材料科学基础第2章习题解答.pptVIP

24.给定Al的膨胀系数为25×10-6℃-1，SiC为4.3×10-6℃-1， 预计哪种材料具有较高的熔点？ 键能曲线 Assumptions: Melting temperature is related to the depth of the bond energy curve and thermal expansion coefficient is related to the asymmetry of the curve. Solution: Since deep energy wells tend to be more symmetric, materials with high melting tempera-tures tend to have low expansion coefficients. Thus, we expect SiC to have a higher Tm than Al. Comments: This prediction is consistent with experiment - Tm(SiC) = 2700?C and Tm(Al) = 660?C. 25．解释为什么难以找到一种材料既具有高的刚度又具有高的线膨胀系数？ Assumptions: Stiffness is related to the curvature (second derivative) of the bond energy curve at its minimum and expansion coefficient is related to asymmetry of the curve. Solution: Bond energy curves that are sharply curved have large second derivatives and high stiffness. However, such curves will also be relatively symmetric so they will exhibit low coefficients of thermal expansion. In order to obtain a stiff material with a high expansion coefficient it would require a tightly curved but asymmetric shape. This combination is difficult to achieve. 键能曲线 26．许多氧化物陶瓷或离子固体具有弹性模量约为 6.9×104MPa，与组成无关，为什么？ Elastic modulus is one of the properties that can be determined from the bond energy curve. If several materials have similar bond characteristics then we should expect those materials to display similar modulus values. This is the case with many oxide ceramics and other ionic solids. 键能曲线 弹性模量是表征材料在发生弹性变形时所需要施加力的大小。在给定应力下，弹性模量大的材料只发生很小的弹性应变，而弹性模量小的材料则发生比较大的弹性应变。结合能是影响弹性模量的主要因素，结合键能越大，则弹性模量越大。 27．窗户玻璃或氧化物玻璃的弹性模量为6.9×104MPa，“塑料”玻璃则为6.3×103MPa，在一定的载荷下，塑料玻璃比氧化物玻璃多弯曲多少？ Given: E(oxide)=10 E(polymer) Data: Deflection is inversely proportional to modulus (stiffness). Solution: The material with the higher modulus will deflect less under the same load. Therefore, the plastic will display 10 times as much deflection as the oxide. 键能曲线 28．画出具有负线性膨胀系数材料的键-能曲线和键-力曲线 的草图。 键能曲线 29．在纯Ti中两原子中心到中心的间距为2.