CN101935877B - Method for synthesizing mullite whiskers by normal pressure sintering - Google Patents

Abstract

The invention relates to a method for synthesizing mullite whisker by normal pressure sintering, which is a second phase whisker material which can be used for reinforcing and toughening metal and ceramic matrixes at the temperature of more than 1200 ℃ and is prepared by utilizing the high-temperature creep resistance of the mullite whisker. The mullite whisker synthesized by the normal pressure powder sintering method has the advantages of simple process, high dispersion degree, uniform particle size distribution, high length-diameter ratio and high purity, and can be used as a reinforcing and toughening whisker of metal and ceramic materials.

Description

A method for synthesizing mullite whiskers by atmospheric pressure sintering

technical field

The invention relates to a method for synthesizing mullite whiskers by normal-pressure sintering, which belongs to the field of material preparation.

Background technique

Mullite whiskers have outstanding advantages such as high temperature resistance, wear resistance, oxidation resistance, low thermal expansion coefficient, and excellent thermal shock resistance, and their excellent high temperature creep resistance makes them particularly suitable for use as metals and ceramics in high temperature environments. The material is reinforced and toughened using whiskers.

There have been many literature and patent reports on the preparation of mullite whiskers. Japan Okada et al. prepared Al ₂ O ₃ -SiO ₂ xerogels with tetraethyl orthosilicate and aluminum nitrate, and calcined them with AlF ₃ in a closed container to synthesize mullite whiskers (see Materials Science Communication.8[9 ] (1989): 1052-1054). The method adopts ethyl orthosilicate and aluminum nitrate, which are expensive, and the sol-gel preparation process is complicated and the operation period is long. Chinese patent application CN92102982.9 discloses a method for preparing high-purity mullite whiskers. In this method, aluminum salt is dissolved in an alcohol solution, and silicon is added dropwise at a molar ratio of SiO ₂ : _{Al 2} O ₃ =2:3. Alcoholate, and add HF solution, the obtained gel is dried, crushed, sieved, calcined, and finally mullite whiskers with various aspect ratios are obtained. The preparation process of the method adopts two calcinations, the energy consumption is large and the preparation process is complicated, and the mullite whisker yield is not high. US patents 4,910,172 and 4,948,766 disclose methods for preparing mullite whiskers by powder sintering (Al ₂ O ₃ , SiO ₂ , AlF ₃ ). The mullite whiskers prepared by the method are unevenly distributed, the calcination temperature is high, and the long diameter of the grown mullite whiskers is relatively short. Xu Xiaohong etc. used kaolin, industrial aluminum hydroxide, AlF ₃ , V ₂ O ₅ as raw materials to in situ synthesize mullite whiskers (see in situ synthesis of mullite whiskers and microstructure research, Journal of Wuhan University of Technology, 2007, 28(2):89-92). The mullite whiskers prepared by the method contain many impurities and low purity, and are difficult to be applied to strengthening and toughening metals and ceramic materials.

The present invention uses the gas formed by aluminum fluoride at high temperature to carry out gas-solid reaction, which is beneficial to the anisotropic growth of crystals, and synthesizes whiskers growing in a specific direction. During the high-temperature heating process, vanadium pentoxide will volatilize and promote gas-solid reaction. The mass transfer process of solid reaction and activation increases the lattice defects of alumina, activates the lattice, and promotes the formation of mullite. The whiskers grown in the present invention are grown by the following gas-solid reaction.

Al(OH) ₃ →Al ₂ O ₃ +H ₂ O

(1)

6AlF ₃ +3O ₂ →6AlOF+12F (2)

Al ₂ O ₃ +2F→2AlOF+0.5O ₂ (3)

2SiO ₂ +8F→2SiF4+2O ₂ (4)

6AlOF+2SiF ₄ +3.5O ₂ →3Al ₂ O ₃ 2SiO ₂ +14F (5)

The mullite formed in this reaction process is precipitated and nucleated after reaching a certain saturation in the high-temperature aluminum fluoride gas. In the gas phase environment provided by the gas, the mullite nuclei grow according to the crystal growth habit to form columnar mullite. whiskers.

Contents of the invention

The purpose of the present invention is to provide a method for synthesizing mullite whiskers by normal pressure sintering, using the powder atmospheric pressure sintering method to synthesize mullite whiskers, and utilizing the high temperature creep resistance of mullite whiskers to prepare mullite whiskers that can be produced at 1200 Above ℃, it is used to strengthen and toughen the second phase whisker material of metal and ceramic matrix.

The present invention is realized through the following technical solutions

Composition and mass percentage of raw materials and additives

Raw material industry aluminum hydroxide 75.5%-85.5%, silicon dioxide 14.5%-24.5%, the total is 100%,

The additive aluminum fluoride is 2-6% of the raw material weight, and vanadium pentoxide is 2-6% of the raw material weight;

Prepare mullite whiskers according to the above formula and complete as follows

(1) Ball milling: raw materials and additives are batched according to the mass percentage of formula, then add in the ball mill according to raw materials: alcohol: ball=1-4: 4-8: 8-12, carry out wet ball milling 6-10h, obtain particle size as 75-100um powder, alcohol as the ball milling medium;

(2) Drying: Dry the ball-milled powder in an oven at 80-100°C for 4-6 hours, remove the alcohol in the ball-milled powder, and obtain a uniformly mixed powder;

(3) Mixed grinding: Add 3-5% of raw material weight to the uniformly mixed powder, polyvinyl alcohol solution (PVA) with a concentration of 10-15% by weight, mix and grind with a grinding rod for 15-45min and then enter Aging in a closed container for 12-24 hours;

(4) Tablet compression: compress the aged powder on a powder tablet press with a pressure of 15-25MPa to form a tablet with diameter×height=10-15mm×5-10mm;

(5) Sintering: Put the pressed sheet in a closed alumina ceramic crucible, raise the temperature from room temperature to 800-1000°C in a muffle furnace at a rate of 3-5°C/min, and heat-preserve and sinter for 1- After 2 hours, increase the temperature at a rate of 5-10°C/min to 1250°C-1450°C for 2-3h, heat preservation and sintering for 2-3h, and cool down with the furnace to obtain mullite whisker products.

The invention has the advantages of simple process for synthesizing mullite whiskers by the atmospheric pressure powder sintering method, and the synthesized mullite whiskers have the advantages of high degree of dispersion, uniform particle size distribution, high aspect ratio and high purity, It can be very well used as a reinforced and toughened whisker for metal and ceramic materials.

Description of drawings

Figures 1 and 2 are electron microscope scanning display diagrams of the structure of the mullite whisker product in Example 1; Figure 3 is an electron microscope scanning display diagram of the structure of the mullite whisker product in Example 2.

Detailed ways

Embodiment 1: raw material adopts industrial aluminum hydroxide 75.5%, silicon dioxide 24.5%, add the vanadium pentoxide of 2% aluminum fluoride of raw material quality and 4% of raw material quality, by raw material: alcohol: ball=2: Add 4:8 into a planetary ball mill and carry out wet ball milling for 8 hours to obtain a mixed powder with a particle size of 75-100um. The ball-milled powder is dried in an oven at 100°C for 4 hours, and the powder in the ball-milled powder is removed. Alcohol, to obtain a uniformly mixed powder; then add this powder to a polyvinyl alcohol solution with a concentration of 15% of 3% by weight of the raw material mass, mix it with a grinding rod for 25 minutes, and age it for 24 hours. Press the tablet machine into 10-15mm×5-10mm flakes, put them into a closed alumina ceramic crucible, and put them into a muffle furnace from room temperature to 800°C at a rate of 3°C/min, and keep warm for 2 hours Then, raise the temperature to 1250°C at a rate of 5°C/min and keep it warm for 3 hours, and then cool down with the furnace to obtain the mullite whisker product. The generated mullite whiskers are well dispersed, the whisker length is 20-30 μm, the diameter is 1.5-2 μm, and the aspect ratio reaches 12-15, which is suitable for strengthening and toughening metal and ceramic materials. See Figure 1 and Figure 2.

Embodiment 2: The raw material adopts industrial aluminum hydroxide 80.5%, silicon dioxide 19.5%, additives aluminum fluoride and vanadium pentoxide are respectively 6% and 2% of the raw material quality, wet ball milling in a planetary ball mill to obtain a mixed Uniform powder, bake at 80°C for 6 hours, then add 4% by weight of polyvinyl alcohol solution with a concentration of 12% to the powder, mix it with a grinding rod for 40 minutes, put it into a closed container and age it for 18 hours, Press it into 10-15mm×5-10mm tablets with a powder tablet press, put them in a closed alumina ceramic crucible, and put them in a muffle furnace from room temperature to 900°C at a heating rate of 4°C/min. After 1.5h, the temperature was raised to 1350°C at a rate of 8°C/min and held for 2.5h, and then the sample was cooled with the furnace to obtain the desired product. Scanning electron microscopy shows that the generated mullite whiskers are well dispersed, the whisker length is 50-70 μm, the diameter is 2.5-4 μm, and the aspect ratio reaches 15-20, which is suitable for strengthening and toughening metal and ceramic materials, as shown in Figure 3 .

Claims (1)

1. the method for a normal pressure-sintered mullite synthesizing whisker is characterized in that: the present invention realizes through following technical proposals,

1), raw material and additive are formed and mass percent

Raw material: industrial aluminium hydroxide 75.5%-85.5%, silica 1 4.5%-24.5%, total amount is 100%,

Additive: ALUMNIUM FLUORIDE is the 2-6% of raw material weight, Vanadium Pentoxide in FLAKES be raw material weight 2-6%;

2), accomplish according to the following steps according to above-mentioned formulation mullite crystal whisker

(1), ball milling: raw material and additive are prepared burden by the mass percent of prescription, and press raw material again: alcohol: ball=1-4: 4-8: 8-12 adds in the ball mill, carries out wet ball grinding 6-10h, obtains the powder that granularity is 75-100um, and alcohol is ball-milling medium;

(2), oven dry: the powder that ball milling is good is in baking oven, and under 80-100 ℃, oven dry 4-6h sloughs the alcohol in the powder behind the ball milling, obtains the powder that mixes;

(3), mix grinding: in the powder that mixes, add the 3-5% of raw material weight, concentration expressed in percentage by weight is the polyvinyl alcohol solution of 10-15%, with going into ageing 12-24h in the encloses container behind the grinding rod mixed grinding 15-45min;

(4), compressing tablet: the pressure with 15-25MPa on the powder type tabletting machine of the powder after the ageing is pressed into the sheet stock of diameter * highly=10-15mm * 5-10mm;

(5), sintering: the sheet stock that presses is placed in the airtight alumina-ceramic crucible; Temperature rise rate with 3-5 ℃/min in retort furnace is raised to 800-1000 ℃ from room temperature; Behind the heat preservation sintering 1-2h, the temperature rise rate with 5-10 ℃/min is raised to 1250 ℃-1450 ℃, heat preservation sintering 2-3h again; Furnace cooling obtains the mullite crystal whisker product.

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