Preparation method of strontium titanate heat source core block

Abstract

The application relates to a preparation method of a strontium titanate heat source core block, which comprises the following steps: mixing strontium carbonate and titanium dioxide by adopting a wet mixing mode to obtain slurry; drying the slurry, and then performing first heat treatment to obtain a mixture; performing cold pressing forming on the mixture to obtain a green body; and performing second heat treatment on the green body to obtain the strontium titanate core block. In the preparation method, a strontium titanate conversion process is reduced to three steps of mixing, drying and heat treatment, so that the process flow is greatly shortened. Moreover, the cold pressing forming can be performed in an oil press, the related complex equipment and precise operation are omitted, and the production efficiency is improved.

Description

Technical Field

[0001] This invention relates to radioactive source cores, and particularly to the preparation of strontium titanate thermal source cores. Background Technology

[0002] Strontium-90 is a long-lived, high-energy beta emitter used as an energy source for isotopic power supplies. It can be used to create long-lived isotopic heat / power sources, which are used abroad to power unmanned scientific stations. Strontium-90 compounds used as fuel need to meet requirements for power density, thermal stability, and solubility (especially in seawater). Studies have shown that strontium titanate (SrTiO3) offers the most ideal combination of performance characteristics. It has very low solubility in water, good thermal stability, and a power density of 0.7–0.9 W / cm³. 3 This is also sufficient to meet the design requirements of the thermal / power source. Therefore, to date, SrTiO3 is the most ideal chemical form for the manufacture of Sr-90 thermal / power sources.

[0003] Strontium titanate is a cubic white crystal with a typical perovskite crystal structure. Its molar mass (based on the natural abundance of each element) is 183.49 g / mol, and its melting point is as high as 2060℃. Strontium titanate is a widely used electronic functional ceramic material with advantages such as high dielectric constant, low dielectric loss, and good thermal stability, making it widely used in the electronics, machinery, and ceramics industries. Simultaneously, as a functional material, it possesses unique electromagnetic properties and redox catalytic activity, finding wide application in photocatalysis fields such as photocatalytic water splitting for hydrogen production, photocatalytic degradation of organic pollutants, and photochemical cells. Therefore, the synthesis process of strontium titanate has been extensively explored.

[0004] The commonly used synthesis method was developed in the 1950s by Leon Merker and Langtry E. Lynd at the National Lead Company in the United States. High-purity titanium tetrachloride, strontium chloride, and high-purity oxalic acid solution react at 60–70 °C for 1 hour to obtain strontium oxalate oxytitanium [SrTiO(C₂O₄)₂·4H₂O]; the strontium oxalate oxytitanium oxytitanium is then washed, dried, and calcined to obtain strontium titanate powder. The reaction equation is as follows:

[0005] SrCl2+TiCl4·2H2C2O4+5H2O→SrTiO(C2O4)2·4H2O↓+6HCl

[0006] SrTiO(C2O4)2·4H2O→SrTiO3+4H2O+2CO2↑+2CO↑

[0007] In the preparation of the heat source pellets, strontium-90 raw material supplied in the form of strontium carbonate (SrCO3) is mainly used. For example, Oak Ridge National Laboratory (ORNL) in the United States has developed a hot-pressing sintering process for preparing heat source pellets. Nitric acid is used to dissolve... 90 SrCO3 is obtained 90 Sr(NO3)2; A slurry is prepared by adding 5% excess TiO2 (stoichiometric ratio) to a mixed solution of (NH4)2CO3 and ammonia, and then adding this slurry to... 90 Obtained from Sr(NO3)2 solution 90 SrCO3·TiO2 precipitate. The precipitate is calcined at 1000℃ to form... 90 SrTiO3 was hot-pressed and sintered at 1400℃ and 10000psi (68.95MPa) to obtain a heat source core of a certain size. The reaction equation is as follows:

[0008] SrCO3+2HNO3→Sr(NO3)2+H2O+CO2↑

[0009] Sr(NO3)2+(NH4)2CO3+TiO2→SrCO3·TiO2+2NH4NO3

[0010] SrCO3·TiO2→SrTiO3+CO2↑

[0011] The above process can achieve mass production of SrTiO3 heat source core blocks, but it involves many steps and is quite complex, especially since it requires hot pressing and sintering, which places high demands on instruments and equipment and is not convenient to be carried out in a hot chamber.

[0012] Therefore, improvements are still needed in the preparation of the strontium titanate heat source core. Summary of the Invention

[0013] The purpose of this invention is to provide an improved method for preparing a strontium titanate heat source core, which can reduce the preparation process, improve production efficiency, reduce the requirements for process equipment, and reduce construction and use costs.

[0014] The method for preparing the strontium titanate heat source chip proposed in this application includes:

[0015] Strontium carbonate (SrCO3) and titanium dioxide (TiO2) were mixed using a wet mixing method to obtain a slurry;

[0016] The slurry is dried and then subjected to a first heat treatment to obtain a mixture;

[0017] The mixture is cold-pressed to obtain a green body; and

[0018] The green blank is subjected to a second heat treatment to obtain the strontium titanate core block.

[0019] In this application, by using a wet mixing method, the strontium nitrate solution obtained by dissolving strontium carbonate in nitric acid is directly mixed with TiO2 to obtain a slurry, thereby achieving high uniformity, which is beneficial for subsequent processing.

[0020] In some embodiments, the wet mixing process includes dissolving SrCO3 fully in an excess of HNO3 solution to obtain an SrCO3 solution, and then adding TiO2 to obtain the slurry.

[0021] In this application, an excess of HNO3 is added to ensure that SrCO3 can be fully dissolved, while the excess HNO3 will volatilize during the drying process and will not affect the quality of the product.

[0022] In some embodiments, the molar concentration of the HNO3 solution is between 5.9 mol / L and 6.4 mol / L. When the concentration of the HNO3 solution is too high, the reaction rate with the carbonate will be too fast, rapidly generating a large amount of gas, which may cause splashing. When the nitric acid concentration is too low, the amount of solution required to completely dissolve the carbonate will increase, and since natural evaporation drying is used, the drying time will also increase. The given concentrations are suitable values ​​obtained through practical experience.

[0023] In some embodiments, 300-350 mL of HNO3 solution is added for every 100 g of SrCO3.

[0024] In some embodiments, the amount of TiO2 used is at least 110% of its stoichiometry. In this application, in order to increase the conversion rate of Sr during the reaction, the amount of TiO2 used is 10% excess by stoichiometry.

[0025] In some embodiments, the drying is carried out in situ at room temperature by evaporation, for example, for 6 to 8 hours. Considering the thermal effects on the raw material... 90 The heat generated by the decay of Sr accelerates evaporation. Therefore, evaporation at room temperature is used to dry the slurry. For cold experiments, the evaporation time may be longer, but tests have shown that it can still be evaporated to a sufficient dryness. This drying method can be carried out directly in situ after the raw materials are mixed, without the need for additional equipment or operations.

[0026] In some embodiments, the first heat treatment is a thermal decomposition treatment, including: heating to a temperature of ≥700°C at a heating rate of 50 to 200°C / h, and holding at that temperature for 0.5 to 4 hours to convert SrCO3 into strontium oxide (SrO).

[0027] In some embodiments, the cold pressing is performed at room temperature under a pressure of 40 MPa to 180 MPa. The cold pressing of this application can be performed using an automatically controlled hydraulic press. Compared with the hot pressing sintering method used in existing processes, it eliminates the need for complex equipment and precision operations, thus improving production efficiency. Furthermore, the green body obtained through cold pressing is composed of SrO and TiO2.

[0028] In some embodiments, the second heat treatment is calcination, comprising heating to 1100–1400°C at a heating rate of 50–200°C / h and holding at that temperature for 2–8 hours. The green billet undergoes a solid-state reaction between strontium oxide and titanium dioxide through calcination to obtain a SrTiO3 heat source core. During this process, the sintering shrinkage and densification of the strontium titanate core are achieved.

[0029] This invention reduces the strontium titanate conversion process to three steps: mixing, drying, and heat treatment. Wet mixing and drying can be carried out in a metal container, and the first heat treatment is carried out in a ceramic crucible or ceramic boat, which greatly reduces the process flow.

[0030] The reaction equation for this application is as follows.

[0031] SrCO3+2HNO3→Sr(NO3)2+H2O+CO2↑

[0032] 2Sr(NO3)2→2SrO+4NO2↑+O2↑

[0033] SrO + TiO2 → SrTiO3

[0034] The sintering of the core requires a muffle furnace, rather than a large, heavy, and complex hot press furnace, which can greatly reduce the construction cost of the hot chamber.

[0035] According to the non-radiative strontium titanate cold test, the source core density obtained in this application can reach more than 70% TD, and the strontium titanate conversion rate can reach more than 95%. Attached Figure Description

[0036] Figure 1 A flowchart of the preparation method according to this application is shown. Detailed Implementation

[0037] The following detailed description discusses exemplary embodiments. The specific embodiments included herein should not be construed as limiting the invention. Furthermore, while specific language may be used to describe features, actions, and / or structures in the embodiments described herein, the claims are not limited to the described features, actions, and / or structures. Those skilled in the art will understand that other embodiments, including improvements, are within the spirit and scope of the invention.

[0038] Throughout this specification, unless otherwise specified, the terminology used herein should be understood as having the meaning as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0039] The present invention will be described in more detail below through embodiments. It should be understood that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially. The raw materials used in the embodiments are analytical grade, with a purity of ≥99%.

[0040] Example

[0041] An HNO3 solution of approximately 6.4 mol / L is prepared by mixing 200 mL of 68% HNO3 with 280 mL of deionized water.

[0042] Take 100g of SrCO3 and slowly add it to 350mL of prepared HNO3 solution while stirring. Stir until the solid is completely dissolved and no more bubbles are produced.

[0043] Take 60g of TiO2, place it in an open metal container, and slowly drip the solution obtained in the previous step into the container until it is completely drained.

[0044] Let stand at room temperature and evaporate until fully dry.

[0045] The dried solid was ground into a fine powder using a mortar and pestle, placed in a crucible, and then heated to 700°C in a muffle furnace at a heating rate of 50°C / h. The mixture was kept at this temperature for 2 hours to allow for thermal decomposition, and then allowed to cool naturally to room temperature.

[0046] Transfer the product The alloy mold is then used to cold press the alloy into shape at a pressure of 60 MPa using a hydraulic press.

[0047] The green body obtained by cold pressing was placed into a ceramic boat and then placed in a muffle furnace for calcination. It was heated to 1200℃ at a heating rate of 50℃ / h and held for 4h. Then it was naturally cooled to room temperature to obtain 115g of strontium titanate heat source core block.

[0048] Test Example. Characterization of the Obtained Material

[0049] The strontium titanate heat source chip obtained in the example was verified by a cold experiment, and the following test results were obtained.

[0050] project <![CDATA[Density / g·cm -3 > Strontium titanate conversion rate / % Example 4.53 (88.6% TD) 99.8

[0051] Although this disclosure has been described with reference to specific exemplary embodiments thereof, many different variations, modifications, etc. will become apparent to those skilled in the art.

[0052] By studying the accompanying drawings, the disclosure, and the appended claims, those skilled in the art can understand and implement variations of the disclosed embodiments in the practice of this disclosure.

Claims

1. A method for preparing a strontium titanate heat source chip, comprising: Strontium carbonate and titanium dioxide were mixed using a wet mixing method to obtain a slurry; The slurry is dried and then subjected to a first heat treatment to convert strontium nitrate into strontium oxide, resulting in a mixture; The mixture is cold-pressed to obtain a green body; as well as The green billet is subjected to a second heat treatment to obtain the strontium titanate core block. The wet mixing process includes dissolving strontium carbonate in an excess of HNO3 solution to obtain a strontium nitrate solution, and then adding titanium dioxide to obtain the slurry. The cold pressing is performed at room temperature under a pressure of 40 MPa to 180 MPa.

2. The preparation method according to claim 1, wherein, The molar concentration of the HNO3 solution is between 5.9 mol / L and 6.4 mol / L.

3. The preparation method according to any one of claims 1 to 2, wherein, The amount of titanium dioxide used is at least 110% of its stoichiometry.

4. The preparation method according to any one of claims 1 to 3, wherein, The drying process is carried out in situ at room temperature by evaporation.

5. The preparation method according to any one of claims 1 to 4, wherein, The first heat treatment is a thermal decomposition treatment, which includes heating to a temperature of ≥700°C at a heating rate of 50~200 °C / h and holding at that temperature for 0.5 hours to 4 hours.

6. The preparation method according to any one of claims 1 to 5, wherein, The second heat treatment is calcination, which includes heating to 1100-1400 °C at a heating rate of 50-200 °C / h and holding at that temperature for 2 to 8 hours.

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