阅读说明：本技术 一种用于制备超低密度气凝胶的超临界干燥方法及由该方法制得的产品 (Supercritical drying method for preparing ultra-low density aerogel and product prepared by same ) 是由 李健 张丽娟 李文静 张恩爽 赵英民 杨洁颖 张昊 于 2019-09-20 设计创作，主要内容包括：本发明公开了一种用于制备超低密度气凝胶产品的超临界干燥方法,包括：将湿凝胶浸没于放置有机溶剂的超临界干燥设备中并密封；将温度和压力调节为使得所使用的干燥介质呈液态,然后通入干燥介质以将有机溶剂带出；将温度和压力调节为使得干燥介质处于超临界状态,并通入干燥介质以进一步将有机溶剂带出；通过分段降压的方式将温度和压力恢复到常温常压状态。本发明还提供了由该方法制得的气凝胶产品。本发明方法解决了超低密度气凝胶经常规超临界干燥处理后隔热性能衰减严重且难以形成完整块体材料的问题,且可直接在常规设备上实现,本发明产品的密度低于50kg/m<Sup>3</Sup>,产品尺寸稳定性高,隔热性能优异,市场应用潜力大。(3 the invention discloses a supercritical drying method for preparing an ultra-low density aerogel product, which comprises the steps of immersing wet gel in supercritical drying equipment in which an organic solvent is placed and sealing, adjusting the temperature and the pressure to ensure that a used drying medium is in a liquid state, then introducing the drying medium to bring out the organic solvent, adjusting the temperature and the pressure to ensure that the drying medium is in a supercritical state, introducing the drying medium to further bring out the organic solvent, and recovering the temperature and the pressure to a normal temperature and normal pressure state in a sectional depressurization mode.)

1. a supercritical drying process for preparing an ultra low density aerogel product, comprising the steps of:

(1) Immersing the wet gel containing the first organic solvent in supercritical drying equipment in which a second organic solvent is placed in advance, and sealing;

(2) Adjusting the temperature in the supercritical drying equipment to a first temperature and the pressure in the supercritical drying equipment to a first pressure, so that a drying medium used for supercritical drying is in a liquid state under the conditions of the first temperature and the first pressure, and introducing the drying medium in the liquid state into the supercritical drying equipment to perform liquid-state cold circulation so as to take out the organic solvent from the wet gel;

(3) After the first organic solvent reaches diffusion equilibrium inside and outside the wet gel, adjusting the temperature in the supercritical drying equipment to a second temperature and adjusting the temperature in the supercritical drying equipment to a second pressure so that the drying medium is in a supercritical state under the conditions of the second temperature and the second pressure, and introducing the drying medium in the supercritical state into the supercritical drying equipment to perform supercritical thermal cycle so as to further carry out organic solvent out of the wet gel, thereby realizing supercritical drying of the wet gel;

(4) Recovering the temperature and pressure in the supercritical drying equipment to normal temperature and normal pressure states in a sectional depressurization mode, thereby preparing the ultra-low density aerogel product through supercritical drying;

wherein the first temperature is lower than the normal temperature, and the second temperature is higher than the normal temperature.

2. The method of claim 1, wherein:

the first organic solvent and the second organic solvent are independently non-alcohol organic solvents and are liquid at normal temperature and normal pressure;

Preferably, the first organic solvent and the second organic solvent are the same solvent.

3. the method of claim 1, wherein:

the volume ratio of the wet gel to be dried in the supercritical drying equipment to the second organic solvent placed in advance is 1: 2-1: 3;

preferably, the volume ratio of the wet gel to be dried in the supercritical drying apparatus to the supercritical drying apparatus is 1: 10-1: 5;

Further preferably, the drying medium is carbon dioxide.

4. The method of claim 1, wherein the liquid cold cycle comprises the steps of:

(I) Cooling the temperature of the drying medium to the first temperature, preferably, the first temperature is 10-18 ℃;

(II) introducing the cooled drying medium into the closed supercritical drying equipment until the pressure in the supercritical drying equipment becomes the first pressure, preferably, the first pressure exceeds the liquefaction pressure of the drying medium at the first temperature by 0.5-1.0 MPa;

(III) performing a liquid cold cycle process under conditions to maintain the first temperature and the first pressure and to maintain a flow balance until the first organic solvent reaches a diffusion balance inside and outside the wet gel.

5. the method of claim 1, wherein the supercritical thermal cycle comprises the steps of:

(i) stopping introducing the drying medium into the supercritical drying equipment;

(ii) Uniformly heating the supercritical drying equipment to the second temperature at the speed of 0.2-0.4 ℃/min, preferably, the second temperature is 35-40 ℃, and the heating speed is 0.2-0.4 ℃/min;

(iii) Heating the temperature of the drying medium to the second temperature;

(iv) Introducing the heated carbon dioxide into the supercritical drying equipment until the pressure in the supercritical drying equipment reaches the second pressure, preferably, the second pressure is 1-2 MPa higher than the supercritical pressure of the drying medium at the second temperature;

(v) performing the supercritical thermal cycle under conditions that maintain the second temperature and the second pressure and maintain flow balance.

6. The method of claim 1, wherein the step of step-down is performed by: the process of reducing the pressure in the supercritical drying equipment to normal pressure is divided into three stages: (I) reducing the supercritical pressure to 7.3MPa, and controlling the pressure reduction rate to be 0.04-0.08 MPa/min; (II) reducing the pressure from 7.3MPa to 2MPa, and controlling the pressure reduction rate to be 0.1-0.15 MPa/min; (III) reducing the pressure from 2MPa to normal pressure, and controlling the pressure reduction rate to be 0.2 MPa/min.

7. the method of claim 1, wherein:

The aerogel is an ultra-low density aerogel;

it is also preferred that the aerogel is a silica aerogel;

more preferably, the aerogel is an ultra-low density silica aerogel;

it is further preferred that the wet gel for preparing the ultra-low density silica aerogel is prepared by the following method:

(A) Mixing methyl orthosilicate, methanol, deionized water and hydrochloric acid, heating and refluxing, and then carrying out distillation reaction to obtain hydrolyzed ultralow-density aerogel glue solution;

(B) And mixing the ultra-low density aerogel glue solution with the first organic solvent and the sodium hydroxide aqueous solution, then injecting into a closed mold, and standing to finish gel aging.

8. The method of claim 1, wherein:

in the step (A), the molar ratio of methyl orthosilicate to methanol is 1: 2-1: 3, the molar ratio of methyl orthosilicate to deionized water is 1: 1.3-1: 1.8, hydrochloric acid is added in a form that the concentration of hydrochloric acid in an aqueous solution reaches 5 x 10 ^-3 M, more preferably, the reflux temperature is 75-85 ℃, the reflux time is 10-15 h, and/or the distillation temperature is 90-120 ℃, and the distillation time is 8-12 h;

(B) The concentration of the sodium hydroxide aqueous solution is 0.1 to 0.3M; the size of the inner cavity of the closed mould is not less than 400mm multiplied by 20 mm; standing for 24-48 h; the mass ratio of the ultra-low density aerogel glue solution to the first organic solvent is 1: 25-1: 50.

9. the method of claim 1, wherein:

the aerogel product is a flat-plate-shaped silicon dioxide aerogel product with the size not less than 250mm multiplied by 20 mm;

the material density of the aerogel product is 10-50 kg/m ³, and/or

the normal-temperature heat conductivity coefficient of the aerogel product is less than or equal to 0.021W/(m.K).

10. an aerogel product made by the method of any of claims 1-9.

Technical Field

The invention relates to a drying method of aerogel materials, in particular to a supercritical drying method for preparing ultralow-density aerogel and a product prepared by the supercritical drying method.

background

Aerogel is considered to be the best insulating solid material presently discovered. The pore diameter of the aerogel is smaller than the mean free path of air molecules, and no air convection exists in the pores of the aerogel, so that the aerogel has extremely low gaseous heat conduction; meanwhile, the aerogel has extremely high porosity and low volume ratio of solid, so the solid heat conduction is low.

Due to the narrow space and high task cost of ships, space stations, airships and the like, the use of ultra-low density aerogel materials with the density of less than 50kg/m ³ plays an important role in reducing the effective load and improving the safety of the system.

However, the current research on the supercritical drying process only aims at the aerogel material with the conventional density, namely the density of more than 100kg/m ³, for the ultra-low density aerogel with ultrahigh porosity and weak framework strength, the dimensional shrinkage changes violently after the conventional supercritical drying treatment, even the complete block is difficult to maintain, and the heat insulation performance is also seriously attenuated due to the damage of the nano-pore structure.

disclosure of Invention

The invention aims to provide a supercritical drying method suitable for an ultralow-density aerogel material aiming at the defects of the conventional supercritical drying process, so as to solve the problems that the aerogel structure is seriously shrunk and the heat-insulating property is seriously reduced after the conventional supercritical drying process is used for treatment.

the present invention provides in a first aspect a supercritical drying process for the preparation of an ultra low density aerogel product, characterized in that said process comprises the steps of:

(1) Immersing the wet gel containing the first organic solvent in supercritical drying equipment in which a second organic solvent is placed in advance, and sealing;

(2) adjusting the temperature in the supercritical drying equipment to a first temperature and the pressure in the supercritical drying equipment to a first pressure, so that a drying medium used for supercritical drying is in a liquid state under the conditions of the first temperature and the first pressure, and introducing the drying medium in the liquid state into the supercritical drying equipment to perform liquid-state cold circulation so as to take out the organic solvent from the wet gel;

(3) after the first organic solvent reaches diffusion equilibrium inside and outside the wet gel, adjusting the temperature in the supercritical drying equipment to a second temperature and adjusting the temperature in the supercritical drying equipment to a second pressure so that the drying medium is in a supercritical state under the conditions of the second temperature and the second pressure, and introducing the drying medium in the supercritical state into the supercritical drying equipment to perform supercritical thermal cycle so as to further carry out organic solvent out of the wet gel, thereby realizing supercritical drying of the wet gel;

(4) Recovering the temperature and pressure in the supercritical drying equipment to normal temperature and normal pressure states in a sectional depressurization mode, thereby preparing the ultra-low density aerogel product through supercritical drying;

Wherein the first temperature is lower than the normal temperature, and the second temperature is higher than the normal temperature.

The present invention provides, in a second aspect, an aerogel product made according to the method of the first aspect of the invention.

The invention provides a supercritical drying method of an ultralow-density aerogel material, which can realize effective protection of an ultralow-density aerogel nano-skeleton structure in a drying process through optimized process control steps. Firstly, compared with the conventional drying mode that the supercritical drying equipment is directly heated to enable the drying medium such as carbon dioxide to enter the supercritical state, the supercritical drying equipment is introduced with the liquid drying medium such as liquid carbon dioxide in advance to carry out cold circulation so as to replace most of the organic solvent in the aerogel, so that the structural damage of the aerogel caused by the rapid removal of the organic solvent under the action of high temperature and concentration difference can be effectively avoided. The wet gel to be dried is soaked in advance with liquid carbon dioxide, and the solvent in the wet gel may be replaced with carbon dioxide in advance before supercritical drying. Although supercritical carbon dioxide can eliminate a gas-liquid interface when a solvent is removed from an aerogel, before the carbon dioxide reaches a supercritical state, a heating and pressurizing process that carbon dioxide is not in the supercritical state is always performed, and in the process, an organic solvent in wet gel is rapidly removed from a gel framework under the action of high temperature and concentration difference, which is a main reason that the structure of low-density gel is difficult to maintain. The above problems can be avoided by replacing the solvent in the wet gel with carbon dioxide in advance.

Second, by controlling the rate of temperature rise of the drying medium, such as carbon dioxide, into the supercritical drying state, the structural shrinkage of the aerogel due to thermal stress is reduced while keeping the bulk material from changing significantly in size. And finally, controlling the decompression rate in the supercritical drying equipment after the supercritical drying in a segmented manner, so that a drying medium such as carbon dioxide fluid is slowly released from the inside of the aerogel, and the structural damage of the aerogel caused by the internal and external pressure difference is avoided.

Compared with the prior art, the supercritical drying method provided by the invention solves the problems that the nano-skeleton structure of the ultra-low density aerogel block material with the density lower than 50kg/m ³ is easy to damage in the drying process and the heat insulation performance is seriously attenuated.

Drawings

FIG. 1 is a photograph of the product obtained in the example. FIG. 1, Panel A, shows a photograph of a product (set on a white plastic bowl) made in example 1 of the present invention; fig. 1, panel B, shows a photograph of the product (contained in a transparent plastic bag) produced in comparative example 1 of the present invention.

Detailed Description

in order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be described more clearly and completely in conjunction with the embodiments of the present invention, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.

The present invention provides in a first aspect a supercritical drying process for the preparation of an ultra low density aerogel product, characterized in that said process comprises the steps of:

(1) immersing the wet gel containing the first organic solvent in supercritical drying equipment in which a second organic solvent is placed in advance, and sealing;

(2) adjusting the temperature in the supercritical drying equipment to a first temperature and the pressure in the supercritical drying equipment to a first pressure, so that a drying medium used for supercritical drying is in a liquid state under the conditions of the first temperature and the first pressure, and introducing the drying medium in the liquid state into the supercritical drying equipment to perform liquid-state cold circulation so as to take out the organic solvent from the wet gel;

(3) After the first organic solvent reaches diffusion equilibrium inside and outside the wet gel, adjusting the temperature in the supercritical drying equipment to a second temperature and adjusting the temperature in the supercritical drying equipment to a second pressure so that the drying medium is in a supercritical state under the conditions of the second temperature and the second pressure, and introducing the drying medium in the supercritical state into the supercritical drying equipment to perform supercritical thermal cycle so as to further carry out organic solvent out of the wet gel, thereby realizing supercritical drying of the wet gel;

(4) Recovering the temperature and pressure in the supercritical drying equipment to normal temperature and normal pressure states in a sectional depressurization mode, thereby preparing the ultra-low density aerogel product through supercritical drying;

wherein the first temperature is lower than the normal temperature, and the second temperature is higher than the normal temperature.

in some preferred embodiments, the first organic solvent and the second organic solvent are independently non-alcoholic organic solvents, and are in a liquid state at normal temperature and pressure, such as acetonitrile, diethyl ether, acetone, and the like. In other preferred embodiments, the first organic solvent and the second organic solvent are the same solvent.

In other preferred embodiments, the volume ratio of the wet gel to be dried to the second organic solvent previously placed in the supercritical drying apparatus is 1: 2-1: 3. it is also preferable that the volume ratio of the wet gel to be dried in the supercritical drying apparatus to the supercritical drying apparatus is 1: 10-1: 5. it is also preferred that the drying medium is carbon dioxide, since the supercritical temperature of carbon dioxide is only 31 ℃, which can reduce the thermal shrinkage of the ultra low density aerogel during high temperature supercritical cycling.

in other preferred embodiments, the liquid cooling cycle comprises the steps of:

(I) cooling the temperature of the drying medium to the first temperature, preferably, the first temperature is 10-18 ℃;

(II) introducing the cooled drying medium into the closed supercritical drying equipment until the pressure in the supercritical drying equipment becomes the first pressure, preferably, the first pressure exceeds the liquefaction pressure of the drying medium at the first temperature by 0.5-1.0 MPa;

(III) performing a liquid cold cycle process under conditions to maintain the first temperature and the first pressure and to maintain a flow balance until the first organic solvent reaches a diffusion balance inside and outside the wet gel.

in other preferred embodiments, the supercritical thermal cycle comprises the steps of:

(i) stopping introducing the drying medium into the supercritical drying equipment;

(ii) uniformly heating the supercritical drying apparatus to the second temperature at a rate of 0.2 to 0.4 ℃/min, e.g., 0.3 ℃/min, preferably the second temperature is 35 to 40 ℃, e.g., 38 ℃;

(iii) Heating the temperature of the drying medium to the second temperature;

(iv) introducing the heated carbon dioxide into the supercritical drying equipment until the pressure in the supercritical drying equipment reaches the second pressure, preferably, the second pressure is 1-2 MPa higher than the supercritical pressure of the drying medium at the second temperature;

(v) Performing the supercritical thermal cycle under conditions that maintain the second temperature and the second pressure and maintain flow balance.

In other preferred embodiments, the step of stepwise reducing the pressure is performed by: the process of reducing the pressure in the supercritical drying equipment to normal pressure is divided into three stages: (I) reducing the supercritical pressure to 7.3MPa, and controlling the pressure reduction rate to be 0.04-0.08 MPa/min (such as 0.06 MPa/min); (II) reducing the pressure from 7.3MPa to 2MPa, and controlling the pressure reduction rate to be 0.1-0.15 MPa/min; (III) reducing the pressure from 2MPa to normal pressure, and controlling the pressure reduction rate to be 0.2 MPa/min.

in other preferred embodiments, the aerogel is an ultra low density aerogel; it is also preferred that the aerogel is a silica aerogel; more preferably, the aerogel is an ultra-low density silica aerogel; it is further preferred that the wet gel for preparing the ultra-low density silica aerogel is prepared by the following method:

(A) mixing methyl orthosilicate, methanol, deionized water and hydrochloric acid, heating and refluxing, and then carrying out distillation reaction to obtain hydrolyzed ultralow-density aerogel glue solution;

(B) and mixing the ultra-low density aerogel glue solution with the first organic solvent and the sodium hydroxide aqueous solution, then injecting into a closed mold, and standing to finish gel aging.

In other more preferred embodiments, in step (A), the molar ratio of methyl orthosilicate to methanol is 1: 2 to 1: 3, the molar ratio of methyl orthosilicate to deionized water is 1: 1.3 to 1: 1.8, hydrochloric acid is added in a form of reaching a concentration of 5 × 10 ^-3 M in an aqueous solution, more preferably, the reflux temperature is 75 to 85 ℃, the reflux time is 10 to 15 hours, and/or the distillation temperature is 90 to 120 ℃, and the distillation time is 8 to 12 hours;

(B) the concentration of the sodium hydroxide aqueous solution is 0.1 to 0.3M; the size of an inner cavity of the closed mold is not less than 400mm multiplied by 20mm, and the standing time is 24-48 h; the mass ratio of the ultra-low density aerogel glue solution to the first organic solvent is 1: 25-1: 50.

In some more specific embodiments, the wet gel is prepared by the following method:

(A) mixing methyl orthosilicate, methanol, deionized water and hydrochloric acid in a certain stoichiometric ratio in a round bottom flask, heating, refluxing and distilling for several hours to react to obtain hydrolyzed ultralow-density aerogel glue solution, wherein the molar ratio of the methyl orthosilicate to the methanol is 1: 2-1: 3, the molar ratio of the methyl orthosilicate to the deionized water is 1: 1.3-1: 1.8, the hydrochloric acid is added in a form that the concentration of the hydrochloric acid in an aqueous solution reaches 5 x 10 ^-3 M, the refluxing temperature is 75-85 ℃, the refluxing time is 10-15 h, the distilling temperature is 90-120 ℃, and the distilling time is 8-12 h.

^{3 3}(B) Fully mixing the hydrolyzed ultra-low density aerogel glue solution, a first organic solvent for dilution and a 0.2M sodium hydroxide solution, injecting the mixture into a closed mold with the inner cavity size not lower than 400mm multiplied by 20mm, standing for 24-48 h to finish gel aging, wherein the first organic solvent can be a non-alcohol organic solvent such as acetonitrile, diethyl ether, acetone and the like, and the mass ratio of the hydrolyzed ultra-low density aerogel glue solution to the first organic solvent influences the density of the final ultra-low density aerogel.

In other preferred embodiments, the aerogel product is a flat-plate-shaped silica aerogel product with the size of not less than 250mm × 250mm × 20mm, the material density of the aerogel product is 10-50 kg/m ³, and/or the normal-temperature thermal conductivity coefficient of the aerogel product is not more than 0.021W/(m · K).

the invention provides an aerogel product prepared by the method in the first aspect, preferably, the aerogel product is a flat-plate-shaped silicon dioxide aerogel product with the size of not less than 250mm multiplied by 20mm, the material density of the aerogel product is 10-50 kg/m ³, and/or the normal-temperature thermal conductivity coefficient of the aerogel product is not more than 0.021W/(m.K).

In some more specific embodiments, the method of the present invention is used to prepare an ultra-low density aerogel block having dimensions of no less than 250mm x 20mm, the method comprising the steps of:

(1) a second organic solvent, which is identical to the first organic solvent for dilution used for preparing the wet gel, is previously placed in a supercritical drying apparatus (e.g., a drying pot). Reducing the amount of second organic solvent used will help reduce the drying time, while ensuring that the second organic solvent is sufficient to submerge the wet gel. Preferably, the volume ratio of the wet gel to the second organic solvent is controlled to be 1: 2-1: 3;

(2) And (3) putting the wet gel to be dried into supercritical drying equipment, and sealing the supercritical drying equipment. Preferably, the volume ratio of wet gel to supercritical drying apparatus is 1: 10-1: 5;

(3) cooling a carbon dioxide storage tank serving as a drying medium by using a precooler, preferably, reducing the temperature of the carbon dioxide from normal temperature to 10-18 ℃;

(4) Introducing cooled carbon dioxide into the closed supercritical drying equipment until the pressure in the supercritical drying equipment exceeds the liquefaction pressure of the carbon dioxide at the temperature by 0.5-1 MPa;

(5) And opening a liquid outlet valve of the supercritical drying equipment to enable the second organic solvent pre-placed in the supercritical drying equipment to flow into the separation kettle along with the liquid carbon dioxide. Preferably, the cold circulation process is maintained for 24-48 h, such as 36 h;

(6) And stopping introducing the carbon dioxide into the supercritical drying equipment, closing a liquid outlet valve of the supercritical drying equipment, and then starting a heating device of the supercritical drying equipment until the supercritical drying equipment is uniformly heated to be above the supercritical temperature of the carbon dioxide. In order to reduce the thermal shrinkage of the aerogel in the heating process as much as possible, the heating temperature is preferably 35-40 ℃, and the heating rate is controlled to be 0.2-0.4 ℃/min, such as 0.3 ℃/min;

(7) starting a carbon dioxide gas circuit heat exchange device to enable the temperature of carbon dioxide in a gas circuit to reach the temperature of supercritical drying equipment, namely 35-40 ℃;

(8) and introducing the heated carbon dioxide into the supercritical drying equipment until the pressure in the supercritical drying equipment exceeds the supercritical pressure of the carbon dioxide at the temperature of the supercritical drying equipment by 1-2 MPa. Maintaining the temperature and the pressure of the supercritical drying equipment, and performing supercritical thermal circulation for 10-15 h;

(9) And (4) after circulation, carrying out constant-temperature and constant-speed depressurization on the supercritical drying equipment, and after the pressure of the supercritical drying equipment is recovered to the normal pressure and cooled, obtaining the dried ultra-low density aerogel block material. Preferably, the supercritical pressure is reduced to 7.3MPa, the pressure reduction rate is controlled to be 0.04-0.08 MPa/min, the pressure reduction rate is reduced to 2MPa from 7.3MPa, the pressure reduction rate is controlled to be 0.1-0.15 MPa/min, the pressure reduction rate is reduced to normal pressure from 2MPa, and the pressure reduction rate is controlled to be 0.2 MPa/min.

the process according to the invention will be further illustrated by way of example, without however the scope of protection of the invention being limited to these examples.