41 research outputs found
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 provoÄenje toplote kroz razliÄite kompozitne zidove i toplotni bilans - termalna efikasnost sa obe strane zidova. Navedene su prednosti i neophodnost postavljanja termoizolacije i dati su i analizirani primeri viÅ”e kombinacija za dva naÄina pasivne izolacije. Predstavljeni su brojni termoizolacioni materijali i izraÄunata je vrednost ukupnog koeficijenta prolaza za razne kombinacije. Konstatovano je da je kod konstrukcije zidova bolje koristiti Å”uplju opeku od pune i da je efikasnije postaviti termoizolaciju na spoljaÅ”njoj strani zida, jer samo tada može ostvariti neprekinut izolacioni omotaÄ
Pregled rezultata teorijskih pristupa fononskom inženjeringu termodinamiÄkih osobina razliÄitih 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 fiziÄkih (mehaniÄkih, elektromagnetnih, optiÄkih, itd.) karakteristika. TermodinamiÄke osobine su povezane sa ponaÅ”anjem fononskog podsistema koji uvek postoji. Fononi su kolektivne mehaniÄke oscilacije molekula ili atoma kristalne reÅ”etke i predstavljaju najvažniji sistem ekscitacija u ovim strukturama.Nezavisno od tipa kristalne reÅ”etke termodinamika njenih podsistema (elektrona, eksitona, spinskih talasa itd.) je odreÄena kada je odgovarajuÄi podsistem u termodinamiÄkoj ravnoteži sa fononima. Uloga fonona u nanostrukturama je znaÄajnija nego u tzv. balk strukturama, posebno Å”to je u nanostrukturama (filmovi, superreÅ”etke, žice, taÄke) kretanje fonona ograniÄeno u nekoliko dimenzija. Zbog ograniÄenosti dimenzija kod nanostruktura se javljaju novi efekti kao Å”to je dimenziono kvantovanje, konfiniranje fonona itd. Uticaj graniÄnih uslova na zakon disperzije fonona dovodi do promene karakteristika termodinamiÄkih i transportnih osobina. Dakle, menjanjem graniÄnih uslova se može uticati na toplotne i transportne osobine ovih struktura, Å”to se danas popularno naziva fononski inženjering. U radu su analizirani i uporeÄivani toplotni kapaciteti i koficijenti toplotne provodnosti (kao merljive veliÄine) za razne strukture, kako na niskim tako i na visokim temperaturama, kao i posledice na njihovu praktiÄnu primenu
Proces difuzije fonona kroz kristalne strukture
Instead of usual approach, applying displacement-displacement Greenās functions, the momentum-momentum Greenās functions will be used to calculate the diffusion tensor. These functions enter into Kuboās 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 uobiÄajenog pristupa, primenom Grinovih funkcija tipa pomeraj-pomeraj, tenzor difuzije izraÄunat je primenom Grinovih funkcija tipa impuls-impuls. Ove funkcije figuriÅ”u u Kubo formuli koja definiÅ”e difuzione osobine sistema. IzraÄunavanje tenzora difuzije zahteva reÅ”avanje sistema diferencnih jednaÄina. U radu je pokazano da elementi tenzora difuzije ukazuju na diskretnost zakona disperzije elementarnih pobuÄenja, i - Å”to je joÅ” znaÄajnije - ne zavise od temperature. Rezultati sprovedenih izraÄunavanja pokazali su da je tenzor difuzije fononskog podsistema u kristalnim strukturama dijagonalan i da svojstvene vrednosti imaju viÅ”e vrednosti pri nižim frekvencijama. Navedeni zakljuÄci su od velikog znaÄaja, jer potvrÄuju makroskopske teorije toplotnog provoÄenja koje tvrde da je koeficijent difuzije temperaturski invarijatan. Ova Äinjenica, kao i posledice koji iz nje mogu proizaÄi, imaju ogroman potencijalni znaÄaj 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: Å etrajÄiÄ, Jovan P., Stevo JaÄimovski, and SiniÅ”a M. VuÄenoviÄ. 2017. āDiffusion of Phonons through (along and across) the Ultrathin Crystalline Filmsā. Physica A-Statistical Mechanics and Its Applications 486: 839ā48. [https://doi.org/10.1016/j.physa.2017.06.003
Biomolekularno prepoznavanje - o moguÄim kvantnim prilazima
Two unresolved issues of the (semi)classically addressed problems in molecular biophysics are unreasonably long time necessary for the change of biopolymer conformations and long-range directedness of selective biomolecular recognition processes - implying their essential quantum origin. In this paper several possible quantum approaches to biomolecular recognition are considered: Theory of Non-Radiative Resonant Structural Transitions, Model of Quantum Decoherence, and Resonant Recognition Model. These approaches might be of fundamental importance in understanding underlying macroscopic quantum-holographic Hopfield-like control mechanisms of morphogenesis, and their backward influence on the expression of genes, with significant potential psychosomatic implications.Dva nerazreÅ”ena pitanja semi-klasiÄno postavljnih problema u molekularnoj biofizici jesu nerazumno dugo vreme potrebno za izmenu biomolekularnih konformacija i dugo-dometna usmerenost selektivnih procesa biomolekularnog prepoznavanja. U radu je razmotreno nekoliko moguÄih kvantnih prilaza reÅ”avanju ovih problema. Predloženi kvalitativni scenario je dovoljno opÅ”ti i Äini dobru osnovu za principijelno reÅ”enje problema biopolimernog sklupÄavanja u nativnu konformaciju pri visoko selektivnim procesima protein/receptor biomolekularnog prepoznavanja, implicirajuÄi makroskopsku kvantnu nelokalnost na bioloÅ”kom Äelijskom nivou. (BaziÄna nelokalnost se može proÅ”iriti i na makroskopski kvantni nivo bioloÅ”kog organizma na Å”ta ukazuje makroskopska kvantna mikrotalasna rezonantna terapija akupunkturnog sistema.) Kvantna priroda ovih procesa ilustrovana je na primeru neradijativnih strukturnih prelaza, modelu kvantne dekoherencije i modelu rezonantnog prepoznavanja uz diskusiju implementirajuÄeg mehanizma elektronsko-konformacione sprege u kljuÄ-brava uklapajuÄim konformacionim prelazima biomolekularnog prepoznavanja protein/supstrat. Na osnovu ovih prilaza u stanju smo da reprodukujemo kako egzistenciju i stabilnost (stacionarnih) polimernih konformacija tako i kratka vremena za kvantno -mehaniÄke procese u konformacionim prelazima u selektivnim procesima biomolekularnog prepoznavanja. PoÅ”to ovi procesi dovode do dinamiÄke modifikacije viÅ”e-elektronske hiperpovrÅ”i energija-stanje Äelijskog protein/receptor ansambalskog biomolekularnog makroskopskog kvantnog sistema, to otvara moguÄnost razmatranja Äelijskog biomolekularnog prepoznavanja kao Hopfildove kvantno-holografske asocijativne neuronske mreže. Ovi prilazi mogu biti od fundamentalnog znaÄaja za razumevanje baziÄnih makroskopskih kvantno-holografskih Hopfildovih kontrolnih mehanizama morfogeneze i njihovog povratnog uticaja na ekspresiju genoma
UtvrÄivanje starosti umetniÄkih 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 odreÄivanja starosti umetniÄkih boja. Ona se zasniva na upotrebi zatvorenih Markovljevih grafova sa tri Äelije. Merenja sadržaja molekula vode u okolini može da se urade samo za prostor u kojem se nalazi umetniÄko platno. To znaÄi da je metoda nedestruktivna
MoguÄa stanja nosilaca naelektrisanja u tankim viÅ”eslojnim 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 ponaÅ”anje (spektri i stanja) elementarnih nosilaca naelektrisanja u anizotropnim perovskitnim strukturama, kakve su savremene superprovodne keramike. Translaciona simetrija atomskih (jonskih) rasporeda sistema elektrona (ili Å”upljina) je naruÅ”ena atomskim/jonskim/molekulskim raspraÅ”ivanjem (spaterovanjem) i dopiranjem, kao i postojanjem dveju graniÄnih povrÅ”i. Ovo je model nosilaca naelektrisanja kod visoko-temperaturskih superprovodnika u kojem se posmatrano naruÅ”enje simetrije normalno na CuO ravni tretira kao perturbacija. OdreÄene su jedno-ÄestiÄne 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 znaÄajno smanjeni samo u jednom pravcu. OdreÄivanje koeficijenta toplotne provodnosti je vrÅ”eno odreÄivanjem koeficijenta difuzije fonona, fononske specifiÄne toplote i masene gustine za datu strukturu. Koeficijent difuzije je naen metodom Grinovih funkcija, koriÅ”Äenjem Kubo formule. IzvrÅ”ena su poreÄenja redukovanih toplotnih provodnosti filma i neprekidnih (balk) struktura za Å”iri temperaturski interval. U poslednjem delu je analiziran uticaj promena na granicama na toplotnu provodnost