<<
>>

THE BASIC MAINTENANCE OF WORK

In introduction the urgency of a theme of the dissertation is proved, research problems are formulated, the practical importance and scientific novelty of the received results are noted.

In chapter 1 the basic experimental and theoretical results, and also the results of computer modelling connected with dimensional dependences of melting points and crystallisation, i.e.

with a fusion-crystallisation hysteresis nanochastits are considered. In a finishing part of the head on the basis of the made analysis of available publications conclusions are drawn and the primary goals of research are supplied.

In the second chapter taken the central place in yielded work, theoretical bases of a method of molecular dynamics and results of its application to metal nanochastitsam are stated. In particular, are discussed
The multipartial potentials of internuclear interaction used at modelling of metal systems. The basic maintenance of the yielded chapter is connected with application of two alternative variants of the thermodynamic approach to research of dimensional dependences of melting points and crystallisation. The dynamic variant answers uniform heating nanochastits for definition of melting point and to uniform cooling with certain speed of change of temperature for a crystallisation temperature finding, and quasistatic - relaxations nanochastits at the fixed temperatures.

Despite a number of interesting theoretical concepts, and also results of computer experiments, laws and mechanisms of a hysteresis of fusion and crystallisation nanochastits remain not quite clear. Our results presented in yielded work, yield the serious bases for revision of some made before conclusions. In particular, any hysteresis, including a hysteresis of magnetisation and a wetting hysteresis, testifies to nonequilibrium character of considered process. Accordingly, and, registered at heating and cooling nanochastits, it is impossible to consider temperatures as equilibrium. Originally in work [1] we have drawn a conclusion, that reduction of speed of change of temperature nanochastits, i.e. speeds of their heating and cooling, it is possible to eliminate finally a fusion-crystallisation hysteresis, i.e. to achieve that melting point has coincided with crystallisation temperature, i.e. was equilibrium temperature of phase change. However, from the theoretical point of view [2], equilibrium temperature of phase change That should answer infinitely protjazhyonnoj to a phase. Besides, even crystallisation in a volume phase demands presence of some overcooling, and fusion is accompanied by some overheat of a crystal though in a quantitative sense this effect is less essential, than effect of overcooling. Considering noted above, in our work [3] we have come to conclusion, that, melting point and crystallisation temperature depend on speed of change of temperature, but completely a fusion-crystallisation hysteresis in the computer experiments connected with heating and cooling nanochastits, to eliminate it is impossible.

Obviously, earlier in works of other authors influence of speed of change of temperature on structural transfomations in nanoklasterah and dimensional

Dependence of melting point it was not investigated, though, certainly, intuitively aspired to reduce this influence. So, in work [4] temperature step change was used, i.e.

the temperature changed in steps on small size, then the particle ran down at constant temperature, and then the temperature changed again jump etc. At the same time, it is easy to show, that centre speed of change of temperature and in these computer experiments made approximately 1012 К/c = 1 ТК/c, i.e. was to usual measures very high. In the yielded dissertational work influence of speed of change of temperature on a fusion-crystallisation hysteresis in nanochastitsah Ni, Au and Al was investigated. Originally this problem dared with use of program CSEG and
PSS [5], and then the received results have been confirmed with use of program LAMMPS and MPA. The same way of registration of melting points and crystallisations on jumps on temperature dependence of specific cohesive energy was thus used. It is possible to name such approach thermodynamic, and its variant answering to registration and in the conditions of fade of temperature - dynamic.

On fig. 1 dependences and for nanoklasterov the specified are presented

The metals, received with use PSS. Despite a number

The remarkable specific features inherent nanoklasteram of various metals, fig. 1 shows the general law: melting point increases, and crystallisation temperature decreases with growth (on the module) speeds of change of temperature, and at heat rates and coolings of order 1 T To / with which earlier as we, and other authors considered comprehensible to maintenance kvaziravnovesnogo a current condition nanoklasterov, distinction between and exceeds 100 K.Odnako in process of reduction of heat rates and cooling curve meltings and crystallisations start to approach, showing to that, that at distinction between and decreases 10 times (to 10 To and less) in comparison with size.

Thus, our last MD results, including dependences and on speed of change of temperature, testify that the fusion-crystallisation hysteresis is defined first of all by nonequilibrium conditions of heating and cooling nanochastits at their fusion and crystallisation, accordingly. Reduction it is possible to reduce a hysteresis to a minimum, but not to eliminate it completely. Similar conclusions become in work [6]. Remains only not quite clear why the appreciable hysteresis of fusion and crystallisation nanochastits was observed and in direct (laboratory) experiments where speed of change of temperature was on many orders less, i.e. made, obviously, no more [7, 8].

The regular error connected with not by quite correct definition of temperature nanochastits at research of their fusion and crystallisation is represented to the most probable.

Qualitatively, the results received with use of program LAMMPS and presented on fig. 2, will be co-ordinated with results presented on fig. 1, i.e. testify that a fusion-crystallisation hysteresis, i.e. temperature difference characterising it increases with increase in speed of change of temperature. However, some specific features answering to use of program LAMMPS and MPA are revealed also. On fig. 2. Dependences () and for nanochastits the nickel, containing 531, 5017 are presented and

50141 atoms. At modelling MPA-PARAMETRIZATION [9] was used.

Fig. 1. Dependences of temperatures

Meltings Tt (▲) and Hardware crystallisations (▼) from heat rates and the coolings received with

Use of program CSEG and PSS: and - for nanoklasterov Ni, containing N = 5 0 0 atoms; - for nanoklasterov Au (N = 1 0 0 0), in - for nanoklasterov Al (N = 1 5 0 0).

Shaped lines answer

To the temperature interpreted as temperature of balance between crystal and liquid phases

On fig. 3 dependences Tt () and the Hardware (), found with use PSS and MPA are compared. It is visible, that the dependences received with use of alternative multipartial potentials and two various computer programs, will well be co-ordinated with each other. The best consent takes place for melting points.

The variant of the thermodynamic approach named us kvaziravnovesnym (quasistatic) and providing relaxation of modelled objects at fixed temperatures, allows to vary a necessary relaxation time, i.e. to provide equilibrium (or kvaziravnovesnoe) a final condition nanochastits. For realisation of the yielded approach the spherical fragment of the GTSK-LATTICE is exposed to a relaxation in a current 1 - 100 nanoseconds, changing temperature with in advance set step 6Т (such designation is used for difference of a designation temperature inkrementa from size of a hysteresis of fusion-crystallisation DTts). Kvaziravnovesnoj cбудет to answer melting point T^e jump on temperature dependence of specific cohesive energy and = U/N. Thus

In a vicinity of melting point modelling was carried out with smaller step δTи with relaxation time increase.

Fig. 2. Dependences Tm (T) (▲) and Tc (T) (▼) for nanochastits Ni, containing 531 (), 5017 () and 50141 () the atoms, programs LAMMPS received with use and MPA

Shaped lines answers

To equilibrium temperature of fusion

cдля nanochastitsy data

The size.

In

Fig. 3. Comparison

Dependences Tm () and Tc (t) found with use PSS and MPA for nanochastits the nickel, containing

500 atoms. Points ▲ (PSS) and Δ (MPA) answer melting point; points ▼ (PSS) and V (MPA) - crystallisation temperature

On fig. 4 dimensional dependence of equilibrium temperature nanochastits the gold, received in ours MD experiments with use of program LAMMPS and MPA, is compared to available experimental data. It is visible, that ours MD results will well be co-ordinated with experimental data, especially for nanochastits radius r0

<< | >>
A source: Talyzin Igor Vladimirovich. MOLECULAR DYNAMIC INVESTIGATION OF THERMODYNAMIC AND KINETIC ASPECTS OF MELTING AND CRYSTALLIZATION OF METAL NANOPARTICLES. ABSTRACT of dissertation for the degree of candidate of physical and mathematical sciences. Tver - 2019. 2019

More on topic THE BASIC MAINTENANCE OF WORK:

  1. THE BASIC MAINTENANCE OF WORK
  2. the Basic maintenance of work
  3. THE BASIC MAINTENANCE OF WORK
  4. the BASIC MAINTENANCE of WORK
  5. THE BASIC MAINTENANCE OF WORK
  6. THE BASIC MAINTENANCE OF WORK
  7. THE BASIC MAINTENANCE OF WORK
  8. THE BASIC MAINTENANCE OF WORK
  9. II. THE BASIC MAINTENANCE OF WORK
  10. the Basic maintenance of work
  11. THE BASIC MAINTENANCE OF WORK
  12. THE BASIC MAINTENANCE OF WORK
  13. THE BASIC MAINTENANCE OF WORK
  14. THE BASIC MAINTENANCE OF WORK
  15. THE BASIC MAINTENANCE OF WORK
  16. THE BASIC MAINTENANCE OF WORK
  17. THE BASIC MAINTENANCE OF WORK