CN112403407A

CN112403407A - Method for preparing aerogel through negative pressure and microwave drying

Info

Publication number: CN112403407A
Application number: CN202011360750.1A
Authority: CN
Inventors: 巩和一; 刘文洁; 陈江涛
Original assignee: 巩和一; 刘文洁; 陈江涛
Current assignee: AVIC Research Institute (Guangdong) Co.,Ltd.
Priority date: 2020-11-27
Filing date: 2020-11-27
Publication date: 2021-02-26

Abstract

The invention relates to a method for preparing aerogel by negative pressure and microwave drying. The method comprises placing gel to be dried into a device which can be pumped to generate negative pressure and is also provided with a microwave generator, generating microwave when the device is pumped to generate negative pressure, and volatilizing water and solvent in the gel under the action of the microwave. After the water and the solvent in the gel are completely separated out and dried, the gap left by the water and the solvent in the gel is occupied by air, and the aerogel with high porosity and low density is obtained. The method can adopt conventional equipment, and in a short time, water and solvent are completely separated out under the condition that the gel is not collapsed in an internal network structure, so that the high price and danger of supercritical drying equipment are avoided, the yield of the aerogel is greatly improved, and the production cost of the aerogel is greatly reduced.

Description

Method for preparing aerogel through negative pressure and microwave drying

Technical Field

The present invention belongs to the field of new material preparation. Is a preparation method of the aerogel with high porosity, high specific surface area and low density.

Background

Regardless of the drying method employed, the resulting material can be referred to as an aerogel, so long as the liquid in the wet gel is replaced with gas while the network structure of the gel remains substantially unchanged. It is usually light nanometer solid material with nanometer level superfine particles mutually aggregated to form nanometer porous network structure and gaseous dispersing medium filled in the network gap. More than 90% of the constituent substances are gases, so that the solid is one of the lowest-density and lightest-weight solids in the world to date.

The unique properties of the aerogel impart its unique properties. At present, the aerogel material becomes the foundation of technologies such as microelectronics, information, aviation detection, hydrogen fuel storage, optics, solar energy utilization and the like, widely permeates into various fields of modern science and technology, and the development of aerogel materials with special functions and special purposes is an important component of high and new technology. The aerogel has countless pores and folds and can be used for absorbing and removing pollutants in water, such as lead, watermarks and even leaked crude oil on the sea; aerogel is used as the most excellent heat-insulating material, is called as a final heat-insulating material, can insulate heat at the temperature of more than 1300 ℃, and is very suitable for energy conservation in the fields of spaceflight, military affairs, industry and buildings. In the energy field, some forms of aerogels made of platinum accelerate hydrogen production, so that they can be used to produce hydrogen-based fuels, achieving high efficiency and environmental protection. Meanwhile, the aerogel can also obviously improve the combustion rate of the lignite, so that the pressure of energy conservation and emission reduction in the steel and coal industry with serious pollution can be effectively relieved. By combining the above characteristics, the material can be made into an excellent material integrating invisibility, dryness insulation, infrared wave absorption, explosion prevention and heat insulation. The journal of science of the united states lists aerogels as one of ten popular sciences.

The preparation of aerogels, at present, is mainly carried out by the following steps: the raw materials are subjected to sol, gel, aging and replacement to obtain gel which can be dried; and then obtaining the aerogel through ultra-zero boundary drying.

The drying process of the gel is very complicated, a large amount of liquid solvent exists in the structure of the gel network, and the liquid forms a meniscus in capillary pores of the gel network to generate additional pressure. As the capillary porosity decreases, the additional pressure increases. The strong capillary forces cause the particles to further contact, squeeze, aggregate and contract, collapsing the gel network structure. The most effective way to eliminate the gel-damaging effect of liquid surface tension is to drive off the liquid in the gel pores under supercritical fluid conditions. The supercritical fluid has both gas property and liquid property, the gas-liquid interface disappears, the surface tension does not exist, and at the moment, capillary additional pressure does not exist in the pores of the gel, so that the original network structure of the gel can be maintained by drying under the supercritical fluid condition, and the agglomeration and the coagulation of nano particles are prevented. However, supercritical drying requires a high pressure of 7.3MPa, and the conditions are very harsh, and conventional equipment cannot complete the supercritical drying. Therefore, the aerogel can not be produced in large scale and in batch, and the production cost is high, so that the aerogel product is extremely expensive and difficult to popularize and use.

Disclosure of Invention

The negative pressure and microwave drying method provided by the invention can be used for producing aerogel by adopting conventional equipment, realizes the continuous and large-scale production of aerogel, greatly reduces the price of aerogel products, and lays a foundation for popularization and use.

The invention is realized by the following method:

the method for preparing the aerogel by negative pressure and microwave drying is adopted, the gel to be dried is put into a device which can be pumped to generate negative pressure (P) and is also provided with a microwave generator, and the microwave is generated when the negative pressure is pumped to act on the gel to be dried. The microwave has very good penetrability, and when the microwave penetrates into the gel, the microwave can generate a certain phase with the gelThe interaction is carried out at microwave frequency of 2450 MHz, so that water and solvent in the gel generate 24 hundred million vibration per second, and the molecules generate friction with each other. The water and the solvent are converted into an excited state from a high-speed rotation state under the action of a microwave field, which is a high-energy unstable state, and the water and the solvent are gasified and separated out (an volatile force P is generated)₂). As the water and solvent in the gel volatilize under the action of microwave, additional pressure (P) is generated in capillary pores of the gel network₁). The negative pressure in the closed space balances out the additional pressure (P) generated by the capillary pores left by the water and the solvent in the gel when the water and the solvent are separated out₁≈P+P₂). The shrinkage forces of the capillary pores do not cause the particles to further contact, squeeze, aggregate and shrink, that is, the gel network structure does not collapse. And after the water and the solvent in the gel are completely volatilized, separated and dried, the aerogel with high porosity and low density can be obtained.

The method for preparing the aerogel by adopting negative pressure and microwave drying comprises the steps of putting gel to be dried into a device which can be pumped to generate negative pressure and is provided with a microwave generator, generating microwave when the negative pressure is generated in the device by pumping to act on the gel to be dried, exciting water and a solvent in the gel to volatilize and be pumped out by the negative pressure, balancing internal force caused by left gaps when the water and the solvent volatilize in the gel due to the existence of the negative pressure, avoiding collapse of a network structure in the gel until the water and the solvent in the gel are all volatilized and pumped out, and obtaining the aerogel with low density, high porosity and extremely low heat conductivity coefficient.

Drawings

FIG. 1 is a device of the present invention having a negative pressure and provided with a microwave generator;

1-device, 2-microwave generator, 3-device gate, 4-gel, 5-air exhaust pipe, 6-water discharge valve (or air inlet valve), P-negative pressure, P₁Capillary force, P₂-volatile power

Detailed Description

The invention is further illustrated by the following examples. It should be understood that the method described in the examples is only for illustrating the present invention and not for limiting the present invention, and that simple modifications of the preparation method of the present invention based on the concept of the present invention are within the scope of the claimed invention. All the starting materials and solvents used in the examples are commercially available products.

1. Gel preparation

The hydrophilic silica aerogel with low density and large porosity is prepared by an acid-base two-step catalysis method by adopting tetraethoxysilane as a precursor for preparing the silica aerogel and a mixed solution of water and ethanol as a solvent. If hydrophobic silica aerogel needs to be prepared, trimethylchlorosilane is adopted as a hydrophobic agent in the preparation process.

Firstly, raw materials are mixed according to the molar ratio of ethyl orthosilicate: water: ethanol ═ 1: 4: 4, fully mixing and uniformly stirring, adjusting the pH value of the sol to 3-4 by using dilute hydrochloric acid, adding dilute ammonia water to adjust the pH value of the sol to 6-7 after the reaction is carried out for a period of time, and mixing and stirring in a stirring kettle. And conveying the stirred gel liquid to an aging box for sealing, and aging for 1-72 hours. Adding ethanol, soaking the gel in the ethanol until the gel is submerged, aging at 50 deg.C for one day, and cooling to room temperature.

2. Solvent replacement

Soaking the colloid in n-hexane as a displacement solvent, completely submerging the gel, and performing displacement for more than 5 times, each time for 2 hours. If hydrophobic treatment is needed, 10-15% trimethylchlorosilane normal hexane solution is slowly poured into the gel, the gel is completely submerged, the container is sealed to avoid volatilization of the surface treating agent, and the reaction is carried out at room temperature for two days. The gel to be dried can be obtained through the steps.

3. Drying by negative pressure and microwave

In the device which can extract air to generate negative pressure and is provided with a microwave generator (a small amount of air can be always put in through a drain valve, and the amount of the air is based on the guarantee of the negative pressure value), gel to be dried is put in.

And (3) starting to exhaust the device to enable the space in the device to generate negative pressure, starting a microwave generator when the negative pressure reaches-0.03 MPa, controlling the temperature of the gel at 30-50 ℃ under the action of microwaves and controlling the microwave power to be 900W, so that water and a solvent in the gel begin to separate out.

The capillary pores left in the gel due to the evolution of water and solvent in the gel create additional pressure, which inward additional pressure, otherwise known as capillary force, causes the lattice structure in the gel to collapse. At this point, the negative pressure within the device and the outward forces created by the volatilization of the water and solvent balance the inward capillary forces.

The behavior of the device at this time is: the negative pressure value (absolute value) of the closed space tends to become smaller (namely the negative pressure is towards the direction of minus 0.02 MPa), and the air suction quantity is adjusted by the control device, so that the negative pressure of the closed space is always kept near minus 0.03MPa until the negative pressure value is not required to be adjusted and is not changed. At this time, the water and solvent in the gel are completely volatilized under the action of the microwave, and the negative pressure and the microwave quantity are kept unchanged for more than 50 minutes in order to fully enter air into the grid structure in the gel and dry the grid structure. The aerogel with good elasticity, high porosity, large specific surface area, low density and extremely high heat-insulating property can be obtained.

In addition, as shown in fig. 1, 1 is a device which can be pumped to generate negative pressure and is also provided with a microwave generator, 2 is a microwave generator arranged in the device; in order to make the negative pressure in the device uniform, a plurality of layers of air extraction holes are arranged on the side wall of the device in a layering manner; for the convenience of automatic control, the door of the 3 rd device should be arranged at the top; 4, placing the gel to be dried in a tray, wherein the bottom of the tray is provided with supporting legs so as to leave a space between the bottom of the tray and the bottom of the device; the bottom of the device is designed to incline to one side and is provided with an adjustable drain valve (6) to facilitate the accumulation and the removal of the redundant liquid; similarly, 5 is that the extraction opening is arranged on the upper part of the sealing device.

Firstly, placing gel to be dried in a tray, placing the tray in the device by a robot (or manual operation), closing the door of the device, starting to exhaust air to generate negative pressure (P) in the space of the device, starting a microwave generator, and generating volatile force P by separating out water and solvent in the gel under the action of microwaves₂The remaining capillary pores generate an additional pressure P due to the precipitation of water and solvent from the gel₁Negative pressure value (absolute) of space in deviceValue) has a tendency of diminishing, and the negative pressure in the space of the automatic adjusting device is always kept near a certain numerical value (ensuring P) in the range of 0.01-0.06 MPa through the control device₁≈P+P₂) So that the force of contraction of the capillary pores does not cause the particles to further contact, squeeze, aggregate and contract, collapsing the gel network structure. Until the negative pressure value is not needed to be adjusted, the change is not generated and is kept for more than 50 minutes. The aerogel with good elasticity, high porosity, large specific surface area, low density and extremely high heat-insulating property can be obtained.

Claims

1. Aerogel, characterized in that: when the gel is dehydrated of water and solvent, the space network structure of the gel is filled with gas and takes the solid appearance.

2. The aerogel production method according to claim 1, wherein: and (3) passing the gel to be dried through a device which generates negative pressure and is provided with a microwave generator, separating out water and solvent in the gel and reserving a space grid structure of the gel to obtain the aerogel with low density, high porosity and extremely low heat conductivity coefficient.

3. A method for the preparation of aerogels according to claim 2, characterized in that: the negative pressure range is 0.01-0.06 MPa.

4. A method for the preparation of aerogels according to claim 2, characterized in that: the power of the microwave generator is controlled to be 0.01-150 kW, and the temperature in the device is controlled to be 20-60 ℃.

5. A method for the preparation of aerogels according to claim 2, characterized in that: and controlling the reaction time of the negative pressure and the microwave generator to be 0.01-12 hours.

6. Aerogel characterized by being obtained by the process according to any one of claims 2 to 5.

7.1 or 6 has wide application in the aspects of spaceflight, heat insulation, energy storage and the like.