Technical principle of nano porous insulation board

1, development overview
The utility model relates to a vacuum insulation board, which is formed by filling the core material with good thermal insulation property into a gas isolating structure, and then sealing them after vacuum. The research and development of VIP board began in the 50s of last century, it has been more than sixty years of history, after the year of 90s, many large companies in Europe and America have carried out the research of the VIP board, and have made rich achievements in scientific research. Now the VIP board at home and abroad has been widely used in refrigerators, freezers, cold storage, refrigerated trucks, refrigerated containers, medical incubator, field wall insulation, such as in Japan, more than 70% of the domestic refrigerator insulation layer using VIP plate, VIP plate has been widely used in Europe and the United States in the frozen ship transport box in.

2, VIP board structure
The vacuum insulation panel usually consists of three parts: gas insulation structure, core material and getter, its structure as shown below.

1 gas insulated structure, 2 core insulation material, 3 gas adsorption material

Gas insulation structure: usually bag structure, hereinafter referred to as the "barrier bag", is the core material vacuum encapsulated in a vacuum state, so this structure has good gas barrier capability, but also has a certain strength to protect the vacuum chamber can not be damaged. The commonly used air barrier materials are thin steel plate, multilayer polyester film and multilayer polyester film containing metal layer.
The core layer insulation materials: hereinafter referred to as "core", commonly used for porous materials, the first is to play a supportive role, avoid under vacuum conditions in gas insulation structure collapse, followed by preventing heat radiation, in addition to its porous material, the contact surface of small, low thermal conductivity. The commonly used core materials are powder silica, aerosols, polyurethane foam, porous polystyrene foam and superfine glass fiber.

Gas adsorption materials: hereinafter referred to as the "getter", the main role is to absorb the residual air in VIP panel to maintain a certain degree of vacuum. Barrier bag vacuum sealing, there will still be a very small amount of gas will penetrate into the bag, or core material in the use of the process will slowly release some gas, all these gases will destroy the bag vacuum affect the thermal conductivity of VIP plate. Therefore, you must use a special getter absorbs nitrogen, oxygen, carbon dioxide and water vapor gas board of VIP to maintain a high degree of vacuum bag.

3, adiabatic principle

VIP The plate is made of vacuum thermal insulation principle, that is, under low air pressure, the thermal conductivity of the material will become very small. VIP plate heat conduction can be regarded as one-dimensional plate heat conduction, the thermal conduction process is as follows.

One dimensional heat conduction through VIP plate
According to the structural characteristics of the VIP plate and the thermal resistance formula R= Delta / lambda, the total thermal resistance of the VIP plate can be expressed as the sum of the thermal resistance of the core material and the two sides of the wall

Among

λ by VIP Total thermal conductivity;

λ₀ by Apparent thermal conductivity of core material;

λ₁，λ₂ The thermal conductivity of the two sides of the VIP plate;

δ by VIP Total plate thickness;

δ₀ by Core thickness;

δ₁，δ_{2 The thickness of the side wall of the VIP plate;}

The apparent heat conductivity coefficient 0 of the core material is composed of the following four parts:

λ₀=λ_g+λ_c+λ_s+λ_r

Type，λ_g Thermal conductivity of air;

λ_c Convective heat transfer coefficient;

λ_{s Thermal conductivity of thermal insulation materials;}

λ_r  Radiation heat transfer coefficient of core material;

Due to the VIP board in the rarefied air, air heat conduction ability is very weak, usually also the core material is a porous material, space partition distance is relatively short, further weaken the heat transfer of air, so the air heat conduction is approximately zero,That is λ_g=0; 气体被分隔或者密闭在无数微小的空间之内，抽真空后The gas is separated or closed within a tiny space, the mean free path of air molecules inside the bag between the core material is greater than the pores between the vacuum, the heat exchange between gas molecules is almost non-existent, so heat convection can be neglected,That is λ_c=0.Therefore, the apparent thermal conductivity of the core layer can be expressed as: λ₀=λ_s+λ_r

4, the impact of factors
The thermal conductivity coefficient is a parameter which can be used to characterize the thermal conductivity of the material, which is W/m K. The coefficient of thermal conductivity of different materials is different, even if the same material, thermal conductivity is also affected by many factors. The following is a brief description of the vacuum, temperature and moisture content of the core of the impact of lambda, which is conducive to us to find a way to reduce the thermal conductivity, improve the insulation performance of VIP board.
(1) vacuum
The VIP board has such excellent thermal insulation performance, to a large extent thanks to the vacuum inside the bag. Because the VIP core material is powder or fiber, gas heat conduction plays a very important role. The relationship between apparent thermal conductivity and residual gas pressure of core material is shown below.

From the above, it is found that when the residual gas pressure drops from atmospheric pressure to 10Kpa, the apparent thermal conductivity remains constant, that is, the thermal conductivity is independent of the gas pressure in the insulating layer. When the pressure is further reduced, that is, when the pressure is 1~ 103Pa, the thermal conductivity is almost linear with the residual gas pressure, and the thermal conductivity decreases with the decrease of pressure. When the pressure is lower than 1Pa, the thermal conductivity is due to the radiation heat transfer and solid heat conduction because of the heat conduction and solid heat conduction of the gas molecules. Therefore, the VIP board production process, the choice of the VIP board vacuum plate generally take less than 1Pa can be further evacuated for reducing thermal conductivity is of no practical significance.

(2) temperature

λ= 4 GdεσT²

In the formula, G is the geometric factor of the stomata, and D is the mean pore diameter (m),εThermal emission rate of pores,σ For the Stephen Pohl Seidman constant, T is the temperature. Therefore, the VIP board in the low temperature environment than in the high temperature environment to achieve better insulation effect.

(3) Moisture content of core material

Core material for porous structure is easy to absorb moisture, aging will also be part of the decomposition of water and gas. The air thermal conductivity is 0.029W/m / K, and the thermal conductivity of water is 0.58W/m / K, is 20 times the air, water and steam and the vacuum deterioration of the VIP board, so the thermal conductivity of core material in water has great influence on the VIP board. The results show that: when the moisture content is greater than 5~10%, insulation material after moisture water occupied the pore space of porous media, due to its thermal conductivity increased rapidly, thermal conductivity of insulation material 0.03W/m K 1% absorption moisture conductivity increased by 25%, and according to the multiple increase. If the insulation material has holes to connect the pores, the water will penetrate into other parts under the action of capillary. Therefore, in the production process of VIP board, the core material must be fully dried.

5, VIP board reliability and life
If you want to ensure that the product factory pass rate, it is necessary to sample the product testing. At present, usually use the following two methods of detection, one is the thermal conductivity using heat flow meter for direct detection of VIP board, this method has the advantages of simple operation and accurate measurement, but each piece of VIP board testing time is about 10 minutes, low detection efficiency should not be used for mass production; the two is the relationship between the number and the degree of vacuum thermal system, through the use of micro vacuum gauge to measure the degree of vacuum in the VIP board to indirectly determine its reliability, this method is convenient and suitable for mass production of online detection, but the detection method is not perfect, and the increase of the pressure sensor will a corresponding increase in product cost, so the current use is not universal.

The process of using VIP plate, due to the external gas through the gas insulation structure slowly penetrate into the VIP plate and the core material in the process of using internal slow release gas, will destroy the vacuum plate, resulting in thermal conductivity of VIP plates increased gradually lead to product failure. This is related to the life of the product. The VIP board products there is no national standard, the following definitions of VIP plate service life, in the standard conditions of environmental temperature of 24 DEG C and a relative humidity of 50%, with thermal conductivity of time gradually increased to 0.010W/m VIP in K (super thermal insulation material thermal conductivity, the upper limit) we think the end of VIP plate life. Experienced this time is the service life of VIP. Now VIP plate life is generally 10~15 years, gas phase method of silica core material for up to 50 years.

The service life of the VIP board is directly related to the vacuum in the board. Therefore, in order to extend the service life of the product, it is necessary to improve the vacuum. We are from the following two aspects: to improve the vacuum structure, improve the barrier properties, barrier bag material increases the getter; process, keep the production environment clean and dry, fully drying the core material, appropriate to extend the exhaust time.