A method for preparing a negative temperature coefficient thermosensitive ceramic sheet

Abstract

The present disclosure provides a preparation method of a negative temperature coefficient thermosensitive ceramic sheet, which comprises adding an additive to a metal oxide-containing water-based slurry to obtain a mixed slurry, then performing directional freezing on the mixed slurry to make ice crystals grow directionally from the bottom of the mixed slurry to the liquid surface, and then performing freeze-drying on the frozen green body to obtain a sheet-shaped ceramic green body, and then performing normal pressure sintering to obtain a sheet-shaped ceramic bulk body with a single spinel structure, and finally performing mechanical separation on the ceramic bulk body to obtain a negative temperature coefficient thermosensitive ceramic sheet. The negative temperature coefficient thermosensitive ceramic sheet prepared by the preparation method of the present disclosure has high density, and the preparation process is simple and easy to operate, and is energy-saving and environmentally friendly.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of heat-sensitive ceramic forming, in particular to a preparation method of a negative temperature coefficient thermistor ceramic sheet. BACKGROUND

[0002] In recent years, the miniaturization and integration trend of electronic products has driven the development of microelectronic technology and surface mounting technology, thus requiring the sheeting of traditional bulk resistance, capacitance, inductance and other sensitive components. The negative temperature coefficient thermistor (NTC) has complex composition and is difficult to be sintered into a dense thermistor ceramic sheet under normal pressure. High-temperature and high-pressure ceramic sintering technology is usually used to sinter a ceramic ingot, which is then sliced and scribed, coated with electrodes, and finally packaged into a component for use.

[0003] However, high-temperature and high-pressure sintering and other technologies have high requirements for equipment and molds, and the sintering temperature of composite functional ceramic materials with excellent NTC characteristics is also high. Different ceramic phases have different sintering temperatures, resulting in a high degree of difficulty in sintering process operation, high cost and high energy consumption. Therefore, a preparation method of a negative temperature coefficient ceramic sheet with high density and low cost is needed, which is simple and easy to operate. SUMMARY

[0004] Therefore, in order to at least partially solve at least one of the above-mentioned technical problems, the present disclosure provides a preparation method of a negative temperature coefficient thermistor ceramic sheet, which can form an ultra-thin thermistor ceramic sheet with high density in situ under normal pressure, has low preparation cost, is simple and easy to operate, and is energy-saving and environmentally friendly.

[0005] In order to solve the above technical problems, the technical scheme provided by the present disclosure is as follows:

[0006] According to an embodiment of the present disclosure, a preparation method of a negative temperature coefficient thermistor ceramic sheet is provided, comprising:

[0007] An additive is added to a water-based slurry containing metal oxides, and after mixing, a mixed slurry is obtained, the metal oxides include MnO2, Ni2O3 and Co2O3, and the additive includes polyethylene glycol and an auxiliary agent, the auxiliary agent being glycerol and / or a glass-ceramic auxiliary sintering agent;

[0008] The mixed slurry is directionally frozen to make ice crystals grow in a direction from the bottom of the mixed slurry to the liquid surface, and a frozen green body is obtained;

[0009] The frozen green body is freeze-dried to make ice crystals sublimate, and a ceramic green body in a sheet form is obtained;

[0010] The ceramic green body is subjected to normal pressure sintering to obtain a ceramic block with a single spinel structure, and the ceramic block is in a lamellar form;

[0011] The ceramic block is subjected to cleavage fracture along the lamellar direction by mechanical separation to obtain a negative temperature coefficient thermosensitive ceramic sheet.

[0012] According to the embodiment of the present disclosure, when the material for preparing the negative temperature coefficient thermosensitive ceramic is added with a dispersing agent, a uniformly dispersed mixed slurry can be obtained; the ice crystals are vertically oriented by directional freezing of the mixed slurry, and the oriented ice crystals push and press the oxide particles into lamellae to obtain a frozen body; the ceramic green body in a lamellar form is clean, dry and impurity-free by freeze-drying of the frozen body; a ceramic block with high density can be obtained by normal pressure sintering of the ceramic green body; and a negative temperature coefficient thermosensitive ceramic sheet with high density and ultra-thin thickness can be obtained by mechanical separation. BRIEF DESCRIPTION OF DRAWINGS

[0013] In order to more clearly illustrate the technical solutions in the embodiments of the present disclosure or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present disclosure, and other drawings can be obtained by those skilled in the art without creative labor.

[0014] Figure 1 Flow chart of the negative temperature coefficient thermosensitive ceramic preparation method of the present disclosure;

[0015] Figure 2 Structure diagram of the directional freezing device in the embodiment of the present disclosure;

[0016] Figure 3 Scanning electron microscope image of the frozen body prepared in Example 1 of the present disclosure;

[0017] Figure 4 X-ray diffraction spectrum of the ceramic sheet prepared in Example 1 of the present disclosure;

[0018] Figure 5 Microstructure scanning electron microscope image of the ceramic sheet prepared in Example 1 of the present disclosure.

[0019] In the above drawings, the meanings of the reference signs are as follows:

[0020] 1. Mixed slurry;

[0021] 2. Freezing mold;

[0022] 3. Heat conduction device;

[0023] 4. Cold source;

[0024] 5. Cold source tank. DETAILED DESCRIPTION

[0025] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. It should be understood, however, that the description is merely exemplary and is not intended to limit the scope of the present disclosure. In the following detailed description of the embodiments, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. However, it would be apparent to those skilled in the art that the embodiments, or one or more of the embodiments, can be practiced without these specific details. In other instances, well-known structures and methods have been omitted to avoid unnecessarily complicating the present disclosure.

[0026] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the term "includes" and / or "including", as well as

[0027] The thermal sensitive ceramic density is a key technical index affecting the performance and stability of the NTC thermistor, and the sintering process of the dense ceramic is a key to the molding of the NTC sensitive ceramic body with a complex phase structure, which will directly affect the performance and stability of the thermal sensitive ceramic. Therefore, the present disclosure proposes a new molding technology for obtaining a dense sensitive ceramic chip by using a full physical self-assembly method according to the principle of freeze casting, which realizes the chip molding process of the ceramic sheet that meets the requirements of high density and ultra-thin chip molding, and also takes into account energy saving and environmental protection, easy operation, low cost and other characteristics at normal pressure, which has important development significance for developing NTC temperature sensors with world leading level.

[0028] According to the embodiments of the present disclosure, a preparation method of a negative temperature coefficient thermal sensitive ceramic sheet is provided, and the flow is as shown in Figure 1 The preparation method comprises the following steps:

[0029] Step S1: adding an additive to a water-based slurry containing metal oxides, and obtaining a mixed slurry after mixing, the metal oxides including MnO2, Ni2O3 and Co2O3, and the additive including polyethylene glycol and an auxiliary agent, the auxiliary agent being glycerol and / or a glass ceramic auxiliary sintering agent;

[0030] Step S2: directional freezing of the mixed slurry to make ice crystals grow in a direction from the bottom of the mixed slurry to the liquid surface, and obtaining a frozen green body;

[0031] Step S3: freeze-drying of the frozen green body to make ice crystals sublimate, and obtaining a ceramic green body in a sheet shape;

[0032] Step S4: normal pressure sintering of the ceramic green body to obtain a ceramic block body with a single spinel structure, the ceramic block body being in a sheet shape;

[0033] Step S5: Use mechanical separation to cleave and fracture along the lamellar direction of the ceramic block to obtain a negative temperature coefficient thermistor ceramic sheet.

[0034] According to embodiments of this disclosure, a dispersant is added during the preparation of negative temperature coefficient thermistor ceramic materials to obtain a uniformly dispersed mixed slurry; directional freezing of the mixed slurry allows ice crystals to grow vertically and oriented, and the oriented ice crystals displace and press oxide particles into sheets to obtain a frozen green body; freeze-drying of the frozen green body yields a clean, dry, and impurity-free ceramic green body; atmospheric pressure sintering of the ceramic green body yields a ceramic block containing ultra-thin, dense sheets; and mechanical separation yields a high-density and ultra-thin negative temperature coefficient thermistor ceramic sheet.

[0035] According to an embodiment of this disclosure, in step S1, the solid phase weight fraction of the metal oxide in the water-based slurry is 45% to 55%, for example, it can be 45%, 50%, 55%, etc.

[0036] According to embodiments of this disclosure, if the solid content of the metal oxide in the water-based slurry is less than 45%, it will cause low density and low strength in the ceramic powder sheeting process, making it unable to meet the requirements of subsequent industrial processes; if the solid content is too high, it will be difficult to form a dense lamellar structure, which will increase the preparation cost and cause unnecessary waste.

[0037] According to embodiments of this disclosure, the directional freezing operation in step S2 is performed using a directional freezing device, such as... Figure 2 As shown, the device includes a freezing mold 2, a heat-conducting device 3, and a cold source tank 5. The freezing mold 2 is suitable for containing the mixed slurry 1; the cold source tank 5 is suitable for containing the cold source 4; one end of the heat-conducting device 3 is connected to the bottom of the freezing mold 2, and the other end is installed in the cold source tank 5, which is suitable for transferring the heat from the cold source 4 in the cold source tank 5 from the bottom of the freezing mold 2 to the mixed slurry 1 in the freezing mold 2, applying a freezing gradient from the bottom to the liquid surface to the mixed slurry 1. However, the structure of the directional freezing device is not limited to this, as long as it can apply a freezing gradient from the bottom to the liquid surface to the mixed slurry 1.

[0038] Furthermore, in step S2, the directional freezing operation includes:

[0039] The mixed slurry is poured into a pre-cooled freezing mold 2, and a freezing gradient from the bottom of the mixed slurry 1 to the liquid surface is applied to the mixed slurry 1 using a refrigerant as a cold source 4, so that ice crystals grow in a specific direction, thus obtaining a directional frozen green body.

[0040] According to an embodiment of the present disclosure, the freezing mold 2 is made of a low-temperature-resistant material, preferably, the freezing mold 2 is made of polytetrafluoroethylene; the cold source 4 is a refrigerant, preferably, the cold source 4 is liquid nitrogen; the cold source 4 is stored in a cold source tank 5, the cold source tank 5 is made of a low-temperature-resistant material, preferably, the cold source tank 5 is a polytetrafluoroethylene tank; the cold source 4 transfers heat to the mixed slurry 1 in the freezing mold 2 in a uniform and directional manner through the heat-conducting device 3, the heat-conducting device 3 and the freezing mold 2 are made of a material that can conduct heat quickly and uniformly, preferably, the heat-conducting device 3 is a copper block and covers at least the bottom of the freezing mold 2.

[0041] According to an embodiment of the present disclosure, in order to apply a freezing gradient from the bottom to the liquid surface to the mixed slurry 1, the liquid surface of the refrigerant as the cold source 4 is parallel to the bottom of the freezing mold 2 and does not contact the side wall of the freezing mold 2. If the bottom of the freezing mold 2 is slightly inclined or the refrigerant contacts the side wall of the freezing mold 2, the freezing gradient will not be in the direction from the bottom to the liquid surface, and the directional growth of ice crystals cannot be achieved, and the subsequent mechanical separation of ultra-thin ceramic sheets cannot be achieved.

[0042] According to an embodiment of the present disclosure, the mixing ratio between MnO2, Ni2O3 and Co2O3 is Mn 2.25-x Ni 0.75 Co x O4 stoichiometric ratio, 0.8≤x≤1.2, for example, x can be 0.8, 0.9, 1, 1.1, 1.2, etc.

[0043] According to an embodiment of the present disclosure, through experimental design, in order to ensure that the prepared ceramic sheet has a negative temperature coefficient thermosensitive characteristic, the mixing ratio of the metal oxides needs to be kept so that the mixture can form a single spinel phase structure through a solid phase reaction. If the value of x is too high or too low, secondary phases will be easily produced during sintering, and a single spinel structure cannot be formed.

[0044] According to an embodiment of the present disclosure, the preparation method of the present disclosure further comprises:

[0045] MnO2, Ni2O3 and Co2O3 are mixed as initial raw materials, ethanol or water is used as a medium, and wet grinding mixing and drying to remove ethanol or water are sequentially performed to obtain a dried mixture; the dried mixture is dispersed in water according to a preset solid phase weight fraction to obtain a water-based slurry containing metal oxides.

[0046] According to an embodiment of the present disclosure, the initial raw materials are mixed more uniformly through wet grinding mixing, and ethanol or water is used as a wet grinding medium, which can help dissipate heat during wet grinding and avoid phase transition of the materials due to temperature rise.

[0047] According to an embodiment of the present disclosure, the water-based slurry has a solid phase weight fraction of 100%, and the additives include 0.5%-1.5% polyethylene glycol and 0-4% glycerol. The content of the polyethylene glycol may, for example, be 0.5%, 1.0%, 1.5%, etc., and the content of the glycerol may, for example, be 1%, 2%, 3%, 4%, etc.

[0048] According to an embodiment of the present disclosure, the polyethylene glycol and the glycerol as dispersants can make the water-based slurry more uniformly dispersed, obtaining a uniformly stable mixed slurry; meanwhile, the addition of the dispersants can also make the interface between the ice crystals and the ceramic particles after freezing clearer, producing a more distinct layered structure, facilitating the subsequent mechanical separation of the ceramic sheets; the glycerol also helps to promote the densification of the sheet structure during directional freezing.

[0049] According to another embodiment of the present disclosure, the water-based slurry has a solid phase weight of 100%, and the additives include 0.5%-1.5% polyethylene glycol and 0-10% glass-ceramic sintering aid. The content of the glass-ceramic sintering aid may, for example, be 2%, 4%, 6%, 8%, 10%, etc.

[0050] According to an embodiment of the present disclosure, the glass-ceramic sintering aid functions to promote densification during the sintering process of the ceramic, improving the structural strength.

[0051] According to an embodiment of the present disclosure, the conditions for freeze-drying include a drying temperature of -40℃ to -50℃, for example, -40℃, -45℃, -50℃, etc., a vacuum degree less than 10 Pa, and a drying time of 24-48 hours.

[0052] According to an embodiment of the present disclosure, the function of freeze-drying is to sublimate the ice in the sheet-shaped ceramic green body, obtaining a clean and impurity-free ceramic green body.

[0053] According to an embodiment of the present disclosure, the normal-pressure sintering is to heat to 800℃ at a temperature rising rate of 5℃ / min under normal pressure, keep the temperature for 2 hours, and then heat to 1200-1400℃ at a temperature rising rate of 3℃ / min, keep the temperature for 2 hours.

[0054] According to an embodiment of the present disclosure, the normal-pressure sintering can obtain a sheet-shaped ceramic bulk body with a single spinel structure, without the need for high-pressure sintering in the prior art, saving cost and simplifying the process flow.

[0055] According to an embodiment of the present disclosure, the method for mechanical separation is mechanical cutting.

[0056] According to an embodiment of the present disclosure, by mechanically cutting along the sheet direction of the ceramic bulk body, a negative temperature coefficient thermosensitive ceramic sheet can be directly obtained, which is easy to operate.

[0057] The present disclosure forms a sheet-layer green body structure through a pure physical self-assembly process, and a dense ceramic sheet can be obtained through static sintering under normal pressure, without a complex heat treatment process such as degumming, and the operation is simple, low-carbon and environmentally friendly.

[0058] The present disclosure will be further described in detail below with reference to specific examples and with reference to the accompanying drawings. The following examples are only intended to illustrate the present disclosure, but not to limit the present disclosure. In the following examples, the experimental methods are all conventional experimental operations unless otherwise specified. In the following examples, all instruments, consumables and reagents, etc. can be obtained from commercial channels unless otherwise specified.

[0059] Example 1

[0060] A water-based slurry with a solid content of 45% metal oxide is prepared for a negative temperature coefficient thermosensitive ceramic sheet as shown in Figure 1 The following steps are followed:

[0061] Step S1: high-purity MnO2, Ni2O3 and Co2O3 are used as initial raw materials, and the stoichiometric ratio of MnO2, Ni2O3 and Co2O3 is 1:1:1. The oxide powders are weighed, ethanol is used as the medium, and high-energy ball milling is used for wet mixing for 24 hours. Then, the mixture is placed in an air drying oven at 60°C for 12 hours to remove ethanol, and a mixed oxide powder is obtained. 2.25-x Ni 0.75 Co x O4 (wherein x=1) stoichiometric ratio, ethanol as medium, wet mixing with high-energy ball mill for 24 h, then placed in air drying oven at 60°C for 12 hours to remove ethanol, to obtain mixed oxide powder;

[0062] The mixed oxide powder is prepared into a water-based slurry with a solid content of 45%, and 4% glycerol and 1.5% polyethylene glycol dispersant are added by weight ratio. High-energy ball milling is used for mixing at a speed of 2000 r / min for 12 hours to obtain a mixed slurry 1.

[0063] Step S2: pour the mixed slurry 1 prepared in step S1 into a pre-cooled polytetrafluoroethylene mold, i.e. a freezing mold 2, and perform directional freezing at a liquid nitrogen temperature for 6 minutes to produce a directional growth of the frozen green body;

[0064] The frozen green body is observed by scanning electron microscopy to obtain a ceramic green body microstructure as shown in Figure 3 The microstructure of the ceramic green body can be seen from Figure 3 The ceramic powder particles are pushed by the ice crystals growing vertically upward to form a dense sheet structure.

[0065] Step S3: place the frozen green body in an environment with a vacuum degree of <10 Pa at -50°C for freeze-drying for 48 hours to completely sublimate the ice crystals and obtain a sheet-layer ceramic green body;

[0066] Step S4: After drying, the ceramic green body in the form of a sheet layer is heated to 800℃ at a heating rate of 5℃ / min under normal pressure, and then heated to 1200℃ at a heating rate of 3℃ / min, and kept for 2 hours, so as to obtain a ceramic block with a single spinel structure through solid phase reaction, and realize densification and strengthening of the sheet layer structure at the same time;

[0067] Step S5: The ceramic block with a sheet layer structure is cleaved along the sheet layer direction by a mechanical separation method, so as to obtain a negative temperature coefficient thermosensitive ceramic sheet.

[0068] The negative temperature coefficient thermosensitive ceramic sheet prepared is subjected to X-ray diffraction analysis, and an X-ray diffraction spectrum as shown in Figure 4 may be obtained, and it can be judged from the diffraction peak that the negative temperature coefficient thermosensitive ceramic sheet prepared has a single spinel structure.

[0069] The negative temperature coefficient thermosensitive ceramic sheet prepared is subjected to scanning electron microscope observation, and a sheet layer microstructure diagram as shown in Figure 5 may be obtained, and it can be known from Figure 5 that the negative temperature coefficient thermosensitive ceramic sheet prepared by the preparation method of the negative temperature coefficient thermosensitive ceramic sheet of the present disclosure has a dense microstructure, and has no cracks, no obvious shrinkage holes, and no transgranular fracture phenomenon, has a high density, and meets the subsequent use process requirements.

[0070] Example 2

[0071] A negative temperature coefficient thermosensitive ceramic sheet with a water-based slurry solid phase content of 50% metal oxide is prepared as shown in Figure 1 , and the following steps are performed:

[0072] Step S1: High-purity MnO2, Ni2O3 and Co2O3 are used as initial raw materials, and the stoichiometric ratio of Mn 2.25-x Ni 0.75 Co x O4 ((wherein, x=1)) is used to weigh each oxide powder, ethanol is used as a medium, high-energy ball milling is used for wet mixing for 24 hours, and then the mixture is placed in an air drying oven and dried at 60℃ for 12 hours to remove ethanol, so as to obtain a mixed oxide powder;

[0073] The mixed oxide powder is used to prepare a water-based slurry with a solid phase weight fraction of 50%, 4% glycerol and 1.5% polyethylene glycol dispersant are added according to the weight ratio, high-energy ball milling is used for mixing, the rotating speed is 2000r / min, and the mixing time is 12 hours, so as to obtain a mixed slurry 1;

[0074] Step S2: Pour the mixed slurry 1 prepared in step S1 into a pre-cooled polytetrafluoroethylene mold, i.e., a freezing mold 2, and perform directional freezing at a liquid nitrogen temperature for 6 minutes, thereby obtaining a directionally grown frozen body;

[0075] Step S3: Place the frozen body in an environment at -50°C and a vacuum degree of <10 Pa, and perform freeze-drying for 48 hours, so that ice crystals are completely sublimated, thereby obtaining a ceramic green body in a lamellar structure;

[0076] Step S4: Heat the dried ceramic green body in a lamellar structure to 800°C at a heating rate of 5°C / min under normal pressure, keep the temperature for 2 hours, and then heat to 1200°C at a heating rate of 3°C / min, keep the temperature for 2 hours, so that a ceramic bulk body with a single spinel structure is obtained through a solid-phase reaction, and the lamellar structure is densified and strengthened;

[0077] Step S5: Mechanically separate and break the ceramic bulk body with a lamellar structure along the lamellar direction, thereby obtaining a negative temperature coefficient thermosensitive ceramic sheet.

[0078] According to the preparation method of the embodiment, a negative temperature coefficient thermosensitive ceramic sheet similar to the structure of Example 1 can be prepared, without obvious shrinkage holes or transgranular fracture, with high density, and meeting the requirements of subsequent use processes.

[0079] Example 3

[0080] A water-based slurry with a solid-phase content of 50% metal oxide for preparing a negative temperature coefficient thermosensitive ceramic sheet is prepared as shown in Figure 1 , according to the following steps:

[0081] Step S1: Take high-purity MnO2, Ni2O3, and Co2O3 as initial raw materials, and weigh each oxide powder according to the stoichiometric ratio of Mn 2.25-x Ni 0.75 Co x O4 ((wherein, x = 1) ), use ethanol as a medium, and use a high-energy ball mill to perform wet mixing for 24 hours, and then place it in an air drying oven at 60°C for 12 hours to remove ethanol, thereby obtaining a mixed oxide powder;

[0082] Prepare a water-based slurry with a solid-phase weight fraction of 50% from the mixed oxide powder, add 7% of a glass-ceramic sintering aid and 1.5% of a polyethylene glycol dispersant according to the weight ratio, mix using a high-energy ball mill at a speed of 2000 r / min for 12 hours, thereby obtaining a mixed slurry 1;

[0083] Step S2: Pour the mixed slurry 1 prepared in step S1 into a pre-cooled polytetrafluoroethylene mold, i.e., a freezing mold 2, and perform directional freezing at a liquid nitrogen temperature for 6 minutes, thereby obtaining a directionally grown frozen body;

[0084] Step S3: Place the frozen green body in an environment of -50℃ and vacuum degree <10Pa for freeze drying for 48 hours to allow the ice crystals to completely sublimate and obtain a layered ceramic green body.

[0085] Step S4: Under normal pressure, the dried lamellar ceramic green body is heated to 800℃ at a heating rate of 5℃ / min and held for 2 hours. Then, it is heated to 1200℃ at a heating rate of 3℃ / min and held for 2 hours. A ceramic block with a single spinel structure is obtained through solid-state reaction, and the lamellar structure is densified and strengthened at the same time.

[0086] Step S5: The ceramic block with a layered structure is mechanically separated and fractured along the layer direction to obtain a negative temperature coefficient thermistor ceramic sheet.

[0087] According to the preparation method of this embodiment, a sheet structure similar to that of Example 1 can be obtained, but with higher density and structural strength than that of Example 1, which meets the requirements of subsequent use processes.

[0088] Comparative Example 1

[0089] Preparation of negative temperature coefficient thermistor ceramic sheets with a water-based slurry solid content of 50% metal oxide, such as... Figure 1 As shown, follow these steps:

[0090] Step S1: Using high-purity MnO2, Ni2O3, and Co2O3 as initial raw materials, according to Mn 2.25-x Ni 0.75 Co x O4 (where x=1) was weighed according to the stoichiometric ratio of each oxide powder. Using ethanol as the medium, the powder was wet-milled and mixed in a high-energy ball mill for 24 h. The powder was then placed in an air drying oven and dried at 60°C for 12 h to remove the ethanol, thus obtaining the mixed oxide powder.

[0091] A water-based slurry was prepared by mixing oxide powder with a solid phase weight fraction of 50%. 1.5% polyethylene glycol dispersant was added by weight, and the mixture was mixed using a vacuum degassing machine for 2 minutes to obtain mixed slurry 1.

[0092] Step S2: Pour the mixed slurry 1 obtained in step S1 into a pre-cooled polytetrafluoroethylene mold, i.e., a freezing mold 2, and perform directional freezing at liquid nitrogen temperature for 6 minutes to produce a directionally grown frozen preform.

[0093] Step S3: Place the frozen green body in an environment of -50℃ and vacuum degree <10Pa for freeze drying for 48 hours to completely biodegrade the ice crystals and obtain a layered ceramic green body.

[0094] Step S4: Under normal pressure, the dried sheet-like ceramic green body is heated to 800°C at a heating rate of 5°C / min and held for 2 hours. Then, it is heated to 1200°C at a heating rate of 3°C / min and held for 2 hours. Through solid-phase reaction, a sheet-like ceramic block with a single spinel structure is obtained.

[0095] According to the preparation method of this embodiment, a ceramic block with a structure similar to that of Example 1 was obtained, but it has obvious cracks and lower structural strength compared to Example 1, which does not meet the requirements of the subsequent negative temperature coefficient thermistor ceramic sheet process.

[0096] Comparative Example 2

[0097] Preparation of negative temperature coefficient thermistor ceramic sheets with a water-based slurry solid content of 45% metal oxide, such as... Figure 1 As shown, follow these steps:

[0098] Step S1: Using high-purity MnO2, Ni2O3, and Co2O3 as initial raw materials, according to Mn 2.25-x Ni 0.75 Co x O4 (where x=1) was weighed according to the stoichiometric ratio of each oxide powder. Using ethanol as the medium, the powder was wet-milled and mixed in a high-energy ball mill for 24 h. The powder was then placed in an air drying oven and dried at 60°C for 12 h to remove the ethanol, thus obtaining the mixed oxide powder.

[0099] A water-based slurry was prepared by mixing the mixed oxide powder at a solid phase weight fraction of 45%. The mixture was then mixed using a high-energy ball mill at a speed of 2000 r / min for 12 hours to obtain mixed slurry 1.

[0100] Step S2: Pour the mixed slurry 1 obtained in step S1 into a pre-cooled polytetrafluoroethylene mold, i.e., a freezing mold 2, and perform directional freezing at liquid nitrogen temperature for 6 minutes to produce a directionally grown frozen green body.

[0101] Step S3: Place the frozen green body at -50℃ and a vacuum degree <10Pa for freeze drying for 48 hours to allow the ice crystals to completely sublimate and obtain a layered ceramic green body.

[0102] Step S4: The dried lamellar ceramic green body is heated to 800℃ at a heating rate of 5℃ / min and held for 2 hours. Then it is heated to 1200℃ at a heating rate of 3℃ / min and held for 2 hours. A ceramic with a single spinel structure is obtained through solid-state reaction.

[0103] According to the preparation method of the present embodiment, a ceramic body structure different from that of Example 1 is prepared, which has cross-linking between the internal ceramic sheet layers, presents an irregular porous structure, and has loose structure and low density, which does not meet the requirements of subsequent use processes.

[0104] The above-described specific embodiments further specifically describe the purposes, technical solutions and beneficial effects of the present disclosure, and it should be understood that the above-described specific embodiments are merely specific embodiments of the present disclosure and are not intended to limit the present disclosure. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present disclosure shall be included in the protection scope of the present disclosure.

Claims

1. A method for preparing a negative temperature coefficient thermistor ceramic sheet, comprising: Additives are added to an aqueous slurry containing metal oxides, and after mixing, a mixed slurry is obtained. The metal oxides include MnO2, Ni2O3, and Co2O3, and the solid weight fraction of the metal oxides in the aqueous slurry is 45%~55%. The solid weight of the aqueous slurry is 100%. The additives include 0.5%~1.5% polyethylene glycol and 1%~4% glycerol, or the additives include 0.5%~1.5% polyethylene glycol and 2%~10% glass-ceramic sintering aid. The mixed slurry is directionally frozen, causing ice crystals to grow directionally from the bottom of the mixed slurry towards the liquid surface, to obtain a frozen preform; The frozen green body is freeze-dried to sublimate the ice crystals, resulting in a layered ceramic green body. The ceramic green body is sintered at atmospheric pressure to obtain a ceramic block with a single spinel structure, wherein the ceramic block is in the form of lamellar pieces; The negative temperature coefficient thermistor ceramic sheet is obtained by mechanically separating the ceramic block along the lamellar direction.

2. The preparation method according to claim 1, wherein, The directional freezing operation includes: The mixed slurry is poured into a pre-cooled freezing mold, and a freezing gradient from the bottom of the mixed slurry to the liquid surface is applied to the mixed slurry using a refrigerant as a cold source, so that the ice crystals grow in a directional manner to obtain a frozen blank.

3. The preparation method according to claim 1, wherein, The mixing ratio of MnO2, Ni2O3 and Co2O3 is Mn 2.25- x Ni 0.75 Co x The stoichiometric ratio of O4, where 0.8 ≤ x ≤ 1.

2.

4. The preparation method according to claim 1 or 3, further comprising: MnO2, Ni2O3 and Co2O3 were mixed as initial raw materials, and ethanol or water was used as a medium. The mixture was then wet-milled and dried to remove ethanol or water, resulting in a dried mixture. The dried mixture is dispersed in water according to a preset solid weight fraction to obtain the water-based slurry containing metal oxides.

5. The preparation method according to claim 1, wherein, The freeze-drying conditions include: a drying temperature of -40℃ to -50℃, a vacuum degree of less than 10Pa, and a drying time of 24 to 48 hours.

6. The preparation method according to any one of claims 1 to 3 and 5, wherein, The atmospheric pressure sintering process involves heating the temperature to 800°C at a rate of 5°C / min under 1 standard atmosphere, holding it at that temperature for 2 hours, and then heating it to 1200~1400°C at a rate of 3°C / min, holding it at that temperature for 2 hours.

7. The preparation method according to any one of claims 1 to 3 and 5, wherein, The mechanical separation method is mechanical cutting.

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