Developing advanced building materials with both excellent thermal insulating and optical properties to replace common glass (thermal conductivity of ∼1 W m-1 K-1) is highly desirable for energy-efficient applications. The recent development of transparent wood suggests a promising building material with many advantages, including high optical transmittance, tunable optical haze, and excellent thermal insulation. However, previous transparent wood materials generally have a high haze (typically greater than 40%), which is a major obstacle for their practical application in the replacement of glass. In this work, we fabricate a clear wood material with an optical transmittance as high as 90% and record-low haze of 10% using a delignification and polymer infiltration method. The significant removal of wood components results in a highly porous microstructure, much thinner wood cell walls, and large voids among the cellulose fibrils, which a polymer can easily enter, leading to the dense structure of the clear wood. The separated cellulose fibrils that result from the removal of the wood components dramatically weaken light scattering in the clear wood, which combined with the highly dense structure produces both high transmittance and extremely low haze. In addition, the clear wood exhibits an excellent thermal insulation property with a low thermal conductivity of 0.35 W m-1 K-1 (one-third of ordinary glass); thus, the application of clear wood can greatly improve the energy efficiency of buildings. The developed clear wood, combining excellent thermal insulating and optical properties, represents an attractive alternative to common glass toward energy-efficient buildings.
Keywords: building materials; cellulose nanomaterials; clear; thermal insulation; wood nanocomposites.