Razmena toplote kroz kompozitne zidove

We investigated the heat conduction through different composite walls and analysed heat balance - thermal efficiency on both sides of the walls. Advantages are commented and the necessity of thermal isolation is emphasized. Examples of several combinations for two ways of passive isolation are analysed. Numerous materials for thermal isolation are presented and value of the overall coefficient of transmission is calculated for different combinations. It is concluded that for construction of walls it is better to use shallow brick rather than full and that it is more efficient to place thermal isolation on the exterior side of wall because only in that case continual isolating layer can be realized.U radu je razmatrano provođenje toplote kroz različite kompozitne zidove i toplotni bilans - termalna efikasnost sa obe strane zidova. Navedene su prednosti i neophodnost postavljanja termoizolacije i dati su i analizirani primeri više kombinacija za dva načina pasivne izolacije. Predstavljeni su brojni termoizolacioni materijali i izračunata je vrednost ukupnog koeficijenta prolaza za razne kombinacije. Konstatovano je da je kod konstrukcije zidova bolje koristiti šuplju opeku od pune i da je efikasnije postaviti termoizolaciju na spoljašnjoj strani zida, jer samo tada može ostvariti neprekinut izolacioni omotač

Pregled rezultata teorijskih pristupa fononskom inženjeringu termodinamičkih osobina različitih kvantnih struktura

Application of nano-structures requires a knowledge of their fundamental physical (mechanical, electro-magnetic, optical, etc.) characteristics. Thermodynamic properties associated with phonon displacements through the nanosamples are particularly interesting. Independent of the type of lattices, the thermodynamics of their subsystems (electrons, excitons, spin waves, etc.) is determined when the subsystem is in thermodynamic equilibrium with phonons. Phonons are collective mechanical oscillations of molecules or atoms and represent the most important system of excitations. Besides, the acoustical characteristics as well as conductive and superconductive properties etc. could not be realistically explained without phonons. In this paper we will try to observe the difference between the characteristics of different nano-crystalline structures: ultrathin films, composite films, i.e. superlattices, nanorods and quantum dots, we were interested in whether the quantum size effects (quantum confinement), quantum (de)coherence and influence of boundary conditions, strengthen or weaken in nanosamples. Finally, this paper describes how the dimensional confinement of phonons in nanostructures leads to modifications in the electronic, optical, acoustic, superconducting and thermodynamic properties of quantum. Thermal properties of nanostructures have recently attracted a lot of attention. The influence of size effects on thermal conductivity is becoming extremely important for device design and reliability. On the basis of the calculated dispersion law and distribution of phonon states in nanoscopic crystals, free energy and entropy will be calculated. Internal energy as well as heat capacitance will also be analyzed.Primena naostruktura zhteva poznavanje njihovih osnovnih fizičkih (mehaničkih, elektromagnetnih, optičkih, itd.) karakteristika. Termodinamičke osobine su povezane sa ponašanjem fononskog podsistema koji uvek postoji. Fononi su kolektivne mehaničke oscilacije molekula ili atoma kristalne rešetke i predstavljaju najvažniji sistem ekscitacija u ovim strukturama.Nezavisno od tipa kristalne rešetke termodinamika njenih podsistema (elektrona, eksitona, spinskih talasa itd.) je određena kada je odgovarajući podsistem u termodinamičkoj ravnoteži sa fononima. Uloga fonona u nanostrukturama je značajnija nego u tzv. balk strukturama, posebno što je u nanostrukturama (filmovi, superrešetke, žice, tačke) kretanje fonona ograničeno u nekoliko dimenzija. Zbog ograničenosti dimenzija kod nanostruktura se javljaju novi efekti kao što je dimenziono kvantovanje, konfiniranje fonona itd. Uticaj graničnih uslova na zakon disperzije fonona dovodi do promene karakteristika termodinamičkih i transportnih osobina. Dakle, menjanjem graničnih uslova se može uticati na toplotne i transportne osobine ovih struktura, što se danas popularno naziva fononski inženjering. U radu su analizirani i upoređivani toplotni kapaciteti i koficijenti toplotne provodnosti (kao merljive veličine) za razne strukture, kako na niskim tako i na visokim temperaturama, kao i posledice na njihovu praktičnu primenu

Proces difuzije fonona kroz kristalne strukture

Instead of usual approach, applying displacement-displacement Green’s functions, the momentum-momentum Green’s functions will be used to calculate the diffusion tensor. These functions enter into Kubo’s formula defining diffusion properties of the system. Calculation of the diffusion tensor requires solving of the system of difference equations. It is shown that the elements of the diffusion tensor express discrete behaviour of the dispersion law of elementary excitations and, more important - that they are temperature independent.Umesto uobičajenog pristupa, primenom Grinovih funkcija tipa pomeraj-pomeraj, tenzor difuzije izračunat je primenom Grinovih funkcija tipa impuls-impuls. Ove funkcije figurišu u Kubo formuli koja definiše difuzione osobine sistema. Izračunavanje tenzora difuzije zahteva rešavanje sistema diferencnih jednačina. U radu je pokazano da elementi tenzora difuzije ukazuju na diskretnost zakona disperzije elementarnih pobuđenja, i - što je još značajnije - ne zavise od temperature. Rezultati sprovedenih izračunavanja pokazali su da je tenzor difuzije fononskog podsistema u kristalnim strukturama dijagonalan i da svojstvene vrednosti imaju više vrednosti pri nižim frekvencijama. Navedeni zaključci su od velikog značaja, jer potvrđuju makroskopske teorije toplotnog provođenja koje tvrde da je koeficijent difuzije temperaturski invarijatan. Ova činjenica, kao i posledice koji iz nje mogu proizaći, imaju ogroman potencijalni značaj za primenu fononskog inženjeringa, prvenstveno kod savremenih nanostruktura

Diffusion of phonons through (along and across) the ultrathin crystalline films

Instead of usual approach, applying displacement-displacement Green's functions, the momentum-momentum Green's functions will be used to calculate the diffusion tensor. With this type of Green's function we have calculated and analyzed dispersion law in film-structures. A small number of phonon energy levels along the direction of boundary surfaces joint of the film are discrete-ones and in this case standing waves could occur. This is consequence of quantum size effects. These Green's functions enter into Kubo's formula defining diffusion properties of the system and possible heat transfer direction through observed structures. Calculation of the diffusion tensor for phonons in film structure requires solving of the system of difference equations. Boundary conditions are included into mentioned system through the Hamiltonian of the film-structure. It has been shown that the diagonal elements of the diffusion tensor express discrete behavior of the dispersion law of elementary excitations. More important result is that they are temperature independent and that their values are much higher comparing with bulk structures. This result favors better heat conduction of the film, but in direction which is perpendicular to boundary film surface. In the same time this significantly favors appearance 2D superconducting surfaces inside the ultra-thin crystal structure, which are parallel to the boundary surface.This is the peer-reviewed version of the article: Šetrajčić, Jovan P., Stevo Jaćimovski, and Siniša M. Vučenović. 2017. ‘Diffusion of Phonons through (along and across) the Ultrathin Crystalline Films’. Physica A-Statistical Mechanics and Its Applications 486: 839–48. [https://doi.org/10.1016/j.physa.2017.06.003

Utvrđivanje starosti umetničkih boja merenjem sadržaja H2O

The method determining age of artistic paints is proposed. It is based on the use of closed Markov's graphs with three cells. The measurements of contents of water molecules in surrounding area can be done only for the space in which artistic paint is located. This means that method is nondestructive.Predložena je metoda određivanja starosti umetničkih boja. Ona se zasniva na upotrebi zatvorenih Markovljevih grafova sa tri ćelije. Merenja sadržaja molekula vode u okolini može da se urade samo za prostor u kojem se nalazi umetničko platno. To znači da je metoda nedestruktivna

Moguća stanja nosilaca naelektrisanja u tankim višeslojnim superprovodnim keramikama

This paper analyzes behavior (spectra and states) of the elementary charge carriers in anisotropic perovskite structures, such as modern superconducting ceramics. Translational symmetry of the atom (ion) distribution of the electron (or hole) system is broken by atomic/ionic/molecular sputtering and doping, as well as due to existence of two boundary surfaces. This is a charge carrier's model of high-temperature superconductors in which the observed symmetry breaking orthogonal to CuO planes was treated as a perturbation. The single-particle fermion's wave functions and the possible energies of charge carriers were determined.U radu je analizirano ponašanje (spektri i stanja) elementarnih nosilaca naelektrisanja u anizotropnim perovskitnim strukturama, kakve su savremene superprovodne keramike. Translaciona simetrija atomskih (jonskih) rasporeda sistema elektrona (ili šupljina) je narušena atomskim/jonskim/molekulskim rasprašivanjem (spaterovanjem) i dopiranjem, kao i postojanjem dveju graničnih površi. Ovo je model nosilaca naelektrisanja kod visoko-temperaturskih superprovodnika u kojem se posmatrano narušenje simetrije normalno na CuO ravni tretira kao perturbacija. Određene su jedno-čestične fermionske talasne funkcije i mogu e energije nosilaca naelektrisanja

Phonon-Induced Thermodynamic Properties of Ultra-narrow Wires

In this paper, we investigated the influence of size effect on thermodynamic properties of ultra-narrow wires with a simple cubic lattice, by means of two-time dependent Green functions method, adjusted to confined crystalline structures. Poles of Green functions, which defining phonon spectra, are found by solving the secular equation. For different boundary parameters, this problem is presented graphically. The temperature behavior of ultra-narrow wire thermal capacitance is compared to that of bulk structures. It turned out that in low-temperature region thermal capacitance of the ultra-narrow wire is notably lower than in the corresponding bulk sample. How this fact reflects the thermal, conducting and superconducting properties of materials, is discussed in the conclusion

Fononska toplotna provodnost tankih kristalnih filmova

In this paper we have analyzed phonon thermal conductivity of thin crystalline films, i.e. of low-dimensional structures with dimensional parameters significantly decreased in only one direction. Finding of coefficient of thermal conductivity is achieved by deriving phonon coefficient of diffusion, phonon specific heat and specific density for explored structure. Coefficient of diffusion is derived by applying method of Green functions, using Kubo formula. Then reduced thermal conductivities for film and bulk structures were compared for wider temperature interval. In the last section an influence of changes of heat conductivity on the boundaries is analyzed.U radu je analizirana fononska toplotna provodnost u kristalnim filmovima, tj. niskodimenzionim strukturama kod kojih su dimenzioni parametri značajno smanjeni samo u jednom pravcu. Određivanje koeficijenta toplotne provodnosti je vršeno određivanjem koeficijenta difuzije fonona, fononske specifične toplote i masene gustine za datu strukturu. Koeficijent difuzije je naen metodom Grinovih funkcija, korišćenjem Kubo formule. Izvršena su poređenja redukovanih toplotnih provodnosti filma i neprekidnih (balk) struktura za širi temperaturski interval. U poslednjem delu je analiziran uticaj promena na granicama na toplotnu provodnost

Fononska toplotna provodnost tankih kristalnih filmova

In this paper we have analyzed phonon thermal conductivity of thin crystalline films, i.e. of low-dimensional structures with dimensional parameters significantly decreased in only one direction. Finding of coefficient of thermal conductivity is achieved by deriving phonon coefficient of diffusion, phonon specific heat and specific density for explored structure. Coefficient of diffusion is derived by applying method of Green functions, using Kubo formula. Then reduced thermal conductivities for film and bulk structures were compared for wider temperature interval. In the last section an influence of changes of heat conductivity on the boundaries is analyzed.U radu je analizirana fononska toplotna provodnost u kristalnim filmovima, tj. niskodimenzionim strukturama kod kojih su dimenzioni parametri značajno smanjeni samo u jednom pravcu. Određivanje koeficijenta toplotne provodnosti je vršeno određivanjem koeficijenta difuzije fonona, fononske specifične toplote i masene gustine za datu strukturu. Koeficijent difuzije je naen metodom Grinovih funkcija, korišćenjem Kubo formule. Izvršena su poređenja redukovanih toplotnih provodnosti filma i neprekidnih (balk) struktura za širi temperaturski interval. U poslednjem delu je analiziran uticaj promena na granicama na toplotnu provodnost