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Transparent insulation materials: a review

After more than ten years of research and development on a new generation of transparent insulation materials in the more restricted sense, namely honeycomb type and other geometric structures, granular and monolithic aerogel, a critical overview of existing materials is presented. During the last years considerable progress has been made, and still more is to be expected as well on the theoretical understanding of the physical properties as on the actual material development. On the other hand there still are problems when one looks at the application aspect, e.g. temperature stability, inflammability, optical appearance, irregularities of the structures, and production costs. The development objectives in the past have been either to physically optimize a material for an application, or to produce a lower-quality cheap material. An example of the first strategy is the recent development of glass capillary structures. The question here, of course, is, whether production, handling and transport can be made easy enough to meet the target of reasonable costs. It should be emphasized, that material costs are not the key issue, but system costs. A continuous and intensive collaboration between production engineer, system or design engineer, scientist and salesman is probably the only chance to meet the requirements of high quality products, adapted to the application, with reasonable costs. An engineer designing an application with transparent insulation materials without understanding the physics of the material type used may run the risk of deteriorating the performance severely.