V. V. Bakovets

Synthesis of the nanostructured luminophor Y2O3-Eu-Bi by the sol-gel method

The sol-gel method of the formation of the nanostructured luminophor based on Y2O3 doped by Eu3+ and Bi3+ was studied. The mechanism of the dehydration and dehydroxylation of gels and xerogels of the mixed hydroxides, particle sizes, structure, and luminescent properties of the synthesized products based on Y2O3 depend on the chemical nature of a precipitating agent (NaOH or NH4OH) and a washing agent (water and alcohol).

Synthesis of nanostructured nickel oxide

The process of formation of nickel oxide nanostructured powders by annealing nickel hydroxide in the temperature range 200–700°C was studied. Nickel hydroxide was prepared by precipitation with alkali from nickel nitrate solutions. The annealing process was shown to be multi-step. In the first stage the hydrogel Ni (OH)2·nH2O decomposes and partial dehydration of hydroxide occurs. Sizes of the formed particles decrease. At the temperatures above 230°C, further hydrogel decomposition and coalescence of NiO particles proceed. In view of the structural rearrangement of powder at the high temperatures 400–700°C, dehydration process is monitored by the decrease of NiO particles surface area at their coalescence. According to the change in the dehydration mechanism, the hierarchically nanostructured material forms, whose particle sizes are in the range 4–5, 9–12, and 18–40 nm.

On the abnormal efficiency of the luminescence of submicron-sized phosphor Y2O3 : Eu3+

Samples of the Y2O3 : Eu3+ (5–11 at %) phosphor with a high luminescence efficiency have been synthesized by the sol-gel method. The chemical, substructural, and optical properties of the material have been investigated using the thermal analysis, X-ray powder diffraction, electron microscopy, vibrational and Raman spectroscopy, and diffuse reflection methods. The size effect of particles and crystallites on the luminescence parameters of the phosphor has been analyzed. It has been found that the submicron-sized Y2O3 : Eu3+ phosphor containing 9–11 at % Eu3+ exhibits an abnormally high luminescence intensity. It has been shown that these effects are associated with the enrichment of the surface of particles and crystallites with dopant ions due to the decrease in the local symmetry of their environment.

Possibility of adjusting photoluminescence spectra of Ca scheelites (CaMoO4: Eu3+ and CaWO4: Eu3+) to the emission spectrum of incandescent lamps

The peculiarities of the photoluminescence of compounds CaMoO4: Eu3+ and CaWO4: Eu3+ with the scheelite structure associated with a change in the short- and long-range orders of the crystal lattice upon a change in the activator (Eu3+) of the photoluminescence range in the interval 1–4 mol %, in which the photoluminescence of the matrix is preserved in the range 484–557 nm, are investigated using X-ray phase analysis as well as photoluminescence, Raman, and diffuse reflection spectroscopies. The introduction of Eu3+ ions leads to the reconstruction of the lattice so that up to 10% of these ions stimulate the formation of centrosymmetric localization upon the substitution of Ca2+ ions in the noncentrosymmetric positions. It is found that the spectral radiant emittance of the more effective luminophore CaMoO4: Eu3+ can be adjusted to this parameter for an incandescent lamp for the Eu3+ concentration of 1–2 mol %.

Correction of the band gap of Y2O3:Eu3+ phosphor

Submicron samples of Y2O3:Eu3+ phosphor with elevated photoluminescence (PL) efficiency and activator concentration of 9 at % obtained by the sol–gel method were investigated by diffuse reflection spectroscopy and PL spectroscopy. It is found that the diffuse reflection spectrum in the vicinity of the fundamental absorption edge (<300 nm) is distorted by the superposition of the PL of Eu3+ ions, as a result of which the calculated value of optical band gap Eg of the Y2O3 matrix is overestimated. An algorithm for eliminating the PL influence on the absorption edge is proposed, and the correct Eg values are found to be 4.61 ± 0.12 and 4.50 ± 0.12 eV for annealing at 700 and 1300°C, respectively.

Features of the sol-gel process of formation of nanostructured gadolinium oxide

The process of dehydration of Gd(OH)3·nH2O obtained by the sol-gel method from a solution of Gd(NO3)3, to Gd2O3 in the temperature range of 50–700°C was explored. The hydrogel and Gd2O3 structurization is shown to depend on the additives (AF-12 and 2-propanol). The final average particle size of Gd2O3 after annealing at 700°C is 20±2 nm, depending on the conditions of the synthesis. The resulting oxide particles are larger than the particles of yttrium oxide Y2O3 (17±2) obtained under the same conditions of the process due to the higher basicity of gadolinium and its higher coordination number with respect to the OH groups. This promotes the formation of crystalline phases of Gd(OH)3 at lower temperature, 50–250°C, while maintaining a favorable structural short-range order in passing through an amorphous state to a crystalline Gd2O3.

Bandgap-width correction for luminophores CaMoO 4 and CaWO 4

Submicron-powder luminophores CaMoO4 and CaWO4 obtained via solid-phase reactions have been studied using diffuse-reflection (DR) spectroscopy and photoluminescence (PL) spectroscopy. It is found that the diffuse-reflection spectrum in the range of a fundamental absorption edge of <300 nm is distorted by PL overlapping, so that subsequent calculations of optical band gap Eg of luminophores CaMoO4 and CaWO4 result in an overestimation of this value. An algorithm for the correct processing of diffuse-reflection spectra is described. It is based on a subtraction of the photoluminescence spectrum in the range of fundamental absorption. The correct Eg values and energy values for the defect levels in the bandgap of CaWO4 and CaMoO4 are determined to amount to 4.78, 4.83, and 4.86 ± 0.01 eV and 3.97, 4.07, 4.16 ± 0.01 eV, respectively.

Special Features of Localization of Eu 3+ Ion in the Matrix of (Gd x Y 1– x ) 2 O 3 : Eu 3+ Solutions During Sol-Gel Synthesis of the Luminophor

Sol-gel synthesis of the (GdxY1–x)2O3: Eu3+ solid solutions (3.5 mol %) allows for the increase in the energy efficiency of emission of the red luminophor. The study of distribution of the luminescence activator Eu3+ has revealed that these ions substitute the Gd3+ ones in the cationic sublattice of ternary solid solution. The Eu3+ ions are predominantly localized in the noncentrosymmetrical position C2 at x < 0.50 and in centrosymmetrical position C3i at x > 0.50. The increase in the Eu3+ ions concentration in the C3i position stabilizes the cubic high-temperature structure of the solid solution in comparison with the monoclinic phase. The solid solution with x = 0.75 has been found to be the most efficient luminophor.

Influence of Morphological Defects on Thermophysical Properties of γ-Gd2S3

The temperature dependences of the heat capacity (Cp) and the thermal conductivity (κ) in the temperature range from 300 to 773 K of polycrystalline gadolinium sulfide samples (γ-GdSy) with the deviation of the composition from the integer stoichiometric were studied. It was found that the thermal conductivity of gadolinium sulfides decreases monotonically and reaches 0.74 W/(m K) at T = 773 K for the composition y = 1.479, which is much lower than for the known single-crystal samples. The influence of morphological defects (boundaries of crystallites and dislocations) on the intensity of scattering of phonons is studied. It has been established that ceramic samples of gadolinium sulphides have a large heat capacity and a lower thermal conductivity, in comparison with monocrystalline samples of the same composition.

Photoluminescence of submicron-thick CVD (Gd x Y1–x )2O3:Eu3+ (11–14 at % Eu) films

Abstract(GdxY1–x)2O3:Eu3+ (11–14 at % Eu) phosphor films ≤150 nm in thickness have been grown by Ln(dpm)3 chemical vapor deposition on Si(100) substrates, and the chemical and phase compositions of the films have been determined. We have studied photoluminescence spectra of the films and the exact distribution of the Eu3+ activator ions over the cation sites of the solid solutions.