Patent ID: 9731264
Date: 2017-08-15
CPC Classifications: B01J,C01B,F26B

Claim:
1. A method for preparing aerogel or gel by drying under reduced pressure is that: the gel to be dried is placed into a sealed drying apparatus which can be depressurized by air extraction and heated to raise temperature; the method of depressurization by air extraction, gradient depressurization or slow depressurization, or temperature-raising, gradient temperature-raising or slow temperature-raising, or controlling the depressurization by air extraction and the gradient or slow temperature-raising in the meantime is acted upon water and solvents within the aerogel to be dried, and outward force is thus generated for the water and solvents to be volatilized outwards; by controlling rates of reducing pressure by air extraction and raising temperature, an outward force is allowed to be equal to or close to an inward capillary force, such that pores of the gel may not be collapsed or cracked; at the same time, drying is carried out continuously, and finally interior water and solvents are removed completely; the above-mentioned method of reducing pressure by air extraction is described as following: when a vapor-liquid interface appears in the gel starting to be dried, surface tension is formed on the vapor-liquid interface; at the same time, a force pointing to a concave point of the liquid surface is generated on a three-phase interface, that is the force tending to collapse inwards, referred to as capillary force and represented by F1; at the same time, the depressurization also applies outward suction force represented by F2 to the gel; when F1=F2, or F1 is close to F2, as long as a force difference is not enough to make gel with the surface starting to be dried at this moment be collapsed or cracked, until the solvents and water in the gel are extracted completely; the above-mentioned method of raising temperature is described as following: upon action of raising temperature and heating, the water and solvents within the gel in the drying apparatus are discharged continuously, the vapor-liquid interface appears within the gel, and the surface tension is generated on the vapor-liquid interface; at the same time, the three-phase interface appears in the gel, the capillary force appears on the three-phase interface, and the capillary force is proportional to the surface tension, and is inversely proportional to a capillary radius; a second capillary force represented by F3 is the force tending to collapse inwards; outward expansive force represented by F4 is generated by increased vapor pressure of the water and solvents within the gel upon the action of heating; when F3=F4, or F3 is close to F4, as long as a force difference is not enough to make pores of the gel with the surface starting to be dried at a moment be collapsed or cracked, the gel will be gradually dried from the outside to the inside; at this moment, the pores of the gel are kept intact by gradually raising temperature and maintaining that F3=F4 or F3 is close to F4, until the solvents and water in the gel are baked and discharged completely and thoroughly; the above-mentioned method of reducing pressure by air extraction and the raising temperature in the meantime is described as following: upon a double action of the depressurization by air extraction and temperature-raising, the water and solvents within the gel in the drying apparatus are discharged continuously, the vapor-liquid interface appears within the gel, and when the three-phase interface appears in the gel, the surface tension is generated; a third capillary force represented by F5 is the force tending to collapse inwards; at the same time, a second outward force represented by F6 is generated by the water and solvents within the gel upon heating, and a third outward force represented by F7 is generated by the water and solvents within the gel upon depressurization; when F5=F6+F7, or F5 is close to F6+F7, as long as the force difference is not enough to make pores of the gel with the surface starting to be dried at this moment be collapsed or cracked, the gel will be gradually dried from the outside to the inside; in case the pores of the gel will not be collapsed and cracked, the solvents and water in the gel are gradually removed completely by baking and extraction; after completely removing the solvents and water in the gel by the above-mentioned methods, a residual high boiling point volatiles in the gel are removed by drying at a temperature of 180° C.