Материаловедение: свойства металлов. Матросова Т.А - 12 стр.

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УлГТУ | Ульяновск

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12
материала? 4. Из каких источников можно получить те или иные
характеристики материалов? 5. Являются ли справочные свойства материалов
абсолютными или относительными, почему? 6. Дайте определение ударной
вязкости. 7. Как получают проволоку? 8. Дайте определение хрупкости и
назовите материалы, которые могут быть отнесены к хрупким материалам. 9.
Что такое ковкость? 10. Как можно дать определение твердости металла? 11.
Что такое
пластичность? 12. Дайте определение упругости материала.
Topic 4. Application of Concepts from Quantum Mechanics
Quantum mechanics, once restricted to the academic halls, has now become a
bread-and-butter topic. It generally deals with particles of atomic or subatomic sizes.
But from the understanding of the behavior of these particles comes a better
understanding of such phenomena as thermal conductivity, heat capacity, electric
conductivity and the very existence of transistors and thermistors.
Quantum mechanics is a complex subject, largely outside the scope of this text,
yet a brief mention of two of its most important concepts may aid the production-
design engineer in the study of the basic characteristics of materials. One of these is
Planck's quantum hypothesis, later extended by Einstein and others. The other is
Pauli's exclusion principle.
The quantum hypothesis postulates that the bound energy state of particles of
very small size cannot be represented by a continuous function but is discrete in
nature. Thus, between any two permissible energy states of a bound particle there
exists a region of states that are forbidden.
Each of the electrons of any atom has a particular energy level E
1
, E
2
, E
3
, etc.
For an electron to go from one energy state to a higher one, it must receive sufficient
energy to jump through the forbidden-energy regions. Thus if an electron at energy
level E
2
is to go to the higher-energy level E3, it must receive an energy E
2
.
All matter has a spontaneous tendency to be in the lowest possible energy
state. Consequently, the electrons of any atom fill the lowest permissible energy
levels and are only excited to upper empty levels when they are given energy by
means of interaction with electromagnetic radiation, particle bombardment, or an
electric field, or by thermal excitation (collision with neighbouring atoms brought
about be an increased amplitude of atomic vibration caused by an increase in
temperature).
Thus, if an electron is dislodged from its initial energy level, it can only go to
a higher unoccupied level or entirely out of the atom. It may be recalled that the
number of electrons in any atom is equal to the number of protons in the nucleus. If
for any reson an atom is stripped of some or all of its electrons, free electrons in the
vicinity of the ion will rapidly fall into the empty energy levels, emitting their excess
energy in the form of radiation. Many of physical properties of materials are
understud through this behavior of electrons.

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