Final Review Questions I. Fill in the Blanks (20) 2.The Clausius-Mosotti equation establishes the relationship between the macroscopic dielectric constant and the microscopic polarization rate. 3． The nature of thermal expansion of solid materials is that the average distance between particles in the lattice structure increases with increasing temperature. 4． The stronger the interaction between the grid and waves, that is, the greater the probability of collision between phonons, the smaller the corresponding average free path, and the lower the thermal conductivity. 5． Piezoelectricity, ferroelectricity and pyroelectricity in dielectric materials are due to the fact that the corresponding piezoelectrics, ferroelectrics and pyroelectrics are crystals without symmetrical centers. 6． The complex dielectric constant consists of two parts, the real part and the imaginary part, the real part is consistent with the commonly applied dielectric constant, and the virtual part represents the magnitude of the energy loss in the dielectric. 77777667． When the magnetization intensity M is negative, the solid exhibits antimagnetism. 8． The electron magnetic moment is composed of the orbital magnetic moment and spin magnetic moment of the electron. 9． The carriers in inorganic non-metallic materials are mainly electrons and ions. 10． Generalized Hooke's law applies to anisotropic non-uniform materials. 11． If the light reflection loss of a glass is set to m, if x pieces of flat glass are passed continuously, the permeable part should be I0•(1-m)2x. 12． For large, thin plates with central penetration cracks, their geometric form factor Y=
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。 13． If the amount of charge q of the charged particle in the dielectric is set, and after polarization under the action of the electric field, the displacement vector of the positive charge and the negative charge is l, and the dipole moment is ql. 14． The driving force of crack propagation is that the reduction of the elastic strain energy stored in the object is greater than or equal to the surface energy required to form two new surfaces due to cracking. 15. Griffith microcrack theory holds that fracture is not the result of two parts of the crystal being pulled along the entire interface at the same time, but the result of crack propagation. 16． Considering the effect of heat dissipation, the maximum temperature difference allowed to be tolerated by the material can be expressed by the second thermal stress factor. 17． When the temperature is not too high, the form of thermal conductivity in solid materials is mainly phonon thermal conductivity. 18． In the method of representing stress components, the stress component σ, the first letter of the subscript of τ indicates the direction, and the second letter indicates the direction of the stress action. 19． The existence of hysteresis loops is an important basis for determining that the crystal is a ferroelectric body. 20． The sources of atomic magnetic moments are the orbital magnetic moments of electrons, spin magnetic moments, and the magnetic moments of atomic nuclei. The magnetism of the substance is mainly caused by the spin magnetic moment of the electron. 21. According to Griffiths microcrack theory, the fracture strength of a material does not depend on the number of cracks, but on the size of the crack, that is, the fracture strength of the material is determined by the most dangerous crack size or critical crack size. 22. The reason for the thermal expansion lag phenomenon in the complex is that the coefficient of expansion varies greatly between different phases or in different directions of the grain, which produces a large internal stress and causes micro-cracks in the blank. 23. The main ways of plastic deformation of crystals are slippage and twins. 24. Ferroelectric is a crystal with spontaneous polarization and an electrical hysteresis loop under the action of an external electric field. 25. The essence of spontaneous magnetization is the electrostatic exchange interaction between electrons. Second, the explanation of terms (20) spontaneous polarization: polarization is not caused by external electrical sites, but by the internal structure characteristics of polar crystals, so that each unit cell in the crystal has an inherent electric dipole moment, and this polarization mechanism is spontaneous polarization. Hysteresis: When stress acts on the actual solid, it takes a certain amount of time for the solid deformation to be generated and eliminated, and this time-related elasticity is called hysteresis. Gingpo: The thermal vibration of an atom at a lattice point can be described as a result similar to the propagation of mechanical waves, which is called a lattice wave, and one of the characteristics of the lattice wave is that its propagation medium is not a connected medium, but a lattice formed by atoms, ions, etc. Dielectric: refers to all substances that can establish polarization under the action of electric field. Electric dipole: Refers to two signs that are very close but have a distance and are opposite and have equal magnitudes.
Creep (slow deformation): The phenomenon that the strain of a solid material increases with time while maintaining constant stress. It differs from plastic deformation, which usually occurs after the stress exceeds the elastic limit, and creep can occur when the stress is less than the elastic limit as long as the stress is acting for a considerable period of time.
Piezoelectric effect: When a crystal without a symmetrical center is deformed by an external force in a certain direction, polarization phenomenon will occur inside it, and positive and negative opposite charges will appear on its two opposite surfaces. When the external force is removed, it will return to a state of no electricity, a phenomenon called the positive piezoelectric effect. When the direction of force changes, the polarity of the charge also changes. Conversely, when an electric field is applied to the polarization direction of a crystal that does not have a symmetrical center, the crystal will also be deformed, and after the electric field is removed, the deformation of the crystal will disappear, this phenomenon is called the inverse piezoelectric effect, or electrostriction phenomenon.
Electrostrictation: When an electric field is applied in the polarization direction of a crystal that does not have a symmetrical center, the crystal will be deformed, and after the electric field is removed, the deformation of the crystal will disappear, this phenomenon is called electrostriction phenomenon, or inverse piezoelectric effect.
Ferroelectric: Crystals with spontaneous polarization and electrical hysteresis loops under the action of an external electric field.
III. Questions and Answers (5 points per question, 20 points in total)
2. Briefly describe the characteristics of displacement polarization and loose polarization.
A: Displacement polarization is an elastic, instantaneously complete polarization that does not consume energy;
Relaxation polarization is associated with thermal motion, which takes time to complete and is inelastic, thus consuming a certain amount of energy.
3. What is the essential difference between ferromagnetism and ferroelectricity?
Answer: (1) Ferroelectricity is caused by ion displacement, and ferromagnetism is caused by atomic orientation.
(2) Ferroelectricity occurs in asymmetrical crystals, and ferromagnetism occurs in the unbalanced spin of subvalent electrons.
(3) The Curie point of the ferroelectric body is caused by the crystal phase transition, and the ferromagnetic Curie point is caused by the irregular vibration of the atoms that destroys the "exchange" between atoms, so that spontaneous magnetization disappears.
4. Why do metal materials have a large thermal conductivity, while non-metallic materials do not conduct as much as metal materials?
A: Thermal conductivity in solids is mainly achieved by lattice vibrations and free electron movements. Due to the large number of free electrons in metals, and the mass of electrons is very light, heat transfer can be achieved quickly. Although lattice vibrations also contribute to the thermal conductivity of metals, they are only very secondary. In the lattice of non-metallic crystals, such as general ionic crystals, free electrons are few, and lattice vibration is their main thermal conductive mechanism. Therefore, metals generally have a greater thermal conductivity than non-metallic materials.
6． What can generally be done to reduce the loss of light reflection in a lens system consisting of multiple pieces of glass? Why?
Answer: There are multiple pieces of glass composed of lens system, often with refractive index and glass similar glue, so that in addition to the outermost and innermost two surfaces are the relative refractive index of glass and air, the internal interface is the smaller relative refractive index of glass and glue, thereby greatly reducing the reflection loss of the interface.
7． The heat capacity of most inorganic crystalline solids changes with temperature.
A: According to the Debye heat capacity theory, the heat capacity tends to be constant (25J/(K·mo1) above the Debye temperature θD, and is proportional to T3 when it is below θD. Therefore, the θD of different materials is different. The relationship between the heat capacity of inorganic materials and the structure of the material is not large, and the vast majority of oxides and carbides have increased from a low value at low temperatures to about 1273K, which is approximately 25J/K·mol. The temperature increases further, and the heat capacity changes virtually nothing.
8. What are the methods for describing the loss of the medium? Is it consistent in essence?
A: Loss angle tangent, loss factor, loss angle tangent reciprocal, lost power, equivalent conductivity, complex dielectric constant of the complex term. Multiple methods all involve the same phenomenon for materials. That is, the current potential phase of the actual dielectric lags the current phase of the ideal dielectric. So their essence is the same.
9. Briefly describe the measures to improve the thermal shock fracture resistance of ceramic materials.
A: (1) Increase the strength of the material sf, reduce the modulus of elasticity E. (2) Improve the thermal conductivity of the material. (3) Reduce the coefficient of thermal expansion of the material. (4) Reduce the surface heat transfer coefficient h. (5) Reduce the effective thickness of the product rm.
10. Why is only a part of the material consisting of atoms containing an underfilled shell layer ferromagnetic?
Substances containing atoms in an underfilled shell include paramagnetic and ordered magnetic substances. Because the atoms in paramagnetic substances do irregular thermal vibrations, the atomic magnetic moment arrangement is chaotic, macroscopically does not show magnetism; ordered magnetic substances include antiferromagnetism, ferromagnetic and ferromagnetic substances, due to the magnetic moment parallel and reverse parallel arrangement formed by the magnetic ordered atomic arrangement in the antiferromagnetic or ferromagnetic material, the magnetic moment is completely or partially canceled, so only part of the magnetic moment (or spin electron) direction of the same ordered magnetic material has ferromagnetism.
IV. Essay Questions: (This question has two questions, a total of 20 points)
2． Illustrate the meanings represented by the parameters, numbers, and curves in the figure below.
A: Bs - saturation magnetic induction intensity, when the applied magnetic field H increases to a certain extent, the B value will no longer rise, that is, the limit of magnetization of this material.
Br - residual magnetic induction strength, when the applied magnetic field drops to 0, the material still retains magnetism, its strength is Br.
Hc - the coercive force (the strength of the refractory magnetic field), which indicates the ability of the material to maintain magnetization and resist demagnetization. According to this size, you can distinguish between soft and hard magnets.
μ – permeability (=B/H), which indicates the ability of a material to conduct and pass through magnetic field lines.
The Oabc segment represents the magnetization process of the material from macroscopic non-magnetism to magnetism; the cdefghc segment represents the process of magnetization and demagnetization of the substance in the applied magnetic field, because the process of demagnetization lags behind the magnetization curve, so it is also called the hysteresis loop. The space enclosed by the curve has a clear physical significance, that is, the larger the area enclosed by the curve, the greater the coercive force (Hc), the greater the required coercive field, the greater the energy required for magnetization, and the more "hard" the magnetic material; conversely, the smaller the area enclosed by the curve, the more "soft" the magnetic material.
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2. Use the solid energy band theory to explain what is a conductor, semiconductor, insulator, and illustrate it.
A: According to the energy band theory, not all electrons in a crystal, and not all valence electrons are involved in conduction, only the electrons in the conductive band or the holes at the top of the valence band can participate in conduction. As can be seen from the figure below, there is no forbidden area between the conductive band and the valence band in the conductor, and the electrons do not need energy to enter the conduction band, so the concentration of conductive electrons is very large. In the insulator, the valence band and the conduction period are separated by a wide bandgap Eg, and the electron needs to supply energy from the outside world to the valence belt, so that the electron is excited to realize the transition from the valence band to the conduction band, so the concentration of conductive electrons in the conductive band is usually very small.
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Semiconductors and insulators have similar band structures, except that the band gap of semiconductors is narrower (Eg is small) and the electron transition is easier.
V. Calculation Questions (5 points per question, a total of 20 points)
8． Corning 1723 glass (aluminum silicate glass) has the following performance parameters: λ =0.021J/( cm.s.°C); α=4.6×10-6/°C; σp=7.0Kg/mm2, E=6700Kg/mm2, μ=0.25. Find the first and second thermal shock fracture resistance factors.
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