Method for manufacturing conductivity and temperature integrated sensor based on LTCC (Low Temperature Co-Fired Ceramic) process

Abstract

The invention discloses a conductivity and temperature integrated sensor manufacturing method based on an LTCC technology, and belongs to the field of sensor manufacturing. The method comprises the following steps: cutting a raw ceramic tape into raw ceramic chips; a line through hole is formed in the green ceramic chip; three pairs of conductivity electrodes are printed on the upper-layer green ceramic chip in a thick film mode, and line through holes in the upper-layer green ceramic chip are metalized; printing a metal signal lead on the intermediate layer green ceramic chip; printing a platinum resistor for temperature measurement on the lower-layer green ceramic chip; the upper-layer green ceramic chip, the middle-layer green ceramic chip and the lower-layer green ceramic chip are sequentially wound on the rubber tube and are laminated to form a sensor structural body; putting the laminated sensor structural body into a high-temperature furnace, and carrying out sintering molding according to a sintering curve; and welding a lead wire, connecting the lead wire with the switching shell, and performing sealing treatment to form the conductivity and temperature integrated sensor. According to the invention, the previous sheet-shaped lamination method is changed, the conductivity and temperature integrated sensor based on the LTCC cylindrical structure is designed, the manufacturing process is simple, the measurement is convenient, and the accuracy is better.

Description

Technical Field

[0001] The invention relates to a method for manufacturing a conductivity and temperature integrated sensor based on an LTCC process, and belongs to the field of sensor manufacturing. Background Art

[0002] A sensor is a detection device that can sense the information being measured and convert the sensed information into electrical signals or other required forms of information output according to certain rules.

[0003] Liquid type identification, water quality parameter detection, and liquid leakage detection are of great engineering significance in industrial production and environmental monitoring. Typically, in industrial control and environmental monitoring, it is necessary to measure the conductivity and temperature parameters of liquids. Currently, similar sensors are mostly single-parameter detection. When multi-parameter measurement is required, multiple sensors with different functions are generally combined to achieve multi-parameter detection. This setup not only makes measurement cumbersome and increases manufacturing costs, but also easily affects the measurement results due to the inconsistent positioning of each sensor. With the continuous development of solid-state functional materials and integration technology, multi-parameter integrated sensors have become an inevitable development trend.

[0004] The LTCC process is a novel material technology developed by Hughes in 1982. It involves sintering ceramic powder at low temperatures into a dense, precisely defined green tape. The desired circuit pattern is then formed on the tape using processes such as laser drilling, micropore grouting, and precision conductive paste printing. Multiple passive components, such as low-capacitance capacitors, resistors, filters, impedance converters, and couplers, are then embedded in a multilayer ceramic substrate and laminated together. Internal and external electrodes can be made of metals such as silver, copper, and gold. The process is sintered at 900°C, creating a high-density, three-dimensional circuit that does not interfere with each other. In addition to its use in mobile phones, LTCC, with its excellent electronic, mechanical, and thermal properties, has become the preferred method for future electronic component integration and modularization. It has also found widespread application in military, aerospace, automotive, computer, and medical fields. Regarding conductivity sensors fabricated using the LTCC process, the existing invention patent, "A Method for Preparing a Seven-Electrode Conductivity Sensor," with publication number CN113495191B, discloses a method for fabricating a seven-electrode conductivity sensor using both HTCC and LTCC ceramic green tape. Patent number CN202975172U, "Co-fired Conductivity Sensor," discloses a four-electrode conductivity sensor fabricated using the HTCC process. While these two inventions are effective at detecting the conductivity of a single liquid, they can exhibit significant errors in the conductivity of mixed liquids, particularly those with stratified layers. Currently, no reports have been found of an integrated sensor fabricated using the LTCC process that combines conductivity and temperature detection with liquid level rise rate detection.

[0005] Therefore, it is urgent to propose a method for manufacturing a conductivity and temperature integrated sensor based on LTCC process to solve the above technical problems. Summary of the Invention

[0006] The present invention aims to propose a method for fabricating an integrated conductivity and temperature sensor based on LTCC to address the issues of single signal response and low integration of existing sensors. This method also provides a new approach to the design and fabrication of integrated conductivity and temperature sensors. The following is a brief overview of the present invention to provide a basic understanding of certain aspects of the present invention. It should be understood that this overview is not an exhaustive overview of the present invention. It is not intended to identify key or important aspects of the present invention, nor is it intended to limit the scope of the present invention.

[0007] The technical solution of the present invention:

[0008] A method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology, comprising:

[0009] S1, cutting the green porcelain strip into green porcelain sheets, wherein the green porcelain sheets are grouped into three layers, namely, an upper layer green porcelain sheet, a middle layer green porcelain sheet, and a lower layer green porcelain sheet;

[0010] S2, opening wire through holes on the upper layer green porcelain sheet, the middle layer green porcelain sheet and the lower layer green porcelain sheet, and making the wire through holes by punching process;

[0011] S3, thick-film printing three pairs of conductive electrodes on the upper green ceramic sheet as the upper layer, and metallizing the through-holes on the upper green ceramic sheet;

[0012] S4, printing metal signal leads on the intermediate layer green ceramic sheet as an intermediate layer;

[0013] S5, printing a platinum resistance strip on the lower green ceramic sheet as the lower layer to form a platinum resistance for temperature measurement;

[0014] S6, wrapping the upper green ceramic sheet, the middle green ceramic sheet, and the lower green ceramic sheet on the rubber tube in sequence, and laminating them by isostatic pressing to form a sensor structure;

[0015] S7, placing the laminated sensor structure into a high-temperature furnace and sintering it according to the sintering curve;

[0016] S8, welding the leads, connecting them to the adapter housing, and sealing them to form an integrated conductivity and temperature sensor.

[0017] Preferably, the number of the green porcelain sheets is an odd number of at least 3.

[0018] Preferably, the adapter shell is provided with an air hole communicating with the interior thereof, and the adapter shell is provided with a pin header.

[0019] Preferably, the sintering temperature in the high-temperature furnace ranges from 1050°C to 1100°C.

[0020] Preferably, the number of the conductivity electrodes is at least 2 pairs.

[0021] Preferably, the platinum resistance strips are arranged in a shape of a bow or a ring.

[0022] Preferably, the conductive electrode is made of platinum, graphite or gold, and the platinum resistor is made of platinum.

[0023] Preferably, the sintered sensor structure is located in the adapter housing, wherein the upper green ceramic sheet is located in the innermost layer.

[0024] The present invention has the following beneficial effects:

[0025] 1. This invention proposes, for the first time, the concept of designing a cylindrical LTCC substrate and integrating conductivity and temperature sensors, enabling online measurement of liquid temperature and conductivity. Using an isostatic roll-to-roll process, a rubber tube mold allows for precise molding of special-shaped structures, improving interlayer bonding density. Furthermore, given that liquids of the same concentration at different temperatures have different conductivities, temperature information measured by a thermal resistor is used to compensate for conductivity measurements, improving conductivity detection accuracy.

[0026] 2. The present invention only allows the conductivity electrode on the inner wall of the upper raw porcelain sheet to contact the liquid. The outer wall is completely insulated and sealed, and the thermal resistor does not directly contact the liquid, which greatly improves the stability of the sensor.

[0027] 3. The integrated conductivity and temperature sensor of this invention is equivalent to being integrated into a thin plate. Through the stacking design of multiple layers of raw ceramic sheets and functional partitioning, it achieves three-dimensional integration of conductivity detection and temperature measurement. The temperature sensor can accurately measure the temperature of the liquid at the conductivity sensor and effectively perform temperature compensation for the conductivity measurement.

[0028] 4. The different-layer layout of the platinum resistance temperature measuring element and the conductivity electrode of the present invention effectively avoids signal crosstalk, the sensor structure has a high degree of integration, and the volume is reduced. It is particularly suitable for space-constrained scenarios and is suitable for widespread use. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic structural diagram of the upper layer green porcelain sheet, the middle layer green porcelain sheet and the lower layer green porcelain sheet of the present invention;

[0030] Figure 2 This is a schematic diagram of the upper green ceramic sheet of the present invention in use when detecting a rising liquid level;

[0031] Figure 3 is a perspective view of the adapter housing of the present invention;

[0032] Figure 4 It is a structural schematic diagram of the adapter housing of the present invention;

[0033] Figure 5 This is a schematic diagram of the adapter housing of the present invention in use;

[0034] Figure 6 This is a schematic diagram of the state of the conductivity electrode of the present invention when detecting liquid immersion.

[0035] In the figure: 1-upper layer raw ceramic sheet, 2-middle layer raw ceramic sheet, 3-lower layer raw ceramic sheet, 4-conductivity electrode, 5-wire through hole, 6-metal signal lead, 7-platinum resistor, 8-adapter housing, 9-air hole, 10-pin header. DETAILED DESCRIPTION

[0036] To make the objectives, technical solutions, and advantages of the present invention more clearly apparent, the present invention is described below using specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely illustrative and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present invention.

[0037] The connections mentioned in the present invention are divided into fixed connections and detachable connections. The fixed connection refers to a non-detachable connection, including but not limited to conventional fixed connection methods such as hem connection, rivet connection, adhesive connection, and welding connection. The detachable connection refers to but not limited to conventional detachable connection methods such as threaded connection, snap connection, pin connection, and hinge connection. When the specific connection method is not clearly specified, it is assumed that at least one connection method can always be found among the existing connection methods to achieve the function. Those skilled in the art can choose according to their needs. For example, a welded connection is selected for a fixed connection, and a hinge connection is selected for a detachable connection.

[0038] Specific implementation method 1: Combination Figures 1-6 This embodiment describes a method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology, including:

[0039] S1, cutting the green porcelain tape into green porcelain sheets, wherein the green porcelain sheets are grouped into three layers, namely, an upper layer green porcelain sheet 1, a middle layer green porcelain sheet 2, and a lower layer green porcelain sheet 3;

[0040] S2, opening wire through holes on the upper green porcelain sheet 1, the middle green porcelain sheet 2 and the lower green porcelain sheet 3, and using a punching process to make wire through holes 5;

[0041] S3, thick-film printing three pairs of conductive electrodes 4 on the upper green ceramic sheet 1 as the upper layer, and metallizing the through-holes 5 on the upper green ceramic sheet 1;

[0042] S4, printing metal signal leads 6 on the intermediate layer green ceramic sheet 2 as an intermediate layer;

[0043] S5, printing a platinum resistance strip on the lower green ceramic sheet 3 as the lower layer to form a platinum resistor 7 for temperature measurement;

[0044] S6, wrapping the upper green ceramic sheet 1, the middle green ceramic sheet 2, and the lower green ceramic sheet 3 on the rubber tube in sequence, and laminating them by isostatic pressing to form a sensor structure;

[0045] S7, placing the laminated sensor structure into a high-temperature furnace and sintering it according to the sintering curve;

[0046] S8, welding the leads, connecting them to the adapter housing 8, and sealing them to form an integrated conductivity and temperature sensor.

[0047] This embodiment integrates conductivity and temperature detection sensors, monitors the temperature field of the liquid in which the conductivity sensor is located in real time by measuring the resistance value of the thermal resistor, and makes the conductivity detection result more accurate and reliable through temperature compensation method.

[0048] The number of the green porcelain sheets is at least an odd number of 3.

[0049] The adapter housing 8 is provided with air holes 9 connected to its interior. There are four air holes 9 in number and they are arranged symmetrically to ensure that the air pressure inside and outside is consistent, which is convenient for liquid immersion and ensures that the gas in the sensor structure can be effectively discharged as the liquid is immersed. The adapter housing 8 is provided with a pin header 10.

[0050] The sintering temperature in the high-temperature furnace ranges from 1050°C to 1100°C.

[0051] The number of the conductive electrodes 4 is at least two pairs, and they are located on the inner wall of the sensor structure. The conductive electrodes 4 are placed parallel to one side of the upper green ceramic sheet 1 .

[0052] Preferably, this embodiment uses three pairs of metal conductive electrodes 4 for design to form a sensitive element for detecting liquid conductivity, and can realize the change of liquid conductivity at the three pairs of metal conductive electrodes 4.

[0053] The three pairs of conductivity electrodes 4 can calculate the liquid level rising rate based on the time difference between each pair of electrodes detecting the liquid and the distance between each pair of electrodes, that is, the integrated sensor has the function of detecting the liquid immersion speed.

[0054] The platinum resistance strip is located between the upper green porcelain sheet 1 and the lower green porcelain sheet 3 to ensure that it does not come into direct contact with the liquid to be measured after isostatic pressing. The shape and size of the thermal resistor can be arranged in a "bow shape" or "circular ring" shape as needed. The principle of impedance change between the conductivity electrodes 4 caused by immersion in the liquid on the inner wall of the cylindrical sensor is used to detect the liquid conductivity.

[0055] The material of the conductivity electrode 4 is platinum, graphite or gold with strong corrosion resistance, and the material of the platinum resistor 7 is platinum material, which can accurately measure the liquid temperature at the location of the sensor without direct contact with the liquid to be measured, avoiding corrosion and improving the stability of the sensor.

[0056] The sintered sensor structure is located within an adapter housing 8, with the upper green ceramic sheet 1 located in the innermost layer. This effectively integrates the platinum resistor 7 and the conductivity electrode 4 into a single sensor structure. The platinum resistor 7 can accurately measure the temperature of the liquid being detected and perform temperature compensation on the conductivity sensor's measurement results. The sensor can be directly connected to the corresponding signal detection circuit through the adapter housing 8, making it simple to use. To increase the sensitivity of the sensor, the resistance of the platinum resistor 7 can be as large as possible, i.e., printed more densely.

[0057] Specific implementation method 2: Combination Figures 1-6 This embodiment is described based on the first specific embodiment. This embodiment is a method for manufacturing an integrated conductivity and temperature sensor based on the LTCC process. The conductivity electrode 4 is connected to the middle layer green ceramic sheet 2 through the wire through hole 5. Similarly, the lower layer green ceramic sheet 3 is also connected to the outer surface of the middle layer green ceramic sheet 2 through the wire through hole 5 provided in this layer, and then connected to the designed adapter housing 8. The conductivity electrode 4 and the platinum resistor 7 are led out of the lower layer green ceramic sheet 3 through the wire through hole 5 provided in this layer to facilitate connection with the wire.

[0058] The upper green ceramic sheet 1 of this embodiment is formed with three pairs of six conductive electrodes 4, which can be excited by an AC constant voltage source or an AC constant current source, effectively reducing the influence of electrode polarization.

[0059] The liquid level rising rate is calculated based on the time difference between adjacent detections of the conductivity electrodes 4 to the liquid and the distance between adjacent conductivity electrodes 4 .

[0060] Specifically, the three pairs of conductivity electrodes 4 of this embodiment calculate the liquid level rising rate based on the time difference between each pair of electrodes detecting the liquid and the distance between each pair of electrodes, so that it has the function of detecting the liquid immersion speed.

[0061] Specifically, if Figure 6As shown, the method for detecting the liquid immersion speed, i.e., the method for detecting the rising and falling rate of the leaked liquid, calculates the rising rate v of the leaked liquid based on the time difference Δt between the detection signal of the previous conductivity electrode 4 and the detection signal of the next conductivity electrode 4, as well as the distance h between each pair of adjacent conductivity electrodes 4. The calculation formula is:

[0062] v = h / Δt

[0063] In this embodiment, the pads and wire through holes 5 are designed on one side of the green ceramic sheet to facilitate packaging. In this embodiment, resin glue is poured into the sensor structure from the other end of the connecting adapter shell 8, covering the pads. The cross-sectional view of the packaged sensor structure is as follows: Figure 4 shown.

[0064] The green ceramic sheet is made of glass-ceramic composite material through a tape casting process.

[0065] Specific implementation method three: Combination Figures 1-6 This embodiment is described based on the second embodiment. This embodiment is a method for manufacturing an integrated conductivity and temperature sensor based on the LTCC process, including:

[0066] S1, select a 0.06-0.1mm thick glass-ceramic composite green porcelain tape, cut it into green porcelain sheets through a slicing process, the size of the green porcelain sheets is 50mm×70mm, and the green porcelain sheets are grouped into three layers, namely upper green porcelain sheet 1, middle green porcelain sheet 2, and lower green porcelain sheet 3;

[0067] S2, opening wire through holes on the upper green porcelain sheet 1, the middle green porcelain sheet 2, and the lower green porcelain sheet 3, using a punching process to make wire through holes 5, with a hole diameter of 0.07mm to 0.15mm. Specifically, 6 wire through holes 5 are processed on the upper green porcelain sheet 1; 16 wire through holes 5 are processed on the middle green porcelain sheet 2; and 8 wire through holes 5 are processed on the lower green porcelain sheet 3;

[0068] S3, using a screen printing process to thick-film print three pairs of conductive electrodes 4 on the upper green ceramic sheet 1 as the upper layer using a platinum electrode slurry, and metallize the wire-through holes 5 on the upper green ceramic sheet 1;

[0069] S4, printing metal signal leads 6 on the intermediate layer green ceramic sheet 2 as an intermediate layer;

[0070] S5, printing a platinum resistance strip on the lower green ceramic sheet 3 as the lower layer to form a platinum resistor 7 for temperature measurement;

[0071] S6, wrapping the upper green porcelain sheet 1, the middle green porcelain sheet 2, and the lower green porcelain sheet 3 in sequence on the rubber tube and laminating them using an isostatic pressing process to form a sensor structure. The pressure range of the hot isostatic pressing is 10MPa to 100MPa. A steel rod is provided in the middle of the rubber tube. After lamination, the steel rod is pulled out to form a cylindrical sensor structure.

[0072] S7, placing the laminated sensor structure into a high-temperature furnace and sintering it according to the sintering curve: debinding from room temperature to 450°C at a heating rate of 5°C / min, holding the temperature for 2 hours to remove the binder, then heating the temperature to 1100°C at a rate of 10°C / min, and sintering in air for 60 minutes;

[0073] S8, welding leads, that is, using silver paste to weld the wire through-holes 5 on the sintered sensor structure to the silver wire, and then connecting the silver wire to the adapter shell 8 and sealing it. That is, resin glue is poured from the other end connected to the adapter shell 8 to fix the two together, and then encapsulate and form an integrated conductivity and temperature sensor.

[0074] Specific implementation method four: Combination Figures 1-6 This embodiment is described based on the second embodiment. This embodiment is a method for manufacturing an integrated conductivity and temperature sensor based on the LTCC process, including:

[0075] S1, select a 0.06-0.1mm thick glass-ceramic composite green porcelain tape, cut it into green porcelain sheets through a slicing process, the size of the green porcelain sheets is 40mm×60mm, and the green porcelain sheets are grouped into three layers, namely upper green porcelain sheet 1, middle green porcelain sheet 2, and lower green porcelain sheet 3;

[0076] S2, opening wire through holes on the upper green porcelain sheet 1, the middle green porcelain sheet 2, and the lower green porcelain sheet 3, using a punching process to make wire through holes 5, with a hole diameter of 0.07mm to 0.15mm. Specifically, 6 wire through holes 5 are processed on the upper green porcelain sheet 1; 16 wire through holes 5 are processed on the middle green porcelain sheet 2; and 8 wire through holes 5 are processed on the lower green porcelain sheet 3;

[0077] S3, using a screen printing process to thick-film print three pairs of conductive electrodes 4 on the upper green ceramic sheet 1 as the upper layer using a gold electrode slurry, and metallize the through-holes 5 on the upper green ceramic sheet 1;

[0078] S4, printing a silver metal signal lead 6 on the intermediate layer green ceramic sheet 2 as an intermediate layer;

[0079] S5, printing a platinum resistance strip on the lower green ceramic sheet 3 as the lower layer to form a platinum resistor 7 for temperature measurement;

[0080] S6, wrapping the upper green porcelain sheet 1, the middle green porcelain sheet 2, and the lower green porcelain sheet 3 in sequence on the rubber tube and laminating them using an isostatic pressing process to form a sensor structure. The pressure range of the hot isostatic pressing is 10MPa to 100MPa. A steel rod is provided in the middle of the rubber tube. After lamination, the steel rod is pulled out to form a cylindrical sensor structure.

[0081] S7, placing the laminated sensor structure into a high-temperature furnace and sintering it according to the sintering curve: debinding from room temperature to 450°C at a heating rate of 5°C / min, holding the temperature for 2 hours to remove the binder, then heating the temperature to 1100°C at a rate of 10°C / min, and sintering in air for 60 minutes;

[0082] S8, welding leads, that is, using silver paste to weld the wire through-holes 5 on the sintered sensor structure to the silver wire, and then connecting the silver wire to the adapter shell 8 and sealing it. That is, resin glue is poured from the other end connected to the adapter shell 8 to fix the two together, and then encapsulate and form an integrated conductivity and temperature sensor.

[0083] It should be noted that in the above embodiments, as long as the technical solutions are not contradictory, they can be permuted and combined. Those skilled in the art can exhaust all possibilities based on the mathematical knowledge of permutations and combinations. Therefore, the present invention will no longer describe the technical solutions after permutations and combinations one by one, but it should be understood that the technical solutions after permutations and combinations have been disclosed by the present invention.

[0084] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for manufacturing an integrated conductivity and temperature sensor based on LTCC technology, characterized in that: include S1, cutting the raw porcelain strip into raw porcelain sheets, wherein the raw porcelain sheets are grouped into three layers, namely, an upper layer raw porcelain sheet (1), a middle layer raw porcelain sheet (2), and a lower layer raw porcelain sheet (3); S2, opening wire through holes on the upper layer raw porcelain sheet (1), the middle layer raw porcelain sheet (2) and the lower layer raw porcelain sheet (3), and using a punching process to make wire through holes (5); S3, thick-film printing three pairs of conductive electrodes (4) on the upper raw ceramic sheet (1) as the upper layer, and metallizing the through-holes (5) on the upper raw ceramic sheet (1); S4, printing a metal signal lead (6) on the intermediate layer green ceramic sheet (2) as an intermediate layer; S5, printing a platinum resistance strip on the lower raw ceramic sheet (3) as the lower layer, and forming a platinum resistance (7) for temperature measurement; S6, wrapping the upper layer raw porcelain sheet (1), the middle layer raw porcelain sheet (2) and the lower layer raw porcelain sheet (3) on the rubber tube in sequence, and laminating them by isostatic pressing to form a sensor structure; S7, placing the laminated sensor structure into a high-temperature furnace and sintering it according to the sintering curve; S8, welding the lead wire, connecting it to the adapter housing (8), and sealing it to form an integrated conductivity and temperature sensor.

2. The method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology according to claim 1, characterized in that: The number of the green porcelain sheets is at least an odd number of 3.

3. The method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology according to claim 1, characterized in that: The adapter housing (8) is provided with an air hole (9) communicating with the interior thereof, and the adapter housing (8) is provided with a pin row (10).

4. The method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology according to claim 1, characterized in that: The sintering temperature in the high-temperature furnace ranges from 1050°C to 1100°C.

5. The method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology according to claim 1, characterized in that: The number of the conductivity electrodes (4) is at least 2 pairs.

6. The method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology according to claim 1, characterized in that: The arrangement shape of the platinum resistance strip is "bow-shaped" or "circular".

7. The method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology according to claim 1, characterized in that: The conductive electrode (4) is made of platinum, graphite or gold, and the platinum resistor (7) is made of platinum.

8. The method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology according to claim 1, characterized in that: The sintered sensor structure is located in the adapter housing (8), wherein the upper layer of raw ceramic sheets (1) is located in the innermost layer.

9. The method for manufacturing a conductivity and temperature integrated sensor based on LTCC technology according to claim 1, characterized in that: The liquid level rising rate is calculated based on the time difference between adjacent detections of the conductivity electrodes 4 to the liquid and the distance between adjacent conductivity electrodes 4 .

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