Preparation method of high-purity alumina powder for polishing silicon carbide substrate

Abstract

The invention provides a preparation method of high-purity alumina powder for silicon carbide substrate polishing, which comprises the following steps: S1, raw material preparation: using boehmite as a raw material, and then carrying out ball milling treatment on the boehmite; s2, hydrothermal reaction: carrying out hydrothermal reaction on the boehmite in the S1 by using a hydrothermal reaction kettle to generate wormlike alumina powder; s3, purification treatment: carrying out purification treatment on the wormlike alumina powder obtained in S2, and removing impurities to obtain pure alumina powder; and S4, grading treatment: carrying out grading treatment on the pure alumina powder obtained in S3 to obtain the required high-purity alumina powder. The vermicular alumina powder is directly produced by using a hydrothermal reaction kettle, the vermicular alumina powder meets the use requirements of a silicon carbide substrate polishing solution through sanding treatment, and the alumina powder produced by the process is large in specific surface area, good in dispersity and high in removal efficiency, and has the characteristics that grain size growth caused by high-temperature treatment is avoided; and therefore, workpiece scratches caused by the subsequent polishing solution in the use process are reduced.

Description

Technical Field

[0001] The invention relates to the technical field of aluminum oxide powder, and more specifically to a method for preparing high-purity aluminum oxide powder for polishing silicon carbide substrates. Background Art

[0002] CMP of silicon carbide substrates is mainly used to flatten and polish SiC substrates to improve their surface quality. It is suitable for the manufacture of semiconductor devices. The CMP process removes surface defects through chemical reactions and mechanical grinding, making the substrate surface smoother and improving device performance and reliability.

[0003] Alumina powder is a compound composed of aluminum and oxygen, with the chemical formula of Al2O3. It is usually in the form of white powder, with high hardness and melting point. Alumina powder can be used for polishing silicon carbide substrates. This is because alumina powder has high hardness and good grinding performance, which can effectively remove the roughness of the silicon carbide surface and achieve the required finish. In addition, its chemical stability makes it difficult to react with the substrate during the polishing process, ensuring the polishing quality.

[0004] In the CMP process of silicon carbide substrates, the use of aluminum oxide powder with insufficient purity may cause the following hazards: Contamination: Impurities may remain on the substrate surface during the polishing process, causing contamination and affecting subsequent processes and device performance; Chemical reaction: Impure alumina powder may react with the chemical components in the polishing liquid to form undesirable compounds, thereby reducing the polishing effect and efficiency; Uneven polishing: The presence of impurities may cause unstable performance of the abrasive, resulting in uneven polishing and affecting the surface quality; Surface defects: Impurity particles may cause surface scratches or other defects, which may affect the optical and electrical properties of the silicon carbide substrate; Reduced mechanical properties: Abrasives with insufficient purity may lead to reduced mechanical strength of the substrate and increase risks in subsequent processing.

[0005] The present invention uses a hydrothermal reactor to directly produce worm-like alumina powder, which is sand-milled to meet the requirements for use in a silicon carbide substrate polishing solution. The alumina powder produced by this process has a large specific surface area, good dispersibility, and high removal efficiency. It is characterized in that it does not require high-temperature treatment above 1200°C, thereby avoiding high temperature causing the alumina grain size to grow and affecting the use effect of the polishing solution. Summary of the invention

[0006] The present invention aims to solve the technical problems raised by the above-mentioned background technology. The present invention provides the following technical solutions: a method for preparing high-purity alumina powder for polishing silicon carbide substrates, comprising the following steps: S1. Raw material preparation: Bohm stone is used as raw material, and then the Bohm stone is subjected to ball milling treatment; S2, hydrothermal reaction: using a hydrothermal reactor to perform a hydrothermal reaction on the bohm stone described in S1 to generate worm-like alumina powder; S3, purification treatment: purifying the worm-like alumina powder described in S2 to remove impurities to obtain pure alumina powder; S4, grading treatment, subjecting the pure alumina powder described in S3 to grading treatment to obtain the desired high-purity alumina powder.

[0007] In one embodiment of the present invention, in S1, the bohmite is selected from one or more of ferroaluminosilicate bohmite, sodium aluminosilicate bohmite, silicate bohmite and aluminosilicate bohmite.

[0008] In one embodiment of the present invention, in S1, the ball milling process comprises the following steps: S1, washing and drying the bohm stone in sequence; S2, select a planetary ball mill, then put the grinding balls and the bohm stone described in S1 into the ball mill in proportion, set the speed and time of the ball mill, start the ball mill, and allow the bohm stone to be ground under the action of the grinding balls; S3. Wash the ball-milled bohm stone with clean water to remove the grinding balls and impurities, and then dry it in a drying oven to prepare for the subsequent hydrothermal reaction.

[0009] Further, in S2, the diameter of the grinding balls is 10-20 mm; the mass ratio of the bohm stone to the grinding balls is 1:1 to 1:3; the rotation speed of the planetary ball mill is 60-80 rpm; and the ball milling time of the planetary ball mill is 3-6 hours.

[0010] In one embodiment of the present invention, in S2, the generating of worm-like alumina powder comprises the following steps: S1. Reactor setting: Add bohm stone and appropriate amount of water into a hydrothermal reactor, and set appropriate temperature and pressure; S2, reaction conditions: maintaining a specific reaction time to promote reaction and crystal growth; S3. pH adjustment: The morphology of alumina can be affected by adjusting the pH value of the reaction system; S4, temperature and time: by adjusting the reaction temperature and time, the growth rate of the crystal is controlled to obtain a worm-like structure; S5, washing and drying: After the reaction is completed, the product is washed to remove impurities and then dried.

[0011] Furthermore, the temperature of the hydrothermal reactor is 460° C.; the pressure of the hydrothermal reactor is 14.6 MPa; the reaction time of the hydrothermal reactor is 10 days; and the particle size D50 of the worm-like alumina powder is about 150 nm.

[0012] In one embodiment of the present invention, in S3, the purification process comprises the following steps: S1. Pretreatment: drying the worm-like alumina powder to remove moisture; S2. Pickling: Treat the alumina powder with dilute hydrochloric acid or dilute nitric acid to remove metal impurities such as iron, calcium, magnesium, etc. S3, neutralization and washing: rinse repeatedly with deionized water to remove acidic residues and ensure that the pH value is close to neutral; S4, precipitation and separation: add alkaline solution to adjust the pH to precipitate the worm-like alumina powder impurities, then use centrifugation or filtration to separate the precipitate and collect the clear liquid; S5. Drying: Drying in an oven to obtain dry pure alumina powder.

[0013] Furthermore, the drying temperature of the worm-like alumina powder is 60-100° C.; the pickling concentration is 0.1-1M; and the pickling reaction time is 1-2 hours.

[0014] In one embodiment of the present invention, in S4, the classification process comprises the following steps: S1. Physical classification: Pure alumina powder is screened through a standard sieve, and sieves of different mesh sizes are selected to separate the powder into components of different particle sizes; S2. Airflow classification: The alumina powder is passed through an airflow classifier to be classified according to the size and density of the particles to separate fine particles and coarse particles; S3, sedimentation classification: Pour the alumina powder slurry into the sedimentation tower, where larger particles settle faster due to gravity, and fine particles are separated; S4, drying: drying the classified components to remove moisture and obtain high-purity alumina powder; S5. Quality inspection: Conduct quality inspection on alumina powders of different particle sizes to ensure that they meet the required purity and particle size distribution requirements. Beneficial Effects

[0015] The high-purity aluminum oxide powder prepared by the present invention has the following advantages in the polishing of silicon carbide substrates: High purity: After strict purification treatment, metal impurities are removed, the pollution to the silicon carbide substrate is reduced, and the polishing effect and product quality are improved; Controllable particle size: Through grading, alumina powders of different particle sizes can be obtained to meet different polishing requirements. Fine particle size facilitates finer polishing, while larger particle size is suitable for removing larger defects. Uniformity: The worm-like alumina powder directly produced by hydrothermal reaction has good uniformity, ensuring uniform distribution of abrasives during polishing and improving polishing efficiency and consistency; Large specific surface area: The powder has a large specific surface area, which enhances its contact with the polishing liquid and improves the removal efficiency; Good dispersibility: Good dispersibility makes the alumina powder evenly distributed in the polishing liquid, avoiding particle aggregation and thus improving the polishing effect; Efficient removal: Due to its superior properties, alumina powder can effectively remove surface defects of silicon carbide substrates during polishing; Avoid sintering: This method does not require sintering, which avoids the growth of grains at high temperatures, prevents the particles from becoming larger, and reduces the risk of scratches after the polishing liquid is prepared. DETAILED DESCRIPTION

[0016] The present invention provides a method for preparing high-purity aluminum oxide powder for polishing a silicon carbide substrate. The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Embodiment 1

[0017] The method for preparing high-purity aluminum oxide powder for polishing a silicon carbide substrate in this embodiment specifically comprises the following steps: S1. Raw material preparation: using ferroaluminosilicate bohmite as a raw material, and then subjecting the ferroaluminosilicate bohmite to ball milling treatment. When ball milling the ferroaluminosilicate bohmite, firstly wash and dry the ferroaluminosilicate bohmite in sequence, then select a planetary ball mill, and then put grinding balls with a diameter of 10-20 mm and the ferroaluminosilicate bohmite in a mass ratio of 1:1 to 1:3 into the planetary ball mill, set the speed of the planetary ball mill to 60-80 rpm, and the ball milling time to 3-6 hours, then start the planetary ball mill, and grind the ferroaluminosilicate bohmite under the action of the grinding balls, and finally wash the ball-milled ferroaluminosilicate bohmite with clean water to remove the grinding balls and impurities, and then dry it in a drying oven to prepare for the subsequent hydrothermal reaction; S2, hydrothermal reaction: the ferroaluminosilicate bohmite described in S1 is used in a hydrothermal reactor, firstly a hydrothermal reactor is set, the ferroaluminosilicate bohmite and an appropriate amount of water are added into the hydrothermal reactor, then the temperature of the hydrothermal reactor is set to 460°C, the pressure of the hydrothermal reactor is set to 14.6 MPa, and the reaction time of the hydrothermal reactor is set to 10 days to promote the reaction and crystal growth, and then the pH value of the reaction system can be adjusted during the hydrothermal reaction to affect the morphology of alumina, thereby obtaining alumina powder with a worm-like structure, and after the reaction is completed, the product is washed to remove impurities, and then dried to finally obtain a stable worm-like alumina powder with a particle size D50 of about 150 nm; S3, purification treatment: the worm-like alumina powder described in S2 is purified, first the worm-like alumina powder is dried to remove moisture, then the alumina powder is treated with dilute hydrochloric acid with a pickling concentration of 0.1-1M, the pickling reaction time is 1-2 hours, to remove metal impurities such as iron, calcium, magnesium, etc., then repeatedly rinsed with deionized water to remove acidic residues, to ensure that the pH value is close to neutral, then an alkaline solution is added to adjust the pH to precipitate the impurities of the worm-like alumina powder, and then the precipitate is separated by centrifugation, the clear liquid is collected, and finally dried in an oven at a temperature of 60-100°C to obtain dry pure alumina powder; S4, grading treatment, the pure alumina powder described in S3 is graded, firstly the pure alumina powder is sieved through a standard sieve, and sieves of different mesh sizes are selected to separate the powder into different particles, and then the alumina powder is passed through an air flow classifier to be graded according to the size and density of the particles to separate fine particles and coarse particles, and then the alumina powder slurry is poured into a sedimentation tower, and the larger particles settle faster due to gravity, and the fine particles are separated, and then the graded components are dried to remove moisture to obtain high-purity alumina powder, and finally the alumina powder of different particle sizes is quality tested to ensure that it meets the required purity and particle size distribution requirements. Embodiment 2

[0018] The method is basically the same as Example 1, except that the reaction conditions of S1 are as follows: sodium aluminum silicate bohm stone is used as a raw material, and then the sodium aluminum silicate bohm stone is subjected to ball milling treatment. When the sodium aluminum silicate bohm stone is ball milled, the sodium aluminum silicate bohm stone is first cleaned and dried in sequence, and then a planetary ball mill is selected, and then grinding balls with a diameter of 10-20 mm and sodium aluminum silicate bohm stone are put into the planetary ball mill in a mass ratio of 1:1 to 1:3, and the speed of the planetary ball mill is set to 60-80 rpm, and the ball milling time is 3-6 hours. Then, the planetary ball mill is started to grind the sodium aluminum silicate bohm stone under the action of the grinding balls, and finally, the ball-milled sodium aluminum silicate bohm stone is washed with clean water to remove the grinding balls and impurities, and then dried in a drying oven to prepare for the subsequent hydrothermal reaction. Embodiment 3

[0019] The method is basically the same as Example 1, except that the reaction conditions of S1 are as follows: silicate bohm stone is used as a raw material, and the silicate bohm stone is then ball-milled. When the silicate bohm stone is ball-milled, the silicate bohm stone is first cleaned and dried in sequence, and then a planetary ball mill is selected, and then grinding balls with a diameter of 10-20 mm and silicate bohm stone are put into the planetary ball mill in a mass ratio of 1:1 to 1:3, and the speed of the planetary ball mill is set to 60-80 rpm, and the ball milling time is 3-6 hours. Then, the planetary ball mill is started to grind the silicate bohm stone under the action of the grinding balls, and finally, the ball-milled silicate bohm stone is washed with clean water to remove the grinding balls and impurities, and then dried in a drying oven to prepare for the subsequent hydrothermal reaction. Embodiment 4

[0020] The method is basically the same as Example 1, except that the reaction conditions of S1 are as follows: aluminosilicate bohm stone is used as a raw material, and the aluminosilicate bohm stone is subjected to ball milling treatment. When the aluminosilicate bohm stone is ball milled, the aluminosilicate bohm stone is first cleaned and dried in sequence, and then a planetary ball mill is selected, and then grinding balls with a diameter of 10-20 mm and aluminosilicate bohm stone are put into the planetary ball mill in a mass ratio of 1:1 to 1:3, and the speed of the planetary ball mill is set to 60-80 rpm, and the ball milling time is 3-6 hours. Then, the planetary ball mill is started to grind the aluminosilicate bohm stone under the action of the grinding balls, and finally, the ball-milled aluminosilicate bohm stone is washed with clean water to remove the grinding balls and impurities, and then dried in a drying oven to prepare for the subsequent hydrothermal reaction. Embodiment 5

[0021] The method is basically the same as Example 1, except that the reaction conditions of S2 are as follows: the ferroaluminosilicate bohmite described in S1 is used in a hydrothermal reactor, a hydrothermal reactor is first set up, and the ferroaluminosilicate bohmite and an appropriate amount of water are added to the hydrothermal reactor, and then the temperature of the hydrothermal reactor is set to 500°C, the pressure of the hydrothermal reactor is set to 15 MPa, and the reaction time of the hydrothermal reactor is set to 8 days to promote the reaction and crystal growth, and then during the hydrothermal reaction process, the pH value of the reaction system can be adjusted to affect the morphology of alumina, thereby obtaining alumina powder with a worm-like structure, and after the reaction is completed, the product is washed to remove impurities, and then dried to finally obtain a stable worm-like alumina powder with a particle size D50 of about 160 nm. Embodiment 6

[0022] The method is basically the same as Example 1, except that the reaction conditions of S2 are as follows: the ferroaluminosilicate bohmite described in S1 is used in a hydrothermal reactor, a hydrothermal reactor is first set up, and the ferroaluminosilicate bohmite and an appropriate amount of water are added to the hydrothermal reactor, and then the temperature of the hydrothermal reactor is set to 550°C, the pressure of the hydrothermal reactor is set to 16 MPa, and the reaction time of the hydrothermal reactor is set to 10 days to promote the reaction and crystal growth, and then during the hydrothermal reaction process, the pH value of the reaction system can be adjusted to affect the morphology of alumina, thereby obtaining alumina powder with a worm-like structure, and after the reaction is completed, the product is washed to remove impurities, and then dried to finally obtain a stable worm-like alumina powder with a particle size D50 of about 155 nm. Embodiment 7

[0023] The method is basically the same as Example 1, except that the reaction conditions of S2 are as follows: the ferroaluminosilicate bohmite described in S1 is used in a hydrothermal reactor, a hydrothermal reactor is first set up, and the ferroaluminosilicate bohmite and an appropriate amount of water are added to the hydrothermal reactor, and then the temperature of the hydrothermal reactor is set to 450°C, the pressure of the hydrothermal reactor is set to 18 MPa, and the reaction time of the hydrothermal reactor is set to 5 days to promote the reaction and crystal growth, and then during the hydrothermal reaction process, the pH value of the reaction system can be adjusted to affect the morphology of alumina, thereby obtaining alumina powder with a worm-like structure, and after the reaction is completed, the product is washed to remove impurities, and then dried to finally obtain a worm-like alumina powder with a stable particle size D50 of about 152 nm. Embodiment 8

[0024] The method is basically the same as Example 1, except for the reaction conditions of S3: the worm-like alumina powder described in S2 is purified by first drying the worm-like alumina powder to remove moisture, and then treating the alumina powder with dilute hydrochloric acid having a pickling concentration of 0.1 M for 1 hour to remove metal impurities such as iron, calcium, magnesium, etc., and then repeatedly rinsing with deionized water to remove acidic residues to ensure that the pH value is close to neutral, and then adding an alkaline solution to adjust the pH to precipitate the worm-like alumina powder impurities, and then using a centrifugal method to separate the precipitate, collecting the clear liquid, and finally drying in an oven at 100°C to obtain dry pure alumina powder. Embodiment 9

[0025] The method is basically the same as Example 1, except for the reaction conditions of S3: the worm-like alumina powder described in S2 is purified by first drying the worm-like alumina powder to remove moisture, and then treating the alumina powder with dilute hydrochloric acid having a pickling concentration of 0.5 M for 1.5 hours to remove metal impurities such as iron, calcium, magnesium, etc., and then repeatedly rinsing with deionized water to remove acidic residues to ensure that the pH value is close to neutral, and then adding an alkaline solution to adjust the pH to precipitate the worm-like alumina powder impurities, and then using a centrifugal method to separate the precipitate, collecting the clear liquid, and finally drying in an oven at 100°C to obtain dry pure alumina powder. Embodiment 10

[0026] The method is basically the same as Example 1, except for the reaction conditions of S3: the worm-like alumina powder described in S2 is purified by first drying the worm-like alumina powder to remove moisture, then treating the alumina powder with dilute hydrochloric acid having a pickling concentration of 1 M, and the pickling reaction time is 2 hours to remove metal impurities such as iron, calcium, magnesium, etc., then repeatedly rinsing with deionized water to remove acidic residues to ensure that the pH value is close to neutral, and then adding an alkaline solution to adjust the pH to precipitate the worm-like alumina powder impurities, and then using a centrifugal method to separate the precipitate, collecting the clear liquid, and finally drying in an oven at 80°C to obtain dry pure alumina powder.

[0027] In summary, the preparation method of this high-purity alumina powder has multiple advantages, including: 1. High purity: After strict purification treatment, metal impurities can be effectively removed, thereby reducing the pollution to the silicon carbide substrate and improving the polishing effect and product quality; 2. Controllable particle size: Through grading, alumina powders of different particle sizes can be obtained to meet different polishing requirements. Fine particle size helps to achieve finer polishing, while larger particle size is suitable for removing larger defects; 3. Uniformity: The worm-like alumina powder produced by the hydrothermal reaction method usually has good uniformity, which helps to maintain the uniformity of abrasive distribution during the polishing process, thereby improving polishing efficiency; 4. Large specific surface area: The large specific surface area of ​​the powder can enhance its contact with the polishing liquid and improve the removal efficiency; 5. Good dispersibility: Good dispersibility enables alumina powder to be evenly distributed in the polishing liquid, avoiding particle aggregation, thereby improving the polishing effect; 6. Efficient removal: Due to its superior physical properties, alumina powder can effectively remove defects on the surface of silicon carbide substrates during polishing; 7. Avoid sintering: This method does not require a sintering process, which avoids the growth of grains at high temperatures, reduces the risk of grain enlargement, and thus reduces the risk of scratches after the polishing liquid is prepared; This preparation method has significant advantages in the production of high-purity alumina powder and is suitable for polishing applications of silicon carbide substrates.

Claims

1. A method for preparing high-purity alumina powder for polishing silicon carbide substrates, characterized in that: The following steps are involved: S1. Raw material preparation: Bohm stone is used as raw material, and then the Bohm stone is subjected to ball milling treatment; S2, hydrothermal reaction: using a hydrothermal reactor to perform a hydrothermal reaction on the bohm stone described in S1 to generate worm-like alumina powder; S3, purification treatment: purifying the worm-like alumina powder described in S2 to remove impurities to obtain pure alumina powder; S4, grading treatment, subjecting the pure alumina powder described in S3 to grading treatment to obtain the desired high-purity alumina powder.

2. The method for preparing high-purity alumina powder for polishing silicon carbide substrates according to claim 1, characterized in that: In S1, the bohmite is selected from one or more of ferroaluminosilicate bohmite, sodium aluminosilicate bohmite, silicate bohmite and aluminosilicate bohmite.

3. The method for preparing high-purity alumina powder for polishing silicon carbide substrates according to claim 1, characterized in that: In S1, the ball milling process comprises the following steps: S1, washing and drying the bohm stone in sequence; S2, select a planetary ball mill, then put the grinding balls and the bohm stone described in S1 into the ball mill in proportion, set the speed and time of the ball mill, start the ball mill, and allow the bohm stone to be ground under the action of the grinding balls; S3. Wash the ball-milled bohm stone with clean water to remove the grinding balls and impurities, and then dry it in a drying oven to prepare for the subsequent hydrothermal reaction.

4. The method for preparing high-purity alumina powder for polishing silicon carbide substrates according to claim 3, characterized in that: In S2, the diameter of the grinding balls is 10-20 mm; the mass ratio of the bohm stone to the grinding balls is 1:1 to 1:3; the rotation speed of the planetary ball mill is 60-80 rpm; and the ball milling time of the planetary ball mill is 3-6 hours.

5. The method for preparing high-purity alumina powder for polishing silicon carbide substrates according to claim 1, characterized in that: In S2, the generating of worm-like alumina powder comprises the following steps: S1. Reactor setting: Add bohm stone and appropriate amount of water into a hydrothermal reactor, and set appropriate temperature and pressure; S2, reaction conditions: maintaining a specific reaction time to promote reaction and crystal growth; S3. pH adjustment: The morphology of alumina can be affected by adjusting the pH value of the reaction system; S4, temperature and time: by adjusting the reaction temperature and time, the growth rate of the crystal is controlled to obtain a worm-like structure; S5. Washing and drying: After the reaction is completed, the product is washed to remove impurities and then dried to finally obtain stable worm-like alumina powder.

6. The method for preparing high-purity aluminum oxide powder for polishing silicon carbide substrates according to claim 5, characterized in that: The temperature of the hydrothermal reactor is 460° C.; the pressure of the hydrothermal reactor is 14.6 MPa; the reaction time of the hydrothermal reactor is 10 days; and the particle size D50 of the worm-like alumina powder is about 150 nm.

7. The method for preparing high-purity aluminum oxide powder for polishing silicon carbide substrates according to claim 1, characterized in that: In S3, the purification process comprises the following steps: S1. Pretreatment: drying the worm-like alumina powder to remove moisture; S2. Pickling: Treat the alumina powder with dilute hydrochloric acid or dilute nitric acid to remove metal impurities such as iron, calcium, magnesium, etc. S3, neutralization and washing: rinse repeatedly with deionized water to remove acidic residues and ensure that the pH value is close to neutral; S4, precipitation and separation: add alkaline solution to adjust the pH to precipitate the worm-like alumina powder impurities, then use centrifugation or filtration to separate the precipitate and collect the clear liquid; S5. Drying: Drying in an oven to obtain dry pure alumina powder.

8. The method for preparing high-purity aluminum oxide powder for polishing silicon carbide substrates according to claim 7, characterized in that: The drying temperature of the worm-like alumina powder is 60-100°C; the pickling concentration is 0.1-1M; and the pickling reaction time is 1-2 hours.

9. The method for preparing high-purity aluminum oxide powder for polishing silicon carbide substrates according to claim 1, characterized in that: In S4, the classification process comprises the following steps: S1. Physical classification: Pure alumina powder is screened through a standard sieve, and sieves of different mesh sizes are selected to separate the powder into components of different particle sizes; S2. Airflow classification: The alumina powder is passed through an airflow classifier to be classified according to the size and density of the particles to separate fine particles and coarse particles; S3, sedimentation classification: Pour the alumina powder slurry into the sedimentation tower, where larger particles settle faster due to gravity, and fine particles are separated; S4, drying: drying the classified components to remove moisture and obtain high-purity alumina powder; S5. Quality inspection: Conduct quality inspection on alumina powders of different particle sizes to ensure that they meet the required purity and particle size distribution requirements.