CN104692403B - Preparation method of single-phase mullite sol - Google Patents

Abstract

The present invention proposes a preparation method of single-phase mullite sol, which is prepared by preparing fresh aluminum hydroxide precipitation, preparing alumina suspension, introducing silicon source into alumina suspension, and decondensing the mixed suspension to prepare single-phase Mullite sol. The invention avoids the need to suppress the hydrolysis speed of aluminum alkoxide, has no special requirements on process parameters such as pH value and ambient humidity, has simple process, has low requirements on synthesis equipment, and is suitable for large-scale industrial production.

Description

A kind of preparation method of single-phase mullite sol

technical field

The invention relates to a preparation method of single-phase mullite sol, which belongs to the technical field of chemical industry.

Background technique

Mullite is the only stable compound in the Al ₂ O ₃ -SiO ₂ binary system. The stoichiometric formula is 3Al ₂ O ₃ 2SiO ₂ . Natural mullite is very rare, and high-performance mullite is made synthetic. Because mullite has outstanding physical and chemical properties, such as excellent high temperature oxidation resistance, creep resistance, corrosion resistance, low thermal expansion coefficient and thermal conductivity, low dielectric constant and dielectric loss, high mechanical Strength and toughness, etc. Mullite ceramics with mullite as the main crystal phase are considered to be a very ideal high-temperature engineering ceramics. In recent decades, mullite has received more and more attention in the fields of high-temperature thermal structure, electromagnetism and optics, and has broad engineering application prospects.

Because the "sol-gel" method can prepare high-purity mullite powder, mullite fiber, mullite porous material, mullite dense block, mullite coating, etc. at low temperature, mullite sol research times favored by researchers. According to the uniform degree of mixing, mullite sol can be divided into single-phase glue and double-phase glue. The uniformity of single-phase glue mixing is high, which belongs to the mixing at the molecular or atomic level, and the preparation conditions are relatively harsh. The arrangement of the Si-O-Al atomic groups in the amorphous state is similar to the mullite structure at the atomic level, so the precursor can be directly converted from the amorphous state to mullite at about 1000 °C.

The preparation method of several single-phase mullite sols has been reported in the literature patent disclosed at home and abroad, and the preparation method of the most common a kind of mullite single-phase glue is proposed by Hoffman et al (J.Am.Ceram.Soc . 1984, 67(7): 468-471). In short, aluminum nitrate with crystal water is dissolved in ethanol first, and then mixed with ethanol solution of ethyl orthosilicate to obtain single-phase mullite sol. The core of the method is to control the hydrolysis process slowly and gradually through limited crystal water. Chinese patent CN1810722A discloses a low-temperature sintering process for mullite ceramics, wherein mullite single-phase sol is prepared by using this method.

There are literature reports (Journal of sol-gel science and technology 1997, 10: 19-30; Journal of sol-gel science and technology 1999, 15: 191-199), using aluminum tert-butoxide as aluminum source, ethyl orthosilicate The ester is a silicon source, and isopropanol or ethanol is used as a solvent. By controlling the pH value, adjusting the ambient humidity, and controlling slow hydrolysis, a single-phase mullite sol can also be prepared. Chinese patent CN 102602947A reports a method for preparing single-phase mullite sol without using alcohol or water as a solvent. The invention uses soluble aluminum inorganic salt and aluminum alkoxide as the aluminum source, and first dissolves the soluble aluminum salt In an organic solvent, adjust the pH value of the solution with acid, then add aluminum alkoxide to react for 10-40 minutes, add silicon source for hydrolysis for 12-36 hours, and heat for 10-60 hours to form mullite sol. The core of these methods is to realize slow hydrolysis by controlling the system environment.

In addition, the hydrolysis rate of aluminum salt can also be controlled by adding various chelating agents to the system, thereby preparing mullite single-phase sol. For example, Chen Lifu's research group at Xiamen University used aluminum tert-butoxide as the aluminum source, partially hydrolyzed tetraethyl orthosilicate as the silicon source, added ethyl acetoacetate as a chelating agent, and added polyvinylpyrrolidone as a fiber-forming additive to prepare a single phase mullite sol, and prepared gel fibers (Journal of Xiamen University (Natural Science Edition), 2009, 48(4):610-612).

To sum up, in the prior art, in the preparation process of single-phase mullite sol, either the energy consumption is high, the preparation cycle is long, or the process parameters (such as pH value, temperature, ambient humidity) are strictly required, and additional chelating agents need to be added , the process is complicated, the repeatability is poor and the raw material is an expensive alkoxide, which is not ideal.

Contents of the invention

The purpose of the present invention is to overcome the disadvantages of the prior art and provide a single-phase mullite sol preparation method with simple process, low energy consumption, short preparation period, low cost and easy large-scale synthesis.

Technical solution of the present invention: a kind of preparation method of single-phase mullite sol is realized by following steps:

The first step is to prepare fresh aluminum hydroxide precipitate,

The present invention uses inorganic aluminum salts as raw materials to obtain fresh aluminum hydroxide precipitation by adding excess alkali at room temperature, does not use expensive aluminum alkoxides as raw materials, and reduces costs. This step can be prepared by the following process, and can also be prepared by using a general-purpose inorganic aluminum salt to prepare the precipitation of aluminum hydroxide.

Prepare the inorganic aluminum salt into an aqueous solution with a molar concentration of 0.2-2M, and gradually add an inorganic alkali solution with a molar concentration of 0.2-1.5M to it under stirring conditions, until a precipitate is formed, and then stir at room temperature for 30 minutes ~60min, followed by filtration and washing to obtain a solid material.

The present invention has no special limitation on the type of inorganic aluminum salt, and one of common aluminum sulfate, aluminum chloride and aluminum nitrate can be used. The present invention has no special limitation on the type of inorganic alkali, and one of common ammonia water, sodium hydroxide, potassium hydroxide, and ammonium bicarbonate can be used, and ammonia water is the best. In the present invention, the concentration of inorganic aluminum salt and inorganic alkali should not be too large. If the concentration is too large, the efficiency of stirring and dispersing at room temperature is not high, and a certain degree of heating is required, which cannot be operated at room temperature, which increases the cost; and the concentration is too high. Low produces too little precipitate. The present invention provides a preferred concentration range, taking into account room temperature operation and precipitation yield.

The method for extracting solid matter is vacuum filtration through filter paper or centrifugal separation.

The aluminum hydroxide precipitate prepared in this step needs to be prepared within 3 hours before use in the subsequent steps, and it is best to prepare and use now.

The second step is to prepare alumina suspension,

Disperse the fresh aluminum hydroxide precipitate obtained in the first step in absolute ethanol, and stir at room temperature to obtain an alumina suspension;

In this step, absolute ethanol is used as a dispersant, as long as the aluminum hydroxide precipitate can be uniformly dispersed, the general reference amount is that the aluminum hydroxide precipitate and absolute ethanol are in the range of 1:10-20, and it can also be used according to specific conditions. Make a selection. After stirring at room temperature for a certain period of time, a white alumina suspension was obtained.

In the third step, a silicon source is introduced into the alumina suspension,

Keep the stirring conditions at room temperature unchanged, add silicate ester dropwise to the alumina suspension according to the ratio of Al/Si molar ratio 3/1, and continue stirring for a certain period of time after all the silicate ester is added dropwise, and the stirring time is generally not low in 2 hours.

There is no special requirement for the type of silicate in this step, and it can be one of methyl silicate, ethyl orthosilicate, propyl orthosilicate, and butyl orthosilicate, and ethyl orthosilicate is the most commonly used.

The fourth step is to decondense the mixed suspension,

Keep the stirring conditions at room temperature unchanged, add dilute acid with a molar concentration of 1 to 2M to the mixed suspension according to the ratio of Al/H+ molar ratio of 1 to 10:1, and the white color of the suspension can be found to fade quickly during the dropping process , the system appeared light blue opalescence, continued to stir for a certain period of time to obtain light yellow transparent mullite sol.

Approximate amount of dilute acid, the smaller the colloidal particles obtained in the subsequent sol if the acid with higher molar concentration is used for decoagulation. The molar concentration range required in this step is the optimal range for controlling the particle size in the range of 10-200nm. If it exceeds this range, it is possible to obtain a mullite sol with a low concentration and a large particle size or not to obtain a stable sol.

Under the same conditions, the greater the total amount of dilute acid added, the smaller the colloidal particles in the subsequent sol. This step requires that the range of the amount of dilute acid added is the optimum value for controlling the particle size in the range of 10-200nm. If it exceeds this range, it is possible to obtain a mullite sol with a low concentration and a large particle size or not to obtain a stable sol.

The present invention does not have special limitation to the kind of dilute acid, can adopt a kind of in commonly used hydrochloric acid, nitric acid, sulfuric acid, formic acid, glacial acetic acid, is best with nitric acid. When choosing an inorganic acid, it can correspond to the inorganic aluminum salt, so that few ions are introduced into the system, which is convenient for later use, and types of different sources can also be used.

The present invention does not suppress the hydrolysis speed of aluminum alkoxide so that it co-hydrolyzes with silicon ester by various control means, but makes the aluminum oxide suspension co-hydrolyze with silicate while decoagulating by adding acid, thereby Realize high uniformity mixing of Al ³⁺ and Si ⁴⁺ at the molecular or atomic level. In the decondensation stage, the concentration and addition amount of dilute acid are very important for the formation of uniform and stable mullite sol. On the one hand, dilute acid is the inducement of decondensation, and on the other hand, it promotes the hydrolysis of silicate to form a stable sol.

The beneficial effect of the present invention compared with prior art:

(1) The present invention avoids the need to inhibit the hydrolysis rate of aluminum alkoxide, has no special requirements on process parameters such as pH value and environmental humidity, has simple process, low requirements on synthesis equipment, and is suitable for large-scale industrial production;

(2) The raw material kind involved in the present invention is few, only needs tetraethyl orthosilicate, ethanol and a kind of inorganic aluminum salt, a kind of dilute acid, does not need to adjust the proportioning between multiple aluminum sources, and avoids because of technological process To control the hydrolysis rate of aluminum alkoxide, there is no need to add acid and alkali to adjust the pH value of the reaction system, and no need to add chelating agent, which effectively reduces the influence of raw material factors on the formation process of single-phase mullite sol, and has good repeatability . The main aluminum source material involved in the present invention is aluminum inorganic salt, which is cheap and suitable for industrial application;

(3) The whole process of the present invention is carried out at room temperature, does not require heating, has low energy consumption, saves production costs, and is energy-saving and environmentally friendly;

(4) The preparation cycle of the single-phase mullite sol greatly shortened by the present invention, the whole production cycle is no more than 3 days, which is far lower than the existing preparation technology with slow hydrolysis as the core;

(5) The single-phase mullite sol particle size prepared by the present invention is small and uniformly distributed, and the solid content is about 10%. After rotary evaporation or standing volatile solvent concentration, the solid content can reach 30%, which is suitable for low temperature (1000 ℃) to prepare fiber-reinforced mullite-based composite materials, reduce the damage to the reinforcing fibers caused by the high temperature treatment (1200-1300 ℃) required for the mullite of dual-phase rubber, and improve the material performance.

Description of drawings

Fig. 1 is a molding process flow chart of the present invention;

Fig. 2 is the XRD pattern of the powder obtained after the mullite sol described in the present invention is dried at 80°C to dry gel and then heat-treated at 1050°C, where "*" represents the characteristic peak of mullite.

detailed description

The present invention will be described in detail below in conjunction with the accompanying drawings and specific examples.

Example 1

Single-phase mullite sol was prepared by using aluminum nitrate and ethyl orthosilicate as aluminum and silicon sources, and dilute nitric acid as decoagulant.

The preparation is shown in Figure 1 and is achieved through the following steps:

1. Prepare aluminum nitrate into a 1M aqueous solution, and gradually add 0.5M ammonia water dropwise thereto until a precipitate forms, then stir at room temperature for 30 minutes, and then filter and wash to obtain a solid substance.

2. Disperse the solid matter obtained by suction filtration in absolute ethanol at a mass ratio of 1:10, and stir at room temperature for 6 hours to obtain a white alumina suspension.

3. Keep stirring at room temperature, add ethyl orthosilicate dropwise to the alumina suspension according to the Al/Si molar ratio of 3/1, and continue stirring for 2 hours after all the silicate is added dropwise.

4. Keep the stirring at room temperature unchanged, add 2M dilute nitric acid dropwise to the suspension according to the ratio of Al/H+ molar ratio 4/1, during the dropping process, it can be found that the white color of the suspension fades quickly, and the system appears light blue Opalescence, after all the dilute nitric acid is added dropwise, continue to stir for 12 hours to obtain a transparent mullite sol.

The average particle diameter of the sol particles measured by a laser particle size analyzer is 36 nm. It can be seen from Figure 2 that the prepared sample has no impurity phase and is pure mullite powder, and the peak shape is sharp, which proves that the crystallization performance is relatively good. At the same time, the heat treatment temperature of 1050° C. proves that the mullite sol prepared by the present invention is a single-phase glue.

Example 2

Single-phase mullite sol was prepared by using aluminum chloride and tetraethyl orthosilicate as aluminum source and silicon source, and dilute hydrochloric acid as decoagulant.

Aluminum chloride was prepared into a 0.5M aqueous solution, and 0.2M potassium hydroxide aqueous solution was gradually added dropwise thereto until a precipitate formed, then stirred at room temperature for 60 minutes, and then filtered and washed to obtain a solid substance. The solid matter obtained by suction filtration was dispersed in absolute ethanol at a mass ratio of 1:20, and stirred at room temperature for 18 hours to obtain a white alumina suspension.

Keep stirring at room temperature, add ethyl orthosilicate dropwise to the alumina suspension according to the Al/Si molar ratio of 3/1, and continue stirring for 2 hours after all the silicate is added dropwise.

Keep stirring at room temperature, and add 1M dilute hydrochloric acid dropwise to the suspension according to the ratio of Al/H+ molar ratio of 10/1. During the dropping process, it can be found that the white color of the suspension fades quickly, and the system appears light blue opalescence , after the dilute nitric acid was added dropwise, the stirring was continued for 12 hours to obtain a transparent mullite sol. The average particle diameter of the sol particles measured by a laser particle size analyzer is 74 nm.

Example 3

Single-phase mullite sol was prepared by using aluminum sulfate and ethyl orthosilicate as aluminum source and silicon source, and dilute sulfuric acid as decoagulant.

Aluminum sulfate was prepared into a 2M aqueous solution, and 1M ammonia water was gradually added dropwise thereto until a precipitate formed, then stirred at room temperature for 60 min, and then centrifuged and washed to obtain a solid substance. The solid matter obtained by suction filtration was dispersed in absolute ethanol at a mass ratio of 1:10, and stirred at room temperature for 6 hours to obtain a white alumina suspension.

Keep stirring at room temperature, add ethyl orthosilicate dropwise to the alumina suspension according to the Al/Si molar ratio of 3/1, and continue stirring for 2 hours after all the silicate is added dropwise.

Keep stirring at room temperature, add 1M dilute sulfuric acid dropwise to the suspension according to the ratio of Al/H+ molar ratio 1/1, during the dropping process, it can be found that the white color of the suspension fades quickly, and the system appears light blue opalescence , after the dilute nitric acid was added dropwise, the stirring was continued for 12 hours to obtain a transparent mullite sol. The average particle size of the sol particles measured by a laser particle size analyzer is 17 nm.

Example 4

Single-phase mullite sol was prepared by using aluminum nitrate and ethyl orthosilicate as aluminum and silicon sources, and dilute nitric acid as decoagulant.

Aluminum nitrate was prepared into a 1M aqueous solution, and 1.5M ammonium bicarbonate solution was gradually added dropwise thereto until a precipitate formed, then stirred at room temperature for 60 min, and then filtered and washed to obtain a solid substance. The solid matter obtained by suction filtration was dispersed in absolute ethanol at a mass ratio of 1:10, and stirred at room temperature for 18 hours to obtain a white alumina suspension.

Keep stirring at room temperature, add ethyl orthosilicate dropwise to the alumina suspension according to the Al/Si molar ratio of 3/1, and continue stirring for 2 hours after all the silicate is added dropwise.

Keep stirring at room temperature, add 1M dilute nitric acid dropwise to the suspension according to the ratio of Al/H+ molar ratio 3/1, during the dropping process, it can be found that the white color of the suspension fades quickly, and the system appears light blue opalescence , after all the dilute nitric acid was added dropwise, the stirring was continued for 24 hours to obtain a transparent mullite sol. The average particle size of the sol particles measured by a laser particle size analyzer is 45 nm.

The average particle size is directly related to the molar concentration and the amount of dilute acid added. It can be seen from Examples 1 and 4 that for an approximate amount of dilute acid, the smaller the colloidal particles obtained in the subsequent sol are when the acid with a larger molar concentration is used for decoagulation; It can be known from Examples 2, 3, and 4 that the greater the total amount of dilute acid added, the smaller the colloidal particles in the subsequent sol.

Parts not described in detail in the present invention are well-known technologies for those skilled in the art.

Claims (2)

1. a preparation method of single-phase mullite sol, is characterized in that comprising the following steps: The first step is to prepare fresh aluminum hydroxide precipitation; The second step is to prepare alumina suspension, Disperse the fresh aluminum hydroxide precipitate obtained in the first step in absolute ethanol, and stir at room temperature to obtain an alumina suspension; In the third step, the silicon source is introduced into the alumina suspension, Keep stirring at room temperature, add silicate ester dropwise to the alumina suspension according to the Al/Si molar ratio of 3/1, and continue stirring to obtain a mixed suspension after all the silicate ester has been added dropwise; The fourth step is to decondense the mixed suspension, Keep stirring at room temperature, add dilute acid dropwise to the mixed suspension according to the ratio of Al/H ⁺ molar ratio of 1 to 10:1, and continue stirring to obtain a single-phase mullite sol; the molar concentration of the dilute acid is 1 to 10:1. 2M. 2. the preparation method of a kind of single-phase mullite sol according to claim 1 is characterized in that: described first step prepares fresh aluminum hydroxide precipitation, realizes by following steps, A1.1. Prepare an aqueous inorganic aluminum salt solution with a molar concentration of 0.2-2M; A1.2. Add an inorganic alkali solution with a molar concentration of 0.2 to 1.5 M dropwise into the aqueous solution of inorganic aluminum salt prepared in step A1.1 until a precipitate is formed, and then stir at room temperature for 30 to 60 minutes; A1.3. Filter and wash the precipitate to obtain fresh aluminum hydroxide precipitate.