A thick film humidity sensitive resistor and a method of making the same

Abstract

The present application relates to the technical field of ceramic materials, in particular to a thick film humidity resistor and a preparation method thereof. ZnO, Al2O3 and B4C are mixed with a dispersant solution to perform ball milling, ceramic powder is obtained, then the ceramic powder is mixed with an organic solvent, an organic binder and an organic dispersant and is ground, the obtained thick film printing paste is printed and dried on a substrate with prepared interdigital electrodes for multiple times, glue is removed, then sintering is performed in a nitrogen-oxygen mixed atmosphere, and the thick film humidity resistor is obtained. Since B4C is used as a sintering aid, the high impedance of the element caused by no addition of the sintering aid is avoided, the sintering temperature is reduced, sintering is performed in a nitrogen-air atmosphere, the high impedance of the element caused by air sintering is avoided, the low impedance of the element and the loss of humidity sensitivity caused by nitrogen sintering are avoided.

Description

Technical Field

[0001] This invention relates to the field of ceramic materials technology, specifically to a thick-film humidity-sensitive resistor and its preparation method. Background Technology

[0002] Humidity sensors are an important type of sensor that detects humidity levels. They are devices that convert the measured humidity into a usable output signal according to a certain rule. A humidity sensor typically consists of a humidity-sensitive element and a conversion circuit. The humidity-sensitive element is the component that can sensitively sense the measured humidity level and respond accordingly. A humidity-sensitive resistor usually refers to a humidity-sensitive element that converts the ambient relative humidity value into its own impedance value.

[0003] Polymer humidity sensors are widely used due to their simple manufacturing process, ease of mass production, and availability of diverse polymer raw materials. However, polymer-based humidity sensors still suffer from drawbacks such as high hysteresis, long response time, significant humidity drift, short lifespan under high temperature and humidity conditions, and poor heat and contamination resistance. Metal oxide ceramic humidity capacitors can sense humidity, but they suffer from relatively low sensitivity, significant humidity drift, and complex capacitance change signal conversion systems. Oxide ceramic humidity resistors offer faster response times. Compared to polymer humidity sensors, ceramic humidity resistors have higher mechanical strength, a wider operating humidity range, stronger contamination resistance, and longer lifespan, making them highly promising for future applications.

[0004] Humidity-sensitive elements can generally be classified into four types based on their morphology: semiconductor junction type, bulk type, thick film type, and thin film type. Bulk-structured humidity-sensitive elements have more and deeper pores, resulting in high sensitivity, but a longer response time. Thin film elements have a short response time, but fewer pores and lower sensitivity. Thick film humidity-sensitive elements can be regarded as miniaturized porous sintered bodies, attracting much attention due to their good sensitivity, fast response time, high mechanical strength, low substrate requirements, and relatively low manufacturing cost. Their moderate performance makes them more valuable for applications. In particular, with the current high degree of integration and miniaturization of silicon-based integrated circuits, sensors are also developing towards miniaturization and integration. Thick film elements are compatible with the fabrication process of low-temperature co-fired ceramics (LTCC), and can be easily integrated with other resistive, capacitive, and inductive devices and circuit wires in thick film hybrid integrated circuits through multilayer printing and stacked hot pressing, and can be co-fired at a relatively low temperature in one step. At the same time, the thick film element fabrication system can provide a substrate for integrating other sensors, further improving the integration level.

[0005] However, ceramic humidity resistors have high overall impedance in the low humidity range. In low humidity environments, the extremely high impedance is not conducive to the design of impedance-numerical information transmission systems in humidity sensors. Therefore, it is of great significance to design a thick-film humidity resistor with low impedance, compatibility with LTCC process, low firing temperature preparation, and excellent humidity sensing performance. Summary of the Invention

[0006] To address the existing problems, this invention provides a thick-film humidity-sensitive resistor and its preparation method. The resulting resistor has low impedance and humidity sensitivity characteristics, and can be prepared at low sintering temperatures using the LTCC process, thus reducing the sintering temperature.

[0007] This invention is achieved through the following technical solution:

[0008] A method for preparing a thick-film humidity-sensitive resistor, characterized by comprising the following steps:

[0009] (1) ZnO, Al2O3, B4C are mixed with a dispersant solution, ball-milled, sieved, and dried to obtain ceramic powder;

[0010] The amount of B4C added is 0.1 to 0.6 wt% of ZnO;

[0011] (2) The ceramic powder obtained in step (1) is mixed with organic solvent, organic binder and organic dispersant and then ground to obtain thick film printing paste;

[0012] (3) The thick film printing paste obtained in step (2) is drop-coated, spin-coated or printed onto the substrate with interdigitated electrodes, so that it is evenly covered, dried, de-adhesive removed, and sintered to obtain a thick film humidity-sensitive resistor.

[0013] The sintering temperature is 800-900℃, and the sintering atmosphere is a mixture of nitrogen and air with a flow rate ratio of (1-3):1.

[0014] Preferably, the amount of B4C added is 0.2 ± 0.1 wt% of ZnO.

[0015] Preferably, the mass ratio of ZnO, Al2O3 and dispersant solution is 100:(1-5):(100-110); the dispersant solution is ammonium polyacrylate.

[0016] Preferably, the sintering process involves heating to 850±20°C at a rate of 3–6°C / min, holding at that temperature for 1.5–3.5 h, and then cooling to 550–650°C within 0.5–1.5 h.

[0017] Preferably, in step (1), ball milling refers to adding the material and balls to a ball mill at a mass ratio of 1:2 and ball milling at 250±30 r / min for 1 to 3 hours; sieving refers to sieving the ball-milled material through a 400-mesh sieve; drying refers to drying at 110±20℃ for 4 to 6 hours; and after drying, grinding continues until the material can pass through a sieve of 80 mesh or higher.

[0018] Preferably, the organic solvent in step (2) is diethylene glycol butyl ether and terpineol, the organic adhesive is K01D010, and the organic dispersant is polyethyleneimine; the mass ratio of the ceramic powder, diethylene glycol butyl ether, terpineol, K01D010, and polyethyleneimine is (10-15):(2-2.5):(1-1.1):(2-2.5):(0.3-0.4).

[0019] Preferably, the glue removal step (3) refers to raising the temperature from room temperature to 400±50℃ at a rate of (2~3)℃ / min, and then keeping it at that temperature for 1~2h.

[0020] Preferably, the interdigitated electrode in step (3) is a silver electrode, the substrate is an alumina substrate, and the thickness of the substrate is 600-650 μm; the thickness of the humidity-sensitive ceramic on the thick film humidity-sensitive resistor is 70 μm-100 μm.

[0021] The thick-film humidity-sensitive resistor prepared by the method described in this invention.

[0022] The present invention has the following advantages and effects compared with the prior art:

[0023] (1) The present invention uses B4C as a sintering aid to avoid the excessively high impedance of the components caused by not adding sintering aids; it also avoids the complexation reaction between B2O3 and ZnO, Al2O3 and dispersant caused by directly adding B2O3 as a sintering aid, which would result in the ball milling slurry and printing slurry being viscous and unable to be dispersed evenly.

[0024] (2) The present invention uses a nitrogen + air atmosphere for sintering, which reduces the oxygen partial pressure in the sintering atmosphere and avoids the excessively high impedance of the components caused by air sintering; and avoids the excessively low impedance of the components and loss of humidity sensitivity caused by nitrogen sintering.

[0025] (3) The humidity-sensitive resistor prepared by the present invention has excellent humidity-sensitive characteristics, high sensitivity, high stability, low hysteresis and fast response recovery characteristics. Attached Figure Description

[0026] Figure 1 This is a top view of the thick-film humidity-sensitive resistor prepared according to the present invention; 1-The gray part is the interdigitated electrode and the terminal electrode; 2-The white part is the aluminum oxide substrate; 3-The black dashed line indicates the position and range of the thick film coating.

[0027] Figure 2 The impedance curve of the humidity-sensitive resistor prepared in Example 1 of the present invention.

[0028] Figure 3 Impedance curves of the humidity-sensitive resistors prepared in Example 1 and Comparative Examples 1-5 of this invention.

[0029] Figure 4The response-recovery curve of the humidity-sensitive resistor prepared in Example 1 of the present invention.

[0030] Figure 5 The repeated cycle curve of the humidity-sensitive resistor prepared in Example 1 of the present invention.

[0031] Figure 6 The hysteresis curve of the humidity-sensitive resistor prepared in Example 1 of the present invention. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0033] Example 1

[0034] A thick-film humidity-sensitive resistor (structural schematic diagram shown) Figure 1 As shown in the figure, its preparation method is as follows:

[0035] 1) Add 100g ZnO, 3.0g Al2O3, 0.2g B4C, 5.8g ammonium polyacrylate aqueous solution, and 100g deionized water to a ball mill. Add zirconia balls at a mass ratio of 1:2. Adjust the ball mill speed to 250r / min and ball mill for 2 hours. After sieving the zirconia balls, the slurry obtained from ball milling is passed through a 400-mesh sieve. The slurry after sieving is dried in a blower dryer at 110℃ for 5 hours, and then ground again until the material can pass through an 80-mesh sieve to obtain ceramic powder.

[0036] 2) Take 12g of ceramic powder, 0.35g of polyethyleneimine, 1.05g of terpineol, 2.2g of diethylene glycol butyl ether, and 2.25g of K01D010 organic adhesive produced by a certain company, grind for 10 minutes to obtain printing paste;

[0037] 3) The surface of the 635μm thick alumina substrate with printed silver interdigitated electrodes was ultrasonically cleaned with anhydrous ethanol for 5 minutes, then dried. Organic paste was then screen-printed onto the side with the interdigitated electrodes using a 200-mesh screen with a 6.9mm × 6.9mm square printed pattern. The substrate was then dried at 150℃ for 1 minute. This printing and drying process was repeated three times. The substrate was then placed in a muffle furnace for debinding. Specifically, the temperature was increased from room temperature to 400℃ at a rate of 2.5℃ / min, held at that temperature for 60 minutes, and then allowed to cool naturally to room temperature. The debinded sample was placed in a tube furnace, and after a vacuum evacuation and priming with pure nitrogen, the nitrogen flow rate and air flow rate were adjusted to 30 sccm, followed by sintering. The specific sintering operation is as follows: the temperature is raised from room temperature to 850℃ at a rate of 5℃ / min in a tube furnace, held for 120min, cooled to 600℃ within 60min, and then naturally cooled to room temperature. The mixed gas is then turned off to obtain a thick film humidity-sensitive resistor sample.

[0038] The humidity-impedance characteristic curve of the sample in this embodiment is as follows: Figure 2 The curves are shown. Tests revealed the following humidity-impedance characteristics of the thick-film humidity sensor in this embodiment under 22°C, 1kHz, and 1V sinusoidal AC current: impedance 778kΩ at 11% relative humidity, 770kΩ at 23% relative humidity, 754kΩ at 33% relative humidity, 694kΩ at 53% relative humidity, 532kΩ at 75% relative humidity, 395kΩ at 85% relative humidity, and 74kΩ at 97% relative humidity. Samples prepared using the same method showed nearly identical test data with minimal differences.

[0039] The response-recovery curve of this embodiment is as follows: Figure 4 As shown, the response time (calculated from the start of impedance change to the end of the calculation of 90% of the difference between the impedance value corresponding to 11% relative humidity and the impedance value corresponding to 97% relative humidity) is 2.9645 seconds, and the recovery time (calculated from the start of impedance change to the end of the calculation of 90% of the difference between the impedance value corresponding to 97% relative humidity and the impedance value corresponding to 44% relative humidity) is 0.4928 seconds.

[0040] The repeatable cyclic curve of this embodiment between 11% and 97% relative humidity is as follows: Figure 5 As shown, the response value did not change significantly during the 5-cycle test, and the baseline of the component did not change significantly, indicating that the humidity-sensitive resistor prepared in this embodiment has good stability.

[0041] The adsorption-desorption curve of this embodiment is as follows: Figure 6 As shown, the adsorption-desorption curves of the humidity-sensitive resistor have a high degree of overlap, and the maximum humidity hysteresis value measured at 85% RH is 2%.

[0042] Comparative Example 1 (without sintering aid)

[0043] A thick-film humidity-sensitive resistor (structural schematic diagram shown) Figure 1 As shown in the figure, its preparation method is as follows:

[0044] 1) Add 100g of ZnO, 3.0g of Al2O3, 5.8g of ammonium polyacrylate aqueous solution and 100g of deionized water to a ball mill to prepare ceramic powder. The steps are the same as in Example 1.

[0045] 2) The steps for preparing the printing paste are the same as in Example 1;

[0046] 3) The printing, drying, glue removal, and sintering steps are the same as in Example 1.

[0047] The humidity-impedance characteristics of the thick-film humidity sensor in this comparative example under 22℃, 1kHz, and 1V sinusoidal AC power were tested as follows: impedance 11070kΩ at 11% relative humidity, 10329kΩ at 23% relative humidity, 7453kΩ at 33% relative humidity, 2441kΩ at 53% relative humidity, 547kΩ at 75% relative humidity, 236kΩ at 85% relative humidity, and 26kΩ at 97% relative humidity. The humidity-impedance characteristic curve of this comparative example sample is shown below. Figure 3 The curve in “Comparative Example 1” is shown.

[0048] Comparative Example 2 (using B2O3 as a sintering aid)

[0049] A thick-film humidity-sensitive resistor (structural schematic diagram shown) Figure 1 As shown in the figure, its preparation method is as follows:

[0050] 1) Add 100g of ZnO, 3.0g of Al2O3, 0.5g of B2O3 (the amount of boron in 0.5g of B2O3 corresponds to 0.2g of B4C), 5.8g of ammonium polyacrylate aqueous solution, and 100g of deionized water to a ball mill to prepare ceramic powder. The steps are the same as in Example 1.

[0051] 2) The steps for preparing the printing paste are the same as in Example 1;

[0052] 3) The printing, drying, glue removal, and sintering steps are the same as in Example 1.

[0053] During the comparative experiment, it was found that the ball-milled slurry was quite viscous, possibly indicating uneven dispersion; the printing slurry was difficult to mix and required a long dispersion time. Therefore, two comparative samples, Comparative Examples 2-1 and 2-2, were prepared for comparison. The humidity-impedance characteristics of the thick-film humidity sensor in Comparative Example 2-1 under 22℃, 1kHz, and 1V sinusoidal AC current were tested as follows: impedance 1900kΩ at 11% relative humidity, 1650kΩ at 23% relative humidity, 1110kΩ at 33% relative humidity, 436.5kΩ at 53% relative humidity, 145.5kΩ at 75% relative humidity, 70.5kΩ at 85% relative humidity, and 13.5kΩ at 97% relative humidity. kΩ; The humidity-impedance characteristics of the thick-film humidity sensor in Comparative Example 2-2 under 22℃, 1kHz, and 1V sinusoidal AC current are as follows: impedance 1215kΩ at 11% relative humidity, 899.5kΩ at 23% relative humidity, 494.5kΩ at 33% relative humidity, 172.5kΩ at 53% relative humidity, 42kΩ at 75% relative humidity, 22.5kΩ at 85% relative humidity, and 7.4kΩ at 97% relative humidity. The humidity-impedance characteristic curve of this comparative example sample is shown below. Figure 3 The curves for "Comparative Example 2-1" and "Comparative Example 2-2" show significant differences between the two samples, indicating insufficient homogeneity in the samples prepared in this comparative example.

[0054] Comparative Example 3 (with 0.4 wt% B4C added as a sintering aid)

[0055] A thick-film humidity-sensitive resistor (structural schematic diagram shown) Figure 1 As shown in the figure, its preparation method is as follows:

[0056] 1) Add 100g of ZnO, 3.0g of Al2O3, 0.4g of B4C, 5.8g of ammonium polyacrylate aqueous solution, and 100g of deionized water to a ball mill to prepare ceramic powder. The steps are the same as in Example 1.

[0057] 2) The steps for preparing the printing paste are the same as in Example 1;

[0058] 3) The printing, drying, glue removal, and sintering steps are the same as in Example 1.

[0059] The humidity-impedance characteristics of the thick-film humidity sensor in this comparative example under 22℃, 1kHz, and 1V sinusoidal AC power were tested as follows: impedance 4083kΩ at 11% relative humidity, 4087kΩ at 23% relative humidity, 3971kΩ at 33% relative humidity, 3347kΩ at 53% relative humidity, 1946kΩ at 75% relative humidity, 1088kΩ at 85% relative humidity, and 145kΩ at 97% relative humidity. The humidity-impedance characteristic curve of this comparative example sample is shown below. Figure 3 The curve is shown in "Comparative Example 3". Comparative Example 4 (using air as the sintering atmosphere)

[0060] A thick-film humidity-sensitive resistor (structural schematic diagram shown) Figure 1 As shown in the figure, its preparation method is as follows:

[0061] 1) The steps of batching, ball milling, and preparing ceramic powder are the same as in Example 1;

[0062] 2) The steps for preparing the printing paste are the same as in Example 1;

[0063] 3) The printing, drying, and adhesive removal steps are the same as in Example 1. The sample with the adhesive removed is placed in a tube furnace, the input dry air flow rate is adjusted to 60 sccm, and then sintering is performed. The specific sintering operation is the same as in Example 1.

[0064] The humidity-impedance characteristics of the thick-film humidity sensor in this comparative example under 22℃, 1kHz, and 1V sinusoidal AC power were tested as follows: impedance 6079kΩ at 11% relative humidity, 6127kΩ at 23% relative humidity, 5990kΩ at 33% relative humidity, 5731kΩ at 53% relative humidity, 5095kΩ at 75% relative humidity, 4212kΩ at 85% relative humidity, and 146kΩ at 97% relative humidity. The humidity-impedance characteristic curve of this comparative example sample is shown below. Figure 3 The curve is shown in "Comparative Example 4". Comparative Example 5 (using a nitrogen-to-air flow rate ratio of 3:1 as the sintering atmosphere).

[0065] A thick-film humidity-sensitive resistor (structural schematic diagram shown) Figure 1 As shown in the figure, its preparation method is as follows:

[0066] 1) The steps of batching, ball milling, and preparing ceramic powder are the same as in Example 1;

[0067] 2) The steps for preparing the printing paste are the same as in Example 1;

[0068] 3) The printing, drying, and adhesive removal steps are the same as in Example 1. The sample with the adhesive removed is placed in a tube furnace, and the nitrogen flow rate is adjusted to 45 sccm and the air flow rate to 15 sccm before sintering. The specific sintering operation is the same as in Example 1.

[0069] The humidity-impedance characteristics of the thick-film humidity sensor in this comparative example under 22℃, 1kHz, and 1V sinusoidal AC power were tested as follows: impedance 2054kΩ at 11% relative humidity, 2052kΩ at 23% relative humidity, 2002kΩ at 33% relative humidity, 1751kΩ at 53% relative humidity, 1251kΩ at 75% relative humidity, 832kΩ at 85% relative humidity, and 104kΩ at 97% relative humidity. The humidity-impedance characteristic curve of this comparative example sample is shown below. Figure 3 As shown in the curve of “Comparative Example 5”.

[0070] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a thick-film humidity-sensitive resistor, characterized in that, Includes the following steps: (1) ZnO, Al2O3, B4C are mixed with a dispersant solution, ball-milled, sieved, and dried to obtain ceramic powder; The amount of B4C added is 0.2 ± 0.1 wt% of ZnO; (2) The ceramic powder obtained in step (1) is mixed with organic solvent, organic binder and organic dispersant and then ground to obtain thick film printing paste; (3) The thick film printing paste obtained in step (2) is drop-coated, spin-coated or printed onto the substrate with interdigitated electrodes, so that it is evenly covered, dried, de-adhesive removed, and sintered to obtain a thick film humidity-sensitive resistor. The sintering temperature is 800~900℃, and the sintering atmosphere is a mixture of nitrogen and air with a flow rate ratio of 1:

1. The mass ratio of ZnO, Al2O3 and dispersant solution is 100:(1~5):(100~110).

2. The preparation method according to claim 1, characterized in that, The dispersant solution is ammonium polyacrylate.

3. The preparation method according to claim 2, characterized in that, The sintering process involves heating the temperature to 850±20℃ at a rate of 3~6℃ / min, holding it at that temperature for 1.5~3.5h, and then cooling it to 550~650℃ within 0.5~1.5h.

4. The preparation method according to any one of claims 1-3, characterized in that, In step (1), ball milling refers to adding the material and balls to the ball mill at a mass ratio of 1:2 and milling at 250±30 r / min for 1~3 hours; sieving refers to sieving the milled material through a 400-mesh sieve; drying refers to drying at 110±20℃ for 4~6 hours; and after drying, grinding continues until the material can pass through a sieve of 80 mesh or higher.

5. The preparation method according to any one of claims 1-3, characterized in that, The glue removal step (3) refers to raising the temperature from room temperature to 400±50℃ at a rate of (2~3)℃ / min, and then keeping it at that temperature for 1~2 hours.

6. The preparation method according to any one of claims 1-3, characterized in that, The interdigitated electrode in step (3) is a silver electrode, the substrate is an alumina substrate, and the thickness of the substrate is 600~650μm; the thickness of the humidity-sensitive ceramic on the thick film humidity-sensitive resistor is 70 μm~100 μm.

7. The thick-film humidity-sensitive resistor prepared by the preparation method according to any one of claims 1-3.

Images