Building insulation is commonly realized using materials obtained from petrochemicals (mainly polystyrene) or\ud from natural sources processed with high energy consumptions (glass and rock wools). These materials cause\ud significant detrimental effects on the environment mainly due to the production stage, i.e. use of nonrenewable\ud materials and fossil energy consumption, and to the disposal stage, i.e. problems in reusing or\ud recycling the products at the end of their lives. The introduction of the concept of “sustainability” in building design\ud process encouraged researches aimed at developing thermal and acoustic insulating materials using natural\ud or recycled materials. Some of them, such as kenaf or wood fiber, are already commercialized but their diffusion\ud could be further improved since their performance is similar to the synthetic ones. Others are currently under\ud study and their development is only at an early stage. The goal of the paper is to report a state of the art of building\ud insulation products made of natural or recycled materials that are not or scarcely commercialized. Comparative\ud analyseswere carried out considering in particular thermal characteristics in terms of thermal conductivity, specific\ud heat and density. Data on the acoustic performance of the materials were also reported. Life Cycle Assessment\ud data were finally collected, in order to put in evidence the environmental advantages of these materials.\ud Particular attention was paid to researches focused to exploit local materials and even industrial byproducts,\ud since these approaches respectively limit transportation and disposal impacts

No abstract

No abstract

Predictions are made of the electric field necessary for the onset of corona in air for spheres and cylinders from the general breakdown criterion, ; α′ is the net ionization coefficient, r is the radius and Q is a constant. It is found that Q = 104 gives a fair fit to experimental results for both wires and points, and for positive and negative corona, for radii varying from 0.01 to 20 cm. By assuming a linear variation of α′ with the difference of the electric field from the critical field where ionization equals attachment, an analytic formula can be derived for the breakdown field for the onset of corona as a function of the radius of the sphere, which is similar to, but different from, Peek's formula for the onset of corona for cylinders or wires. The formulae also give the breakdown field as a function of temperature and pressure. For Townsend and Streamer breakdown mechanisms, values of Q are typically 3 × 104 and 108, respectively. Thus the value of Q derived from experimental values of the onset field for corona for both wires and points is significantly lower than would be expected from either the Townsend breakdown criterion or the Streamer breakdown criterion. It is suggested that the most likely collision process to explain the observed low value of Q is indirect ionization, for example by collisions between excited molecules.

No abstract