Boron, elastic properties

Boron fibers possess good mechanical properties at low densities, which accounts for their use in composites for lightweight structures. Commercially obtainable boron fibers exhibit an elasticity modulus at room temperature of 400 Gpa, a tensile strength of 3-4 GPa and a thermal expansion coefficient from room temperature to 327°C of 4.9 10 /K. The maximum use temperature is 367°C, the elasticity modulus having dropped to 240 GPa at 627°C. 

Reinforced Structural Resins. Polyesters and polyepoxides are reinforced with E- or S-glass strands, mats, or cloth raising their tensile strength 10-20 times and their modulus of elasticity 10-100 times, approaching the properties of steel. Reinforced with graphite, boron, or the new poly-p-benzamide fiber or whiskers, novel composites surpass steel. 

In the field of nanoscale materials, SIESTA has probably made its largest impact in the study of carbon nanotubes. This is a field which has captivated the attention of researchers for their unusual electronic and mechanical properties. Simulation and theory have played a major role, often providing predictions that have guided the way for experimental studies. Work done with SIESTA has spanned many aspects of nanotube science vibrational properties [239-241], electronic states [242-246] (including the effect of lattice distortions on the electronic states [247-250]), elastic and plastic properties [251-254], and interaction with other atomic and molecular species [255-259]. Boron nitride nanotubes have also received some attention [260, 261]. 

Silicon carbide is noted for its extreme hardness [182-184], its high abrasive power, high modulus of elasticity (450 GPa), high temperature resistance up to above 1500°C, as well as high resistance to abrasion. The industrial importance of silicon carbide is mainly due to its extreme hardness of 9.5-9.75 on the Mohs scale. Only diamond, cubic boron nitride, and boron carbide are harder. The Knoop microhardness number HK-0.1, that is the hardness measured with a load of 0.1 kp (w0.98N), is 2600 (2000 for aAl203, 3000 for B4C, 4700 for cubic BN, and 7000-8000 for diamond). Silicon carbide is very brittle, and can therefore be crushed comparatively easily in spite of its great hardness. Table 8 summarizes some typical physical properties of the SiC ceramics. 

CVD of boron nitride films on silicon or germanium or on printed circuit boards is now a common practice in the electronic industry [154 to 162]. The high thermal conductivity combined with the excellent electrical insulation properties are most valuable for these applications [163] see additional references in Section 4.1.1.10.8, p. 129. The use of a-BN layers is of particular importance in the manufacture of electrophotographic photoreceptors (such as solar cells) and of X-ray lithographic masks (see Section 4.1.1.10.8, p. 129). In the last mentioned application, structural aspects of the deposited films are of importance. In films still containing hydrogen, (N)H moieties are depleted by annealing at about 600°C, while (B)H moieties are depleted above 1000°C [164]. Also, elastic stiffness and thermal expansion of boron nitride films have to be viewed in connection with the temperature-dependent stress of CVD-deposited boron nitride films [165]. Reviews of properties and electronic applications of boron nitride layers have appeared in Polish [166] and Japanese [167]. 

Processing enhancers include mineral oils, glycerol monostearate, and pentaerythritol monooleate. These decrease melt viscosity and/or elasticity at low shear rate and result in shorter cycle times without sacrificing peak impact strength. Ultrafine monospherical silica particles and boron nitride are also used as additives for special purposes because of their high-performance mechanical properties. Combinations of liquid polybutadienes (with and without functional groups) and dialkyl peroxide have been developed as compatibilisers for polyolefin blends (LLDPE/HDPE/ polypropylene). 

---