| Vacuum insulation panel is a novel thermal insulation material that has developed rapidly in recent years.It is based on the principle of vacuum insulation,thus maximizing the degree of vacuum inside the panel and adopting measures in place to eliminate or drastically reduce the modes of heat transfer within the panel,consequently resulting in the production of the lowest-thermal-conductivity material currently.The thermal insulation performance of a VIP is dependent on the pore structure of the core material and decreases with an increase in the internal air pressure and moisture content.European countries,the United States,Japan and South Korea are advanced countries in manufacturing and performance testing technology of the VIPs.And now VIPs are widely applied in the white household appliances,constructions,transportation and other fields.Comparing with the VIP technology of the above countries,China has a weak foundation due to the late start.Also,the key technology has not been fully established yet since quite a number of basic research works need to be carried out.As the background of developing super-insulation,energy-saving,domestic and high-performance VIPs,this thesis focuses on the effect of preparation parameters,composition and ratio of raw materials on the microstructure of the core materials,thermal conductivity and anti-aging properties of the VIPs.The aim of this thesis is to develop low-thermal-conductivity,low-rebound-ratio and low-cost VIPs.In this work,glass wool core materials were obtained by passing the centrifugal glass wool and flame attenuated glass wool through a number of processes(mixing,beating,slurry conditioning,forming,drying and cutting).The optimization of the ratio of glass wool and the number of beating revolutions resulted in a homogenized and high-strength glass wool core material while the reduction of the thickness of the core material layer,the optimization of the number of core material layers,drying condition of glass wool core material as well as the pressure holding time of extraction process resulted in a low-thermal-conductivity VIP.Moreover,in this work,fumed silica composite core materials were obtained via mechanical stirring,stitching and pressing the fumed silica powders,opacifiers,polyester chopped strand fibers and the second phase thermal insulation powders.Optimizing the content of the second phase insulating powders suppressed the large pores in the composite core materials thereby achieving low-bound-ratio-and stable-thermal-conductivity VIPs.The microstructures of the core materials were analyzed by the scanning electron microscope and transmission electron microscope.The specific surface area,porosity,average pore diameter and pore diameter distribution of the core materials were characterized by the pore diameter and specific surface area analyzer and mercury intrusion porosimeter.The thermal conductivity of the VIPs was measured by the thermal conductivity analyzer and the thermal conductivity rapid detector.The VIPs were aged in the programmable constant temperature and humidity testing machine,and the service life of the VIPs were predicted by a theoretical model.The innovative results obtained from this work were as follows:(1)The formation mechanism of glass wool core materials was systematically studied.The effects of the ratio of centrifugal glass wool and flame attenuated glass wool and the number of beating revolutions on the fiber dispersion and formation of the glass wool slurry,microstructure and tensile strength of the glass wool core materials were discussed.The results showed that beating could reduce the flocculation degree of the fibers in the slurry,but too much beating will shorten the fiber length of glass wool.The addition of flame attenuated glass wool to centrifugal glass wool could reduce the average pore diameter of the glass wool core material and make the pore size distribution more concentrated,but it reduces the formation speed of the core material.In a case where the weight ratio of centrifugal glass wool and flame attenuated glass wool was 3:7 and the number of beating revolutions was 5000,the tensile strength of the glass wool core material was 160 N/m and the quality of the core materials was the best.(2)The heat and mass transfer characteristics of glass wool core materials were analyzed.The influence of the thickness and number of core material layers,drying conditions and pressure holding time of extraction process of the core materials on the thermal conductivity of glass wool VIPs were studied.The results showed that the optimal drying temperature and time of glass wool core materials was 150~oC and 60 min.Reduction in the thickness of core material layers and sufficient pressure holding time of extraction process were the key in obtaining a low-thermal-conductivity glass wool VIP.At a pressure holding time of 300 s,the thermal conductivity of the glass wool VIPs with 0.5mm-thickness core material layers was the lowest,i.e.,2.0 mW/(m·K).(3)The new scheme of polyester fiber/multi-particle-size powder composites was put forward as the VIP core material.The effect of the amount of the opacifiers and second phase thermal insulation powders on the pore structure of the composite core materials,the thermal conductivity and rebound ratio of the resulted VIPs were investigated.The results showed that the thermal conductivity of the VIPs was the lowest and the anti-ageing property of the VIPs was the best when the addition of carbon black of 3 wt.%.The addition of 6-46 wt.%rice husk ash and hollow glass microspheres reduced the average pore size of the composite core materials and made the pore size distribution more concentrated.Also,the optimal content of hollow glass microspheres was 6-26 wt.%and the thermal conductivity and the rebound rate of the VIPs was 5.3-7.2 mW/(m·K)and 6-14%,respectively.The optimal content of rice husk ash was 26-36 wt.%and the thermal conductivity and the rebound rate of the VIPs was 5.5-6.2 mW/(m·K)and 35-39%,respectively.(4)Based on the micro/nano pore structure of the core materials,a theoretical contribution ratio model of small pores and large pores on gas thermal conductivity was established.The influence of the pore diameter and ratio of large pores on gas thermal conductivity of the VIPs was analyzed.Also,according to the dependence of thermal conductivity of the core materials on internal air pressure and moisture content,the service life of the associated VIPs was predicted.The results showed that the maximum allowable internal pressure in the VIPs was linear in relation to the pore diameter of large pore in the core material in the lg-lg coordinate system.The moisture content and internal air pressure had a great influence on the thermal conductivity of VIPs at the early stage and the late service stage,respectively.The service life of rice husk ash/fumed silica VIP was 12-55 years while that of glass wool VIPs with desiccants was 11-13 years.Reducing the pore diameter of the core material and the ratio of large pores in the core material was the key in obtaining low-thermal-conductivity and long-service-life VIPs. |
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