Papers by Author: Dj. Veljković

Abstract: Pulsed laser deposition was used to obtain functionally graded bioactive glass coatings on titanium substrates. An UV KrF* (λ=248 nm, τ>7 ns) excimer laser was used for the multi-pulse irradiation of the targets. The depositions were performed in oxygen while keeping substrate temperature at 400°C. We used sintered glass targets in the system SiO2-Na2O-K2O-CaO-MgOP2O5 that differed in SiO2 content, which was either 57 wt.% (6P57) or 61 wt.% (6P61). A glass 6P61 was used as the first layer in direct contact with the metallic substrate, while the outer bioactive layer was made of glass 6P57. Both the bioactive coatings and the bulk glasses were analyzed by Fourier transform infrared spectrometry (FTIR), grazing incidence X-ray diffraction (GIXRD), and scanning electron microscopy (SEM). The FTIR spectra of the glass powders and glass coatings showed the main vibration modes of the Si-O-Si groups. GIXRD analysis confirmed that the glass coatings had an amorphous structure. The SEM micrographs of the glass coatings showed the films to consist of droplets with diameters ranging from 0.2 to 5 μm. SEM was used to determine the rate of apatite formation on the coating when exposed to simulated body fluid (SBF) solution for 7 days. We demonstrated that pulsed laser deposition leads to good glass-metal adhesion on the substrate and well attached bioactive particles on the surface. We consider therefore this method appropriate for forming implants that can develop an apatite layer after immersion in SBF.

Abstract: The influences of temperature and time on sintering behaviour of nanosized HAP powder were investigate in this paper. The calcium hydroxyapatite powder, with the average crystallite size of 34 ± 1 nm, was uniaxially pressed at a pressure of 500 MPa. Obtained green compacts were sintered at temperature ranging from 1000°C to 1200°C in air atmosphere at various times. According to the results of scanning electron microscopy, X-ray and FTIR analyses, it is shown that HAP compacts with dense microstructure and average grain size below 250 nm is obtained.

Abstract: Exciton states in type-II InP/InGaP and GaSb/GaAs self-assembled quantum dots and quantum-dot superlattices subject to a normal magnetic field are calculated. Strain is explicitly taken into account in single particle models of the electronic structure, while an exact diagonalization approach is adopted to compute the exciton states. Strain reverts type II band alignment in InP quantum dots to type I, therefore no transitions between the lowest energy states of different angular momenta are observed. On the other hand, strain increases the barrier for the electron in the conduction band of GaSb/GaAs quantum dots, therefore the exciton, being composed of electron and hole states of various angular momenta, may have a finite angular momentum in the ground state. Consequently, the oscillator strength in the InP single quantum dot and quantum-dot superlattice increases with the magnetic field, while the angular momentum transitions between the bright and the dark exciton states in the GaSb system bring about decay of the oscillator strength when the magnetic field exceeds a certain value.

Abstract: In this paper the synthesis of bioactive cement based on α-Ca3(PO4)2 (α-TCP) and calcium deficient hydroxyapatite (CDHAp), obtained by hydrothermal method from CaCl2, EDTA, NaH2PO4·12H2O and urea at 160oC, was described. According to the results of DTA, SEM, X-ray and FTIR analyses performed in this investigation, it is shown that CDHAp is transformed to β- TCP at 780oC, and to α-TCP at 1400oC. The hardening of the samples, prepared from the mixture of α-TCP and 2.5 % solution of Na2HPO4, in simulated body fluid at 37oC, was followed by the microstructure changes.

Abstract: The intersublevel absorption in n-doped InAs/GaAs self-assembled quantum-dot molecules composed of three quantum dots is theoretically considered. The transition matrix elements and the transition energies are found to vary considerably with the spacer thickness. For s polarized light, decreasing the thickness of the spacer between the dots brings about crossings between the transition matrix elements, but the overall absorption is not affected by the variation of the spacer thickness. For p-polarized light and thick spacers, there are no available transitions in the single quantum dot, but a few of them emerge as a result of the electron state splitting in the stacks of coupled quantum dots, which leads to a considerable increase of the transition matrix elements, exceeding by an order of magnitude values of the matrix elements for s-polarized light.