Preparation method and application of low expansion ceramics

Abstract

The present invention belongs to the field of ceramic technology, and specifically relates to a preparation method and application of low-expansion ceramics. Preparation steps: pre-mixing ceramic powder and adhesive, kneading, extruding, granulating to obtain particles, injecting the particles into a mold, injection molding to obtain injection green body, degreasing, sintering, and obtaining low-expansion ceramics, wherein the ceramic powder includes alumina and eucryptite. Low-expansion ceramics are used in glass sealing processes, and the glass sealing process includes placing glass and metal into a mold and sintering them together, and the material of the mold is the prepared low-expansion ceramic. The low-expansion ceramics prepared by the present invention have high strength and low thermal expansion coefficient, can replace conventional graphite molds, and have good demolding effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of ceramics, and in particular relates to a preparation method and application of low-expansion ceramics. Background Art

[0002] Currently, DM-305 glass and Kovar alloy are primarily used for sealing in glass sealing applications. Traditionally, dry-pressed glass blanks and Kovar alloys are assembled in a graphite mold and then melt-sealed at nearly 1000°C. This glass sealing process presents several major issues: First, the graphite mold material is relatively loose, and graphite easily adheres to the glass surface, affecting the product's electrical insulation properties. Second, the dry-pressed glass blanks used for sealing exhibit poor density and dimensional accuracy, making precision control difficult during assembly and sintering. This, combined with numerous internal bubbles in the glass, severely impacts the pass rate and reliability of sealing products. Therefore, improvements to this sealing process are necessary. To address these issues, we have developed a ceramic material whose expansion coefficient is adjusted using eucryptite. Summary of the Invention

[0003] The present invention addresses the deficiencies of the prior art and provides a method for preparing low-expansion ceramics. Furthermore, the present invention also provides applications thereof based on the method.

[0004] The technical solutions of the present invention are as follows:

[0005] In a first aspect, a method for preparing a low expansion ceramic comprises the following steps:

[0006] The ceramic powder and the binder are pre-mixed, kneaded, extruded, and granulated to obtain particles, which are then injected into a mold, injection-molded to obtain an injection green body, degreased, and sintered to obtain a low-expansion ceramic, wherein the ceramic powder includes alumina and eucryptite.

[0007] In one embodiment, the alumina includes α-Al 2 O 3 and the eucryptite includes β-eucryptite.

[0008] In one embodiment, the weight ratio of alumina to eucryptite is 1-3:1.

[0009] It should be noted that the above technical solution is used to introduce the positive expansion phase α-Al2O3 and the negative expansion phase β-eucryptite to construct a two-phase eutectic system; by adjusting the ratio, a composite structure is formed in which the negative expansion phase is dispersed and evenly embedded in the rigid main crystal phase matrix.

[0010] In one embodiment, the binder includes polycaprolactone, polypropylene carbonate, polyethylene glycol, and glyceryl stearate.

[0011] It should be noted that the present invention introduces the above components, which are different from conventional paraffin-based binders. Polycaprolactone provides backbone flexibility and a low melting point, which can improve molding plasticity and injection stability. Polypropylene carbonate can enhance the wettability and dispersibility of the powder. Polyethylene glycol can be removed in advance during later degreasing, forming microchannels within the material and reducing internal stress and sintering defects during thermal degreasing. Stearyl glycerol has lubricating and demoulding effects, preventing adhesion to the mold surface and reducing mold loss.

[0012] In one embodiment, at 273-1273K, the average linear expansion coefficient of aluminum oxide is 8.8×10 -6 / K, the average linear expansion coefficient of eucryptite is -6.2×10 -6 / K.

[0013] It should be noted that the composite material of alumina and β-eucryptite has two main advantages. On the one hand, at 273-1273K, the average linear expansion coefficient of alumina is 8.8×10 -6 / K, the average linear expansion coefficient of β-eucryptite is -6.2×10 -6 By adjusting the ratio of α / K, the thermal expansion coefficient can be varied to achieve a coefficient close to that of the glass blank and Kovar alloy, thus achieving a matching seal. β-Eucryptite, on the other hand, is one of the few materials with a large negative thermal expansion coefficient, exhibiting excellent thermal stability and thermal shock resistance. The additive nature of thermal expansion coefficients allows materials with different thermal expansion coefficients to be combined to create materials with very low or even zero thermal expansion coefficients, minimizing internal stress in high-temperature materials and increasing their thermal shock resistance.

[0014] In one embodiment, the mixing parameters include: mixing temperature of 120-130° C., rotation speed of 30-50 rpm, and mixing time of 30-50 min.

[0015] In one embodiment, the injection molding parameters include: injection temperature of 145-155° C., mold temperature of 40-50° C., injection pressure of 60-90 MPa, and holding time of 8-15 s.

[0016] In one embodiment, degreasing comprises the following steps:

[0017] The injection green body is immersed in an organic solvent; after soaking, it is dried and then placed in a degreasing furnace for degreasing at 20-500°C under nitrogen protection.

[0018] In one embodiment, the degreasing at 20-500°C comprises the following steps:

[0019] Increase the temperature from 20°C to 200°C at a heating rate of 1.5°C / min, and keep warm for 1-3 hours; increase the temperature from 200°C to 280°C at a heating rate of 1°C / min, and keep warm for 1-3 hours; increase the temperature from 280°C to 370°C at a heating rate of 1°C / min, and keep warm for 1-3 hours; increase the temperature from 370°C to 500°C at a heating rate of 1°C / min, and keep warm for 1-3 hours.

[0020] It should be noted that step-by-step degreasing, first using an organic solvent to remove polyethylene glycol and form microchannels, can reduce the stress of high-temperature degreasing.

[0021] In one embodiment, the sintering temperature is 1000-1500°C.

[0022] Secondly, low expansion ceramics are used in glass sealing processes. The glass sealing process includes placing glass and metal into a mold and sintering them together. The mold material is low expansion ceramics, and the low expansion ceramics are prepared according to the aforementioned preparation method of low expansion ceramics.

[0023] It should be noted that graphite molds are relatively loose and easily adhere to the glass surface, affecting the product's electrical insulation. Ceramic molds offer excellent heat resistance, high strength, excellent insulation, and a long service life, without the risk of powder shedding. As long as the thermal expansion coefficient can be matched to that of glass and Kovar alloy, ceramic molds can completely replace the currently used graphite molds.

[0024] Furthermore, the glass sealing process includes the following steps:

[0025] Prepare the glass blank: dry the sealing glass powder, add lubricant, and then put it into the mold for dry pressing. The dry pressing molding parameters are: pressure 80-100MPa, holding time 40-60s, and then preheat the molded glass blank at 300-500℃ to obtain the glass blank.

[0026] Assembly: The glass blank is placed in a mold made of low-expansion ceramic and then covered with metal. A light pressure is applied to complete the mold.

[0027] Sintering: Place the entire mold in a hot press furnace, gradually heat it from room temperature to 880°C, and keep it warm; cool it down to ≤180°C, and then demold it to obtain a sealed part, that is, a metal-glass sealed part.

[0028] It should be noted that in the above technical solution, the present invention also improves the preparation of the glass blank in the glass sealing process. The lubricant can reduce friction between particles, preheat in advance, discharge gas, and reduce bubbles inside the glass.

[0029] Furthermore, the glass powder includes DM-305, and the metal includes Kovar alloy.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. When preparing low expansion ceramics, the present invention introduces alumina and eucryptite. Alumina is a positive expansion phase, providing structural strength, high hardness, and high temperature resistance, and serves as a rigid skeleton. Eucryptite is a negative expansion phase, which can reduce the thermal expansion coefficient of the entire material. The prepared low expansion ceramic has a low thermal expansion coefficient and high strength. The thermal expansion coefficient (CTE) is reduced from the traditional value of 8.8×10 -6 / K decreased to 5.23×10 -6 / K (25-700°C), and at the same time, high-density sintering with a relative density of ≥94.62% is achieved at low temperature (the sintering temperature in the embodiment can be reduced to 1310°C).

[0032] 2. The present invention prepares a multi-component composite binder. Polycaprolactone serves as the main structural framework, which has strong compatibility. Polypropylene carbonate can adjust flexibility and improve the material's molding toughness. Polyethylene glycol can serve as a pore-forming agent and is removed first during degreasing, forming microchannels, facilitating subsequent high-temperature degreasing. Glyceryl stearate can improve the material's fluidity during injection.

[0033] 3. The low-expansion ceramic of the present invention can be used in glass sealing processes as a mold during sintering. Compared with traditional graphite molds, the low-expansion ceramic of the present invention has a better match with the thermal expansion coefficient of DM-305 glass and Kovar alloy. It will not bond with the glass even at higher demolding temperatures, enabling lossless demolding. The mold has a long life and is reusable, reducing production costs. DETAILED DESCRIPTION

[0034] To better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below with reference to specific examples. The experimental methods used below are conventional methods unless otherwise specified, and the materials and reagents used are all commercially available unless otherwise specified.

[0035] It should be noted that thermal expansion refers to the phenomenon in which an object's volume or length increases with increasing temperature. The relative change in volume or length caused by a 1°C temperature increase is called the object's volumetric expansion coefficient or linear expansion coefficient. Therefore, the thermal expansion coefficient can be further divided into volumetric expansion coefficient and linear expansion coefficient. This application focuses on the linear expansion coefficient within the thermal expansion coefficient, and the thermal expansion coefficient mentioned here can be understood as the linear expansion coefficient.

[0036] Method 1

[0037] The invention provides a method for preparing low-expansion ceramics.

[0038] Examples 1 to 3 are given below. The raw materials refer to Table 1, and the preparation method refers to the following specific steps:

[0039] Step 1: Select the raw materials by weight, referring to Table 1 for the number of parts and the following specific raw materials:

[0040] (1) Ceramic powder:

[0041] Alumina: α-Al2O3, positive expansion main crystal phase, average linear expansion coefficient is 8.8×10 -6 / K (273-1273K);

[0042] Eucryptite: β-eucryptite (β-LAS), a negative expansion phase with an average linear expansion coefficient of -6.2×10 -6 / K (273-1273K);

[0043] (2) Binder: 10 parts polycaprolactone (PCL, molecular weight Mn20000), 5 parts polypropylene carbonate, 4 parts polyethylene glycol, 1 part glyceryl stearate.

[0044] For polyethylene glycol, choose PEG-600, and for polypropylene carbonate, buy Covestro's Desmophen® C2200. For α-Al2O3, choose a particle size of 0.2 μm, and for β-eucryptite, choose a particle size of <5 μm.

[0045] Step 2: Using powder injection molding process:

[0046] (1) Feeding: The ceramic powder and the adhesive are pre-mixed by weight, and then hot-mixed in a twin-screw internal mixer. The specific parameters are: mixing temperature of 120-130 °C, rotation speed of 40 rpm, mixing time of 40 min, extrusion through a die after mixing, granulation after cooling, and obtaining granules.

[0047] (2) Injection: Use an injection molding machine to inject the particles into the mold, set the injection temperature range to 145-155°C, the mold temperature to 40-50°C, the injection pressure to 60-90 MPa, and the holding time to 8-15s to obtain an injection green body.

[0048] (3) Degreasing: Immerse the injection green body in 60℃ n-hexane for 12-24h to remove PEG-600 and form early microchannels; dry it after soaking, and then put it into the degreasing furnace. Under nitrogen protection, heat it from 20℃ to 200℃ at a heating rate of 1.5℃ / min and keep it warm for 2h; heat it from 200℃ to 280℃ at a heating rate of 1℃ / min and keep it warm for 1h; heat it from 280℃ to 370℃ at a heating rate of 1℃ / min and keep it warm for 1h; heat it from 370℃ to 500℃ at a heating rate of 1℃ / min and keep it warm for 2h to obtain the degreased green body.

[0049] (4) Sintering: The degreased green body is heated to 1310°C at a heating rate of 3°C / min, kept at this temperature for 90 min, and then cooled to obtain low expansion ceramics.

[0050] Table 1

[0051]

[0052] Detection:

[0053] In this application, the linear expansion coefficient in the thermal expansion coefficient is examined, and the thermal expansion coefficient mentioned can be understood as the linear expansion coefficient.

[0054] (1) The coefficient of thermal expansion (CTE) of the low expansion ceramics of Examples 1-3 in the range of 25-700°C was measured using a thermomechanical analyzer. The results were:

[0055] Example 1: 6.10×10 -6 / K;

[0056] Example 2: 5.61×10 -6 / K;

[0057] Example 3: 5.23×10 -6 / K.

[0058] (2) The relative density of the low expansion ceramics of Examples 1-3 was measured using an electronic density meter. The results were:

[0059] Example 1: 94.8%;

[0060] Example 2: 94.62%;

[0061] Example 3: 95.15%.

[0062] The above results show that the CTE of the low expansion ceramics prepared by the present invention can be reduced to 5.23×10 -6 / K, which is basically close to the thermal expansion coefficient of Kovar alloy and DM-305 glass blank, with a relative density of ≥94.62% and high strength.

[0063] The present invention adjusts the thermal expansion coefficient of the low-expansion ceramic by adjusting the weight fractions of alumina and eucryptite, thereby obtaining a thermal expansion coefficient close to that of a glass blank and a kovar alloy, and the material strength is high. Therefore, the low-expansion ceramic of the present invention can replace graphite molds in glass matching and sealing processes.

[0064] The following is a further test to see whether low expansion ceramics can be used in glass sealing processes.

[0065] Method 2

[0066] The low expansion ceramics of Example 1, Example 2 or Example 3 are injected using the same mold to form a ceramic mold cavity, thereby obtaining Example 4, Example 5 and Example 6 respectively.

[0067] At the same time, a graphite mold with the same size as the ceramic mold cavity was set as comparative example 1. The dimensions were 30×30×2.66 mm.

[0068] Low expansion ceramics are used in glass sealing processes. The specific steps are as follows:

[0069] Prepare the glass blank: dry the sealing glass powder (DM-305) at 120°C for 4 hours to remove moisture, add 0.5wt% zinc stearate as a lubricant, and then place it in a mold for dry pressing. The dry pressing molding parameters are: pressure 80-100MPa, holding time: 60s. Then preheat the molded glass blank at 400°C for 1h to discharge volatile gases and reduce sintering bubbles to obtain the glass blank.

[0070] Assembly: Place the glass blank into the ceramic mold cavity of Example 4, Example 5, or Example 6, or the graphite mold of Comparative Example 1, then cover with Kovar alloy (4J29). The Kovar alloy should be degreased, pickled, and polished beforehand. Apply a light pressure of 0.1 MPa to ensure a tight fit between the three components to complete the mold.

[0071] Sintering: Place the entire mold in a hot press furnace, heat it from room temperature to 150°C at a heating rate of 3°C / min, and keep it warm for 30 minutes; heat it to 500°C at a heating rate of 3°C / min, and keep it warm for 20 minutes; heat it to 880°C at a heating rate of 2°C / min, and keep it warm for 15 minutes; cool it down to a certain temperature, and then demold it to obtain a sealed part, i.e., a metal-glass sealed part.

[0072] Multiple parallel tests were conducted to record the cooling and demoulding temperature and demoulding conditions, and it was found that:

[0073] In Example 4, the temperature needs to be lowered to ≤140° C. before demoulding, and no bonding occurs during demoulding.

[0074] Example 5 required the temperature to be lowered to ≤160° C. before demoulding, and no adhesion occurred during demoulding.

[0075] Example 6 required the temperature to be lowered to ≤180°C before demoulding, and no bonding occurred during demoulding.

[0076] Comparative Example 1 required the temperature to be lowered to ≤100° C. before demoulding, and bonding occurred during demoulding.

[0077] Taking Example 4 as an example, the above is represented by "≤", which means that in multiple parallel tests, it was found that the temperature needs to be lowered to at least 140°C to achieve lossless demoulding and avoid adhesion between the mold and the glass at high temperature.

[0078] It can be seen from this that the ceramic mold cavity of Example 6 can be demolded at a higher temperature than that of the other examples, and its demolding effect is better at the same temperature.

[0079] The low-expansion ceramic materials used in the ceramic mold cavity of Example 4, Example 5, or Example 6 have different thermal expansion coefficients. As the CTE gradually decreases, the thermal matching between the mold and the glass sealing component after high-temperature sintering is significantly improved, thereby achieving lossless demolding at higher temperatures.

[0080] The ceramic mold made of low-expansion ceramic in this invention solves the problem of graphite molds being loose and prone to graphite adhering to glass surfaces, which affects the electrical insulation performance of the product. Ceramic molds made of low-expansion ceramic material can replace graphite molds within a certain demolding temperature range.

[0081] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A method for preparing low expansion ceramics, characterized in that: The following steps are involved: Ceramic powder and a binder are premixed, kneaded, extruded, and granulated to obtain particles, which are then injected into a mold, injection-molded to obtain an injection green body, degreased, and sintered to obtain a low-expansion ceramic, wherein the ceramic powder includes alumina and eucryptite; the alumina includes α-Al2O3, and the eucryptite includes β-eucryptite; the weight ratio of the alumina to the eucryptite is 1-3:1; and the binder includes polycaprolactone, polypropylene carbonate, polyethylene glycol, and glyceryl stearate.

2. The method for preparing a low expansion ceramic according to claim 1, characterized in that: At 273-1273K, the average linear expansion coefficient of aluminum oxide is 8.8×10 -6 / K, the average linear expansion coefficient of eucryptite is -6.2×10 -6 / K.

3. The method for preparing a low expansion ceramic according to claim 1, characterized in that: The mixing parameters include: mixing temperature of 120-130° C., rotation speed of 30-50 rpm, and mixing time of 30-50 min.

4. The method for preparing a low expansion ceramic according to claim 1, characterized in that: The injection molding parameters include: injection temperature of 145-155°C, mold temperature of 40-50°C, injection pressure of 60-90 MPa, and holding time of 8-15s.

5. The method for preparing a low expansion ceramic according to claim 1, characterized in that: Degreasing involves the following steps: The injection green body is immersed in an organic solvent; after soaking, it is dried and then placed in a degreasing furnace for degreasing at 20-500°C under nitrogen protection.

6. The method for preparing a low expansion ceramic according to claim 5, characterized in that: 20-500℃ degreasing includes the following steps: Increase the temperature from 20°C to 200°C at a heating rate of 1.5°C / min, and keep warm for 1-3 hours; increase the temperature from 200°C to 280°C at a heating rate of 1°C / min, and keep warm for 1-3 hours; increase the temperature from 280°C to 370°C at a heating rate of 1°C / min, and keep warm for 1-3 hours; increase the temperature from 370°C to 500°C at a heating rate of 1°C / min, and keep warm for 1-3 hours.

7. The method for preparing a low expansion ceramic according to claim 1, characterized in that: The sintering temperature is 1000-1500℃.

8. Low expansion ceramics are used in glass sealing process, characterized by: The glass sealing process includes placing glass and metal into a mold and sintering them together. The mold is made of low-expansion ceramic, which is prepared according to the preparation method of low-expansion ceramic according to any one of claims 1-7.

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