Methods to reduce the bulk density of nano-cerium oxide

Abstract

The application discloses a method for reducing the loose bulk density of nano cerium oxide, which comprises the following steps: 1) mixing an aqueous solution of an alkaline precipitant with an aqueous solution of cerium salt under stirring to obtain an initial mixture, and controlling the pH value of the initial mixture to be greater than or equal to 8, and then continuing to stir the mixture for 1-5 hours to obtain a reaction mixture; 2) placing the reaction mixture, and then removing the upper aqueous phase to obtain a lower layer of precipitate; drying the lower layer of precipitate at 175-240 DEG C to obtain dried solid; 3) washing the dried solid with water to obtain washed solid; and drying the washed solid at 175-240 DEG C to obtain nano cerium oxide. The method can reduce the loose bulk density of the obtained nano cerium oxide and improve the polishing quality of the obtained nano cerium oxide on silicon wafers.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for reducing the bulk density of nanometer cerium oxide. BACKGROUND

[0002] Cerium oxide (CeO2) as an important rare earth functional material, due to its excellent redox performance, high chemical stability and good mechanical properties, has a wide range of applications in catalysis, energy storage, sensors and polishing and other fields. Especially in the field of precision polishing, cerium oxide, due to its efficient and low-damage polishing effect on glass, silicon wafer and optical elements, has become one of the irreplaceable polishing materials.

[0003] The performance of the polishing material is closely related to its physical properties, and the bulk density is one of the key parameters affecting the polishing effect. Nanometer cerium oxide with low bulk density usually has higher porosity, larger specific surface area and better suspension, thereby showing better material removal rate (MRR) and better surface quality (low scratches and high smoothness) in the polishing process.

[0004] However, the bulk density of cerium oxide prepared by general traditional high-temperature solid-phase method or conventional precipitation method is high or the prepared cerium oxide is micron or submicron, which limits its application in high-end polishing. For example, a method for preparing rare earth oxide with low bulk density is reported, which comprises: (1) taking a rare earth chloride solution as a raw material; (2) microwave-assisted liquid-phase crystallization to obtain a precursor precipitate; (3) reacting at the precipitation temperature for 1-3 h; (4) heat filtering, washing and drying the precipitate; (5) thermal decomposition to obtain the product. SUMMARY

[0005] Therefore, the purpose of the present application is to provide a method for reducing the bulk density of nanometer cerium oxide. The method can reduce the bulk density of the obtained nanometer cerium oxide on the basis of obtaining nanometer cerium oxide. The polishing liquid formed by the obtained nanometer cerium oxide can obtain a silicon wafer with better surface quality when polishing the silicon wafer.

[0006] The present application achieves the above-mentioned purpose by adopting the following technical solutions.

[0007] On the one hand, the present application provides a method for reducing the bulk density of nanometer cerium oxide, comprising the following steps:

[0008] 1) Under stirring, mix an alkaline precipitant aqueous solution with a cerium salt aqueous solution to obtain an initial mixture, and control the pH value of the initial mixture to be greater than or equal to 8, and then continue to stir and react for 1-5 h to obtain a reaction mixture;

[0009] 2) the reaction mixture is left to stand, and then the upper aqueous phase is removed to obtain a lower precipitate; the lower precipitate is dried at 175-240℃ to obtain a dried solid;

[0010] 3) the dried solid is washed with water to obtain a washed solid; the washed solid is dried at 175-240℃ to obtain the nano ceria. This is conducive to reducing the bulk density of the obtained nano ceria, and reducing the tap density of the obtained nano ceria, and increasing the absolute value of the Zeta potential of the obtained nano ceria on the basis of obtaining the nano ceria; and the obtained nano ceria can be used as a polishing material, and the polishing liquid formed has a smaller surface roughness Ra, i.e. a higher smoothness, and a better surface quality.

[0011] The present application unexpectedly finds that the present application can control the pH value of the aqueous solution of the alkaline precipitant after mixing with the aqueous solution of the cerium salt, and control the reaction to obtain a reaction mixture, and directly separate the reaction mixture by standing and removing the aqueous phase (which can be removed by pouring or sucking), instead of filtering or centrifuging, which is more conducive to reducing the bulk density and tap density of the obtained nano ceria. The present application also finds that the present application does not use the drying at about 100℃ (such as 100℃, 110℃) commonly used in the prior art, but dries the lower precipitate after separation at a specific temperature (175-240℃), and omits the calcination step, which is conducive to directly obtaining nano ceria, and can also reduce the bulk density and tap density of the obtained nano ceria, and improve the polishing quality of the polishing liquid formed on the silicon wafer.

[0012] In the present application, the standing time in step 2) can be 2-5h, preferably 3-4h, and the drying time can be 5-12h, preferably 8-10h. The drying time in step 3) can be 4-12h, preferably 7-10h.

[0013] According to the method for reducing the bulk density of nano ceria of the present application, preferably, the aqueous solution of the alkaline precipitant is selected from one or more of an aqueous solution of an alkali metal hydroxide, an aqueous solution of an alkali metal carbonate, an aqueous solution of an alkali metal bicarbonate and ammonia water. This is conducive to forming a precursor of ceria.

[0014] In the present application, the alkali metal in the aqueous solution of the alkali metal hydroxide, the aqueous solution of the alkali metal carbonate and the aqueous solution of the alkali metal bicarbonate is selected from sodium or potassium.

[0015] According to one embodiment of the present application, the aqueous solution of the alkaline precipitant is selected from one of an aqueous solution of an alkali metal hydroxide, an aqueous solution of an alkali metal carbonate, an aqueous solution of an alkali metal bicarbonate and ammonia water.

[0016] Preferably, the aqueous solution of the alkaline precipitant is an aqueous solution of an alkali metal hydroxide. The aqueous solution of the alkali metal hydroxide is an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide.

[0017] Preferably, the aqueous solution of the alkaline precipitant is an aqueous solution of sodium hydroxide. This is advantageous for obtaining nanometer cerium oxide with higher purity, and for reducing the bulk density and tap density of the obtained nanometer cerium oxide and improving the polishing quality of a polishing liquid formed by the obtained nanometer cerium oxide on a silicon wafer.

[0018] Preferably, the concentration of the aqueous solution of the alkaline precipitant is 5-20 wt%. Preferably, the concentration of the aqueous solution of the alkaline precipitant is 10-15 wt%.

[0019] According to one specific embodiment of the present application, the aqueous solution of the alkaline precipitant is an aqueous solution of sodium hydroxide with a concentration of 10-15 wt%.

[0020] Preferably, the aqueous solution of the cerium salt is selected from one or more of an aqueous solution of cerium chloride, an aqueous solution of cerium nitrate and an aqueous solution of cerium sulfate; and the mass concentration of the aqueous solution of the cerium salt is 0.1-0.2 g / mL.

[0021] In the present application, the aqueous solution of the cerium salt is preferably selected from one of an aqueous solution of cerium chloride, an aqueous solution of cerium nitrate and an aqueous solution of cerium sulfate, and more preferably an aqueous solution of cerium nitrate.

[0022] In the present application, the mass concentration of the aqueous solution of the cerium salt can be 0.1-0.2 g / mL, and preferably 0.15-0.18 g / mL. The aqueous solution of the cerium salt can be formed by dissolving a water-soluble cerium salt in water, and heating can be applied during the dissolving. In the present application, the water-soluble cerium salt used can contain crystal water or not.

[0023] Preferably, in step 1), the aqueous solution of the alkaline precipitant is added dropwise to the aqueous solution of the cerium salt under stirring at 15-40℃ to obtain an initial mixture, and the pH value of the initial mixture is controlled to be 8-11; and then the stirring is continued at 15-75℃ for 1-5 h to obtain a reaction mixture. This is advantageous for reducing the bulk density and tap density of the obtained nanometer cerium oxide.

[0024] In the present application, the raw materials can be mixed at 25-30℃.

[0025] In the present application, the pH value of the initial mixture is preferably controlled to be 9-11, for example, 9, 10, 11. The reaction temperature of the initial mixture can be 15-75°C, for example, 15°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 75°C; the reaction time is preferably 2-4h, more preferably 2-3h. In some embodiments, the initial mixture can be reacted at 15-40°C. In other embodiments, the initial mixture can be reacted at 25-30°C.

[0026] According to the method for reducing the loose bulk density of nano cerium oxide of the present application, preferably, in step 2), the lower layer precipitate is dried at 180-200°C to obtain a dried solid.

[0027] In the present application, the temperature for drying the lower layer precipitate is preferably 180-190°C. This is advantageous for reducing the loose bulk density and tap density of the obtained nano cerium oxide and improving the polishing quality of the nano cerium oxide on silicon wafers.

[0028] According to the method for reducing the loose bulk density of nano cerium oxide of the present application, preferably, in step 3), the dried solid is washed with water for more than 2 times to obtain a washed solid; the washed solid is dried at 180-200°C and ground to obtain nano cerium oxide.

[0029] In the present application, the dried solid is preferably washed with deionized water, purified water or distilled water, more preferably deionized water. According to one specific embodiment of the present application, the dried solid is washed with water for 3 times. After washing, the obtained filter cake is filtered and washed again.

[0030] In step 3), the drying temperature is preferably 180-190°C, for example, 180°C, 185°C, 190°C.

[0031] According to the method for reducing the loose bulk density of nano cerium oxide of the present application, preferably, the particle size of the obtained nano cerium oxide is 50-120nm.

[0032] In the present application, the obtained nano cerium oxide is substantially spherical or spheroidal. In some embodiments, the particle size of the nano cerium oxide is 50-100nm. In other embodiments, the particle size of the nano cerium oxide is 80-110nm. In still other embodiments, the particle size of the nano cerium oxide is 90-100nm. The obtained nano cerium oxide has a loose bulk density of less than or equal to 1.63g / cm 3 , and a tap density of less than or equal to 2.5g / cm 3 . 3 3 .

[0033] ​The method for reducing the loose bulk density of nanometer cerium oxide of the present application can reduce the loose bulk density of the obtained nanometer cerium oxide, reduce the tap density thereof, and improve the Zeta potential of the obtained nanometer cerium oxide on the basis of obtaining nanometer cerium oxide. The nanometer cerium oxide obtained by the method is spherical or quasi-spherical, has a particle size of about 100 nm, and has uniform particle size distribution. The method of the present application omits the calcination step, is conducive to reducing the loose bulk density of the obtained nanometer cerium oxide, improving the polishing quality of silicon wafers, and can save costs. According to the preferred technical solution of the present application, the method comprises the following steps: adding sodium hydroxide solution dropwise into cerium salt aqueous solution, controlling the pH value to be alkaline, continuously reacting to obtain a reaction mixture; layering the reaction mixture by standing, removing the upper aqueous phase, drying the lower precipitate at a specific temperature, washing, and drying again to obtain nanometer cerium oxide with reduced loose bulk density. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 XRD patterns of the nanometer cerium oxide obtained in Example 1 and Comparative Example 1.

[0035] Figure 2 SEM patterns of the nanometer cerium oxide obtained in Example 1 and Comparative Example 1, wherein a pattern is the SEM pattern of the nanometer cerium oxide obtained in Comparative Example 1, and b pattern is the SEM pattern of the nanometer cerium oxide obtained in Example 1.

[0036] Figure 3 Zeta potential patterns of the nanometer cerium oxide obtained in Example 1 and Comparative Example 1. Figure 1 、 Figure 3 In the above description, Example refers to Example 1, and Comparative Example refers to Comparative Example 1.

[0037] Figure 4 Surface roughness Ra results of the polished silicon wafers, wherein a pattern is the polishing result of the polishing liquid formed by using the nanometer cerium oxide obtained in Comparative Example 1, and b pattern is the polishing result of the polishing liquid formed by using the nanometer cerium oxide obtained in Example 1. DETAILED DESCRIPTION

[0038] Some test methods are introduced as follows:

[0039] XRD: tested by using an X’Pert PRO X-ray diffractometer.

[0040] SEM: tested by using a ZEISS Sigma 500 field emission scanning electron microscope.

[0041] Surface roughness Ra: tested by using a Bruker Dimension Icon atomic force microscope.

[0042] Zeta potential: tested by using a Nano Brook 90Plus PALS.

[0043] Polishing test: Tegramin-25 from Stell was used for the test.

[0044] Loose density: GB / T 1482-2010 “Determination of loose density of metal powders” method was used for the test.

[0045] Tapped density: GB / T 5162-2016 “Determination of tapped density of metal powders” method was used for the test.

[0046] Example 1

[0047] 60 g of cerium nitrate hexahydrate was dissolved in 500 mL of deionized water, stirred for 10 min to obtain a cerium nitrate aqueous solution (i.e. a cerium salt aqueous solution).

[0048] 10wt% of sodium hydroxide aqueous solution was added dropwise to the above prepared cerium salt aqueous solution under stirring at room temperature 25℃ to obtain an initial mixture, and the pH value of the initial mixture was controlled to be 11, then the reaction was continued to stir for 2h at 25℃ to obtain a reaction mixture.

[0049] The reaction mixture was allowed to stand for 3h, and then the upper aqueous phase was removed to obtain a lower precipitate. The lower precipitate was dried at 180℃ for 10h to obtain a dried solid.

[0050] The dried solid was washed with deionized water for 3 times to obtain a washed solid. The washed solid was dried at 180℃ for 10h, and then ground to obtain a nano cerium oxide.

[0051] Comparative Example 1

[0052] 60 g of cerium nitrate hexahydrate was dissolved in 500 mL of deionized water, stirred for 10 min to obtain a cerium nitrate aqueous solution (i.e. a cerium salt aqueous solution).

[0053] 10wt% of sodium hydroxide aqueous solution was added dropwise to the above prepared cerium salt aqueous solution under stirring at room temperature 25℃ to obtain an initial mixture, and the pH value of the initial mixture was controlled to be 11, then the reaction was continued to stir for 2h at 25℃ to obtain a reaction mixture.

[0054] The reaction mixture was allowed to stand for 3h, and then the upper aqueous phase was removed to obtain a lower precipitate. The lower precipitate was dried at 180℃ for 10h to obtain a dried solid.

[0055] The dried solid was washed with deionized water for 3 times to obtain a washed solid. The washed solid was dried at 180℃ for 10h, and then ground to obtain a nano cerium oxide.

[0056] The powder cerium oxide was placed in a muffle furnace and calcined at 600°C for 2 hours, and after cooling, was ground to obtain nano cerium oxide.

[0057] Test experiment

[0058] 1. XRD test

[0059] The nano cerium oxide obtained in Example 1 and Comparative Example 1 was respectively tested by XRD, and the results are shown in FIG. 1. Figure 1 .

[0060] It can be seen from FIG. 1 that the XRD diffraction peaks of the two correspond to the cerium oxide diffraction peaks identified in the standard PDF card JCPDS No. 34-0394. The characteristic peaks of the nano cerium oxide obtained in Comparative Example 1 are relatively sharp, indicating that the material has better crystallinity. Figure 1 2. SEM test

[0061] The nano cerium oxide obtained in Comparative Example 1 was tested by SEM, and the results are shown in FIG. 2a. The nano cerium oxide obtained in Example 1 was tested by SEM, and the results are shown in FIG. 2b.

[0062] Figure 2 Figure 2

[0063] It can be seen from FIG. 2a and FIG. 2b that the nano cerium oxide obtained in Example 1 has a rough surface, a particle size of 50-100 nm, and the obtained cerium oxide is basically spherical or spherical-like. The nano cerium oxide obtained in Comparative Example 1 has a smooth surface, and a particle size of 50-100 nm. Figure 2 Figure 2 3. Test of loose bulk density and tap density

[0064] The nano cerium oxide obtained in Example 1 and Comparative Example 1 was respectively tested for loose bulk density and tap density, and Hausner value, and the results are shown in Table 1.

[0065] Table 1

[0066]

[0067] It can be seen from Table 1 that the loose bulk density of the nano cerium oxide obtained in Example 1 is less than that of the nano cerium oxide obtained in Comparative Example 1, and the tap density of the nano cerium oxide obtained in Example 1 is less than that of the nano cerium oxide obtained in Comparative Example 1. The Hausner value is a key indicator for evaluating the flowability of the sample, and the smaller the value, the better the flowability. It can be seen from Table 1 that the nano cerium oxide obtained in Example 1 has better flowability.

[0068] 4. Potential test

[0069]

[0070] ​​​​​The test potentials of the nano-cerium oxide obtained in Example 1 and Comparative Example 1 are shown in the figure. Figure 3 .

[0071] Depend on Figure 3 It can be seen that the absolute value of the Zeta potential of the nano-cerium oxide obtained in Example 1 is 12.58 mV, while the absolute value of the Zeta potential of the nano-cerium oxide obtained in Comparative Example 1 is 6.82 mV. The larger the absolute value of the Zeta potential, the less likely the nano-cerium oxide is to agglomerate. Therefore, the nano-cerium oxide obtained in this invention is less likely to agglomerate.

[0072] 5. Polishing test

[0073] Polishing test method: Polishing was performed using polishing slurries prepared from the nano-cerium oxide obtained in Example 1 and Comparative Example 1, respectively. 2 g of nano-cerium oxide prepared in Example 1 and Comparative Example 1 were placed in 100 mL of deionized water to prepare polishing slurries with a solid content of approximately 2 wt%. The polishing object was a silicon wafer. The polishing machine's upper plate pressure was 80 N, the upper plate rotation speed was 80 rpm, the lower plate rotation speed was 70 rpm, the dropper flow rate was 50 mL / min, and the polishing time was 2 min. After polishing, the polished 2-inch silicon wafer was placed in a beaker and ultrasonically cleaned with alcohol for 5 min. The surface roughness Ra of the polished silicon wafer was tested using the five-point method, with a test range of 5 × 5 μm².

[0074] The surface roughness Ra of the polished silicon wafer is shown in the figure. Figure 4 .

[0075] Figure 4 In Figure a, the silicon wafer was polished using the polishing slurry formed from nano-cerium oxide obtained in Comparative Example 1, and the surface roughness Ra of the polished silicon wafer was 0.922 nm.

[0076] Figure 4 In Figure b, the silicon wafer was polished using a polishing slurry formed from nano-cerium oxide obtained in Example 1, and the surface roughness Ra of the polished silicon wafer was 0.45 nm.

[0077] Depend on Figure 4 It is known that when polishing silicon wafers with the polishing liquid formed by the nano-cerium oxide obtained in this invention, the surface roughness Ra of the polished silicon wafers is smaller, that is, the surface quality of the polished silicon wafers is better.

[0078] This invention is not limited to the above-described embodiments. Any modifications, improvements, or substitutions that can be conceived by those skilled in the art without departing from the essential content of this invention fall within the scope of this invention.

Claims

1. A method of reducing the apparent density of nanoceria, characterized in that, The method comprises the following steps: 1) mixing an aqueous solution of a basic precipitant with an aqueous solution of cerium salt at 15-40℃ under stirring to obtain an initial mixture, and controlling the pH value of the initial mixture to be 11; then continuously stirring the reaction mixture at 25-30℃ for 1-5h to obtain a reaction mixture; wherein the aqueous solution of cerium salt is formed by dissolving a water-soluble cerium salt in water; the aqueous solution of cerium salt is selected from one or more of the following: an aqueous solution of cerium chloride, an aqueous solution of cerium nitrate and an aqueous solution of cerium sulfate; wherein the aqueous solution of basic precipitant is selected from an aqueous solution of alkali metal hydroxide; 2) standing the reaction mixture, then removing the upper aqueous phase to obtain a lower precipitate; drying the lower precipitate at 180-190℃ to obtain a dried solid; wherein the aqueous phase is removed by pouring or sucking, instead of filtration or centrifugation; 3) washing the dried solid with water to obtain a washed solid; drying the washed solid at 180-190℃ to obtain nano ceria.

2. The method of claim 1, wherein, The aqueous solution of basic precipitant is an aqueous solution of sodium hydroxide.

3. The method of claim 1, wherein, The concentration of the aqueous solution of basic precipitant is 5-20wt%.

4. The method of claim 1, wherein, The mass concentration of the aqueous solution of cerium salt is 0.1-0.2g / mL.

5. The method of claim 1, wherein, In step 3), the dried solid is washed with water for more than twice to obtain a washed solid; the washed solid is dried at 180-190℃ and ground to obtain nano ceria.

6. The method of claim 1, wherein, The particle size of the obtained nano ceria is 50-120nm.

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