatus according to embodiment 2. Fig. 9 shows only the vicinity of the object to be measured in the space charge distribution measuring apparatus of this embodiment.

[0031] In the space charge distribution measurement device shown in FIG. 1 of the first embodiment, the area of ​​the detection electrode facing the object to be measured is larger than the area of ​​the bottom surface of the object to be measured. In a space charge distribution measurement device arranged in this manner, when a conductive fluid greater than the required amount is filled between the object to be measured and the detection electrode, a portion of the conductive fluid may spill out from the bottom surface of the object to be measured. The conductive fluid spilling out from the bottom surface of the object to be measured may creep up from the side surface of the object to the upper side and contaminate the top surface of the object to be measured. In the PEA method, the top surface of the object to be measured functions as an insulating part when a voltage is applied, so if the top surface of the object to be measured is contaminated by the conductive fluid, accurate measurement may not be possible.

[0032] 9, in the space charge distribution measuring device of this embodiment, the area of ​​the detection electrode 3 in contact with the object to be measured 1 is made smaller than the area of ​​the bottom surface of the object to be measured 1. Specifically, a protruding sample mounting portion 3a is formed on the detection electrode 3, the object to be measured 1 is placed on the top surface of this sample mounting portion 3a, and a conductive fluid 5 is filled between the top surface of the sample mounting portion 3a and the bottom surface of the object to be measured 1. In other words, the area of ​​the top surface of the sample mounting portion 3a is made smaller than the area of ​​the bottom surface of the object to be measured 1, and a space is formed between the outer periphery of the object to be measured 1 and the detection electrode 3.

[0033] In the space charge distribution measuring device configured as above, even if the amount of conductive fluid 5 filled between the object to be measured 1 and the sample mounting portion 3a of the detection electrode 3 is greater than the required amount and some of the conductive fluid 5 spills over the upper surface of the sample mounting portion 3a, the conductive fluid 5 will not flow downward from the periphery of the sample mounting portion 3a and contaminate the upper surface of the object to be measured 1. As a result, in the space charge distribution measuring device of this embodiment, the upper surface of the object to be measured will not be contaminated by the conductive fluid, and correct measurement is possible.

[0034] 9, a wall 3b is provided on the outer periphery of the detection electrode 3, protruding toward the object to be measured 1. This wall 3b can prevent the conductive fluid 5 that has flowed downward from the periphery of the sample placement portion 3a from flowing out of the detection electrode 3. As a result, workability is improved.

[0035] Fig. 10 is a schematic diagram showing the main part of another space charge distribution measurement apparatus according to this embodiment. Fig. 10 shows only the vicinity of the object to be measured in this space charge distribution measurement apparatus. In this space charge distribution measurement apparatus, as shown in Fig. 10, a side wall 3c on the outer periphery of a sample mounting portion 3a formed in a protruding shape on a detection electrode 3 is perpendicular to the underside of the object to be measured 1.

[0036] In the space charge distribution measurement apparatus configured as above, the distance between the surface of the detection electrode 3 other than the sample mounting portion 3 a facing the DUT 1 with the conductive fluid 5 interposed therebetween and the underside of the DUT 1 is larger than that in the space charge distribution measurement apparatus shown in FIG. 9 , so that a higher pulse voltage can be applied.

[0037] Although various exemplary embodiments are described in this disclosure, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless variations not illustrated are contemplated within the scope of the technology disclosed in this specification. For example, this includes cases where at least one component is modified, added, or omitted, or where at least one component is extracted and combined with components of another embodiment.

[0038] 1 Object to be measured, 2 Voltage application electrode, 3 Detection electrode, 3a Sample placement portion, 3b Wall portion, 3c Side wall, 4 Elastic body, 5 Conductive fluid, 6 Piezoelectric element, 7 Piezoelectric element electrode, 8 Air layer, 9 Grease, 11 DC power supply, 12 Pulse power supply, 13 Amplifier, 14 Oscilloscope, 15 Computer, 16 Dispenser, 20 Equivalent circuit, 21 Power supply, 22, 24 Capacitive component, 23, 25 Resistive component, 26 Bottom surface potential, 100 Space charge distribution measurement device.

Claims

1. In a method for measuring space charge distribution, in which an object to be measured is placed between a voltage application electrode and a detection electrode, a pulse voltage is applied to the voltage application electrode to detect elastic waves generated inside the object to be measured using a piezoelectric element provided on the detection electrode, and the space charge distribution inside the object to be measured is measured based on the waveform of the detected elastic waves, A method for measuring spatial charge distribution, characterized in that a conductive fluid is filled between the detection electrode and the object to be measured.

2. The method for measuring a space charge distribution according to claim 1, characterized in that the charge relaxation time, which is expressed as the product of the dielectric constant and the volume resistivity of the conductive fluid, is smaller than the time required to obtain an initial distribution determined by the product of the period of the pulse voltage and the number of averaging processes required to acquire the waveform of the elastic wave.

3. The method for measuring a spatial charge distribution according to claim 2, characterized in that the charge relaxation time is less than one-tenth of the time it takes to obtain the initial distribution.

4. The method for measuring space charge distribution according to any one of claims 1 to 3, characterized in that the conductive fluid is an ionic molten salt.

5. The method for measuring the spatial charge distribution according to any one of claims 1 to 3, characterized in that the conductive fluid is thixotropic.

6. The method for measuring space charge distribution according to any one of claims 1 to 3, characterized in that the detection electrode has a protruding sample placement portion on which the object to be measured is placed, the conductive fluid is filled between the upper surface of the sample placement portion and the lower surface of the object to be measured, the area of ​​the upper surface of the sample placement portion is smaller than the area of ​​the lower surface of the object to be measured, and there is a space between the outer circumference of the object to be measured and the detection electrode.

7. The method for measuring spatial charge distribution according to claim 6, characterized in that the side wall of the outer periphery of the sample placement portion is perpendicular to the lower surface of the object to be measured.