Crystal structure and photoluminescence of ZnSe and ZnSe:Mn nanocrystals obtained by combustion synthesis

The photoluminescence spectra of ZnSxSe1-x and ZnSxSe1-x : Mn nanocrystals obtained by combustion synthesis for all compositions with the parameter step x = 0.2 were registered. The movement of the maximum of the integral photoluminescence spectrum in ZnSxSe1-x and ZnSxSe1-x : Mn nanocrystals towards higher energies, depending on the parameter x, was noted. It was noticed that in the range of values x = 0.2÷0.4 there is an abrupt change in the half-width of the integral photoluminescence spectrum in ZnSxSe1-x and ZnSxSe1-x : Mn nanocrystals and the signal intensity; this may be due to the crystal lattice transformation. The parameters of the individual photoluminescence spectra of ZnSxSe1-x : Mn nanocrystals were determined by one experimental measurement based on the Tikhonov method and the derivative spectroscopy method. The nature of the individual photoluminescence bands is discussed. The difference between the integral (sum of individual bands) and experimental spectrum arises from the presence of an additional individual band of low intensity in the experimental spectrum. This individual band is located in the region of E = 2.48 eV and is associated with the electronic transitions in Mn2+ ions in the ZnS lattice.

Self-propagating high-temperature synthesis (SHS) of powder compacts is a novel processing technique currently being developed as a route for the production of engineering ceramics and other advanced materials. The process, which is also referred to as combustion synthesis, provides energy- and cost-saving advantages over the more conventional processing routes for these materials. At the same time, the rapid heating and cooling rates provide a potential for the production of metastable materials with new and, perhaps, unique properties. This paper reviews the research that has been, and is being, undertaken in this exciting new processing route for high-technology materials and examines the underlying theoretical explanations which will, eventually, lead to improved control over processing parameters and product quality.

Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide

Mn-doped ZnS nanocrystals based on low dopant concentrations (0–2%) and coated with a shell of Zn(OH)2 have been prepared via soft template and precipitation reaction. The results indicate that the ZnS:Mn nanocrystal is cubic zinc blende structure and its diameter is 3.02 nm as demonstrated by XRD. Measured by TEM, the morphology of nanocrystals is a spherical shape, and their particle size (3–5 nm) is similar to that of XRD results. Photoluminescence spectra under ultraviolet region shows that the volume ratio of alcohol to water in the template has a great effect on the luminescence properties of ZnS:Mn particles. Compared with unpassivated ZnS:Mn nanocrystals, ZnS:Mn/Zn(OH)2 core/shell nanocrystal exhibits much improved luminescence and higher absolute quantum efficiency. Meanwhile, we simply explore the formation mechanism of ZnS:Mn nanocrystals in alcohol and water system and analyze the reason why alcohol and water cluster structures can affect the luminescent properties of nanoparticle.

Low-dimensional nanosized semiconductor materials, such as quantum dots, have attracted considerable interest in the past decade. Due to their size-dependent physical and optical properties, these nanosized semiconductor materials offer various technological applications in photoelectronic devices. Among the II-VI semiconductor materials, zinc selenide (ZnSe) is of special interest due to its intense UV blue light emission at 460 nm wavelength with a band gap of 2.7 eV, which has not been observed in other well-known semiconductor materials, such as CdS and CdSe. In addition, it has been reported that when the surface of ZnSe is passivated by another semiconductor nanocrystal layer such as ZnS, a ZnSe/ZnS core shell quantum dot can be formed, and the quantum yield of the ZnSe/ZnS quantum dot is much higher (about 20 times) than that of bare ZnSe nanocrystallite. The preparation method of ZnSe or ZnSe/ZnS quantum dot often includes a thermal decomposition reaction of organometallic precursors in a hot coordinating organic solvent such as trioctylphosphineoxide (TOPO), leading to a hydrophobic surface on the resulting nanocrystal. Previously, in this lab, water-dispersible ZnSe and ZnSe/ZnS nanocrystals were also prepared by exchanging the hydrophobic nanocrystal surface with polar organic ligands such as mercaptoacetic acid (MAA) and ethylenediaminetetraacetic acid (EDTA) molecules. Manganese-ion doped ZnSe nanocrystals have been prepared in various media. In most cases, emission wavelengths of the ZnSe:Mn nanocrystals were far shifted from that of the undoped ZnSe nanocrystal. They usually emit orangecolored lights (580-600 nm) due to the dopant Mn ions. In this paper we report on the synthesis of Mn ion-doped ZnSe nanocrystal via thermal decomposition reaction from organometallic precursors, and unexpected white light emission from the prepared ZnSe:Mn nanocrystal. Figure 1 presents an HR-TEM image of ZnSe:Mn nanocrystal. In the picture, the measured and calculated average particle size was 3.5 nm. In addition, the appearance of distinct lattice planes in the fringe image with an approximate spacing of 3.4 A suggests that all the solid samples were made of single crystals rather than poly-crystalline aggregate mixtures. An energy dispersive X-ray spectroscopy diagram (EDXS, in Fig. 2) was also obtained to confirm the elemental compositions of the ZnSe:Mn nanocrystal in the solid state. The obtained doping concentration of manganese(II) ions in the ZnSe:Mn nanocrystal was 2.3 atomic %. To determine the doping concentration of Mn ions more precisely, Inductively Coupled Plasma-Atomic Emission Spectrometry (ICPAES) analysis was performed. Three trials of the sample measurements revealed that the average elemental proportion of the Mn ions relative to ZnSe parent crystal was Figure 1. HR-TEM image of ZnSe:Mn nanocrystal, the scale bar represents 5 nm.

The self-propagating high-temperature synthesis (SHS) of industrial refractories from low-cost domestic raw materials (dolomite and silica) using aluminum powder as a reducing agent is investigated. The phase composition, microstructure and combustion wave velocity are studied for different compositions of a powder charge. Differential thermal analysis has revealed that at low heating rates, about 10 K/min, which are typical of traditional furnace synthesis, the self-ignition is impossible because of oxidation of aluminum in air. Thermodynamic modeling has been used for studying the interaction mechanism in the SHS wave. The effect of preliminary mechanical activation of a charge mixture on the SHS wave velocity is investigated. The feasibility of the cost end energy efficient SHS method for producing refractories for hightemperature applications, e.g., furnace lining, in this system is demonstrated. INTRODUCTION Industrial refractory ceramic materials (RCM) and articles made from them are used for a wide range of high temperature applications such as lining of furnaces, casting ladles, etc. They are supposed to possess high heat resistance, thermal stability, and mechanical strength. Such a combination of properties are can be attained in refractories containing aluminum magnesium spinel, MgAl2O4, which has a high melting temperature, Tm=2135 ° , and is chemically stable towards many liquid metals and slags, and silicon carbide, which imparts high heat resistance, electrical and thermal conductivity to a material. In recent years, a number of attempts was made to produce high performance RCM from mineral substances using reduction-oxidation reactions in the regime of self-propagation high-temperature synthesis (SHS). The latter is often termed as combustion synthesis (CS) or solid flame. In this process, a heterogeneous reaction front, being initiated by heating a mixture of reactive powders by a local heat source, e.g., resistance coil or electric arc, propagates progressively through the charge as a combustion wave with temperature TCS leaving behind hot interaction product. The basic feature of SHS is that the heat released due to exothermal reactions in the wave front initiates the thermal reactions in the adjacent layer thus sustaining displacement of the combustion wave. Alternatively, the powder charge is preheated as a whole to a certain temperature at which the reaction starts either throughout the volume providing fast self-heating of the specimen to the final temperature TCS (the so-called thermal explosion mode of CS) or initiates spontaneously at an edge of the sample and then propagates through the preheated material as an SHS wave. SHS is characterized by a high value of TCS reaching 3500 C in highly exothermic systems such as Ti-B, relatively fast velocity of the SHS wave, ~0.1 to 10 cm/s in different systems, a high rate of self-heating, up to 10 K/s, steep temperature gradient, up to 10 K/cm, rapid cooling after synthesis, up to 100 K/s, and fast accomplishment of conversion. It should be noted that traditional furnace synthesis of refractory compounds requires a much longer time, ~1-10 h, for the same initial composition, particle size and close final temperature, and necessitates the use of costly and energy-consuming high-temperature facilities. It has been demonstrated both experimentally and theoretically that in many systems phase and structure formation during SHS proceeds via uncommon interaction mechanisms from the point of view of the classical Materials Science. To improve the degree of conversion in difficult-to-react systems and exert a closer control over CS, in some cases mechanical activation (MA) of starting powder mixtures is used. In this connection, an urgent problem is the development RCM and efficient technologies for their production from low-cost domestic raw materials basing on the cost and energy saving concept of SHS. Thus, the objective of this research is investigating experimentally a possibility of obtaining RCM for furnace lining applications in the regime of SHS using dolomite, CaMg(CO3)2, and silica sand, which occur in Belarus, as reducible compounds and aluminum powder as a reducing agent. RESEARCH METHODOLOGY In this work, a variety of experimental methods was used together with a theoretical study, namely thermodynamic modeling of SHS. Experimental procedure For producing porous RCM, fine-dispersed powders of silica, dolomite and aluminum were used in a different mass ratio (see Table I). In a series of experiments, the green powder mixture was subjected to mechanical activation in a rotary ball mill with a rotation speed of 1 revolution per second for several hours using 5-10 mm diameter wear-resistant steel balls as milling bodies with the ball-to-powder mass ratio of 2 to 4. Table I. Green powder compositions for SHS, wt.% No. Al SiO2 dolomite, CaMg(CO3)2 1 20 30 50

The luminescence spectra of thin films of PbWO4 and Bi2WO6 were investigated. It is shown that these spectra are similar and that they consist of three individual bands in the blue (2.80 eV PbWO4 and 2.93 eV Bi2WO6), green (2.35 eV PbWO4 and Bi2WO6), and red (1.75 eV PbWO4 and 1.90 eV Bi2WO6) spectral regions. The differences in the nature of the absorption centers of excitation energy are established. The distinguishing features displayed by the temperature dependences of the individual emission bands in the PbWO4 films are explained by energy migration between emission centers via transfer of free carriers through the conduction and valence bands.

Burning velocities in catalytically assisted self-propagating high-temperature combustion synthesis systems

The project uses an exothermic combustion synthesis reaction, termed self-propagating high-temperature synthesis (SHS), to produce high quality, reproducible nitride fuels and other ceramic type nuclear fuels (cercers and cermets, etc.) in conjunction with the fabrication of transmutation fuels. The major research objective of the project is determining the fundamental SHS processing parameters by first using manganese as a surrogate for americium to produce dense Zr-Mn-N ceramic compounds. These fundamental principles will then be transferred to the production of dense Zr-Am-N ceramic materials. A further research objective in the research program is generating fundamental SHS processing data to the synthesis of (i) Pu-Am-Zr-N and (ii) U-Pu-Am-N ceramic fuels. In this case, Ce will be used as the surrogate for Pu, Mn as the surrogate for Am, and depleted uranium as the surrogate for U. Once sufficient fundamental data has been determined for these surrogate systems, the information will be transferred to Idaho National Laboratory (INL) for synthesis of Zr-Am-N, Pu-Am-Zr-N and U-Pu-Am-N ceramic fuels. The high vapor pressures of americium (Am) and americium nitride (AmN) are cause for concern in producing nitride ceramic nuclear fuel that contains Am. Along with the problem of Am retention during the sintering phases of currentmore » processing methods, are additional concerns of producing a consistent product of desirable homogeneity, density and porosity. Similar difficulties have been experienced during the laboratory scale process development stage of producing metal alloys containing Am wherein compact powder sintering methods had to be abandoned. Therefore, there is an urgent need to develop a low-temperature or low–heat fuel fabrication process for the synthesis of Am-containing ceramic fuels. Self-propagating high temperature synthesis (SHS), also called combustion synthesis, offers such an alternative process for the synthesis of Am nitride fuels. Although SHS takes thermodynamic advantage of the high combustion temperatures of these exothermic SHS reactions to synthesize the required compounds, the very fast heating, reaction and cooling rates can kinetically generate extremely fast reaction rates and facilitate the retention of volatile species within the rapidly propagating SHS reaction front. The initial objective of the research program is to use Mn as the surrogate for Am to synthesize a reproducible, dense, high quality Zr-Mn-N ceramic compound. Having determined the fundamental SHS reaction parameters and optimized SHS processing steps using Mn as the surrogate for Am, the technology will be transferred to Idaho National Laboratory to successfully synthesize a high quality Zr-Am-N ceramic fuel.« less

FeSi2 is one of the most interesting high-temperature thermoelectric transition materials due to its high transition efficiency, low cost and high oxidation resistance at working temperatures [1]. FeSi2 thermoelectric devices have been prepared conventionally through melting raw metals, crushing, sintering and annealing. In this study, a novel processing technique, i.e. self-propagating high-temperature synthesis (SHS) or combustion synthesis, was used to prepare Fe-Si alloys. The SHS process is superior to other conventional methods in terms of saving energy and processing time. The SHS-products are usually highly pure because at extremely high reaction temperatures the volatile contaminants in the samples mostly vaporize. Owing to the very high cooling rate following SHS reaction, high defect concentrations and non-equilibrium structures generally exist in the SHS-produced materials, rendering the products more reactive and metastable [2]. In this Fe-Si alloy system, direct formation of /3 phase from the melt can be expected during SHS reaction when the cooling rate is high enough. The purpose of this study is to measure the thermal conductivity of Fe-Si alloys synthesized by SHS and to investigate the applicability as a thermoelectric generator material. To obtain /3-FeSi2 by the SHS process, iron (>99%) and silicon (98%) powders were mixed according to the composition shown in Table I, corresponding to /3-FeSi 2 and a little excess Si, by considering the loss of Si during combustion reaction because Si is more volatile than Fe. KNO 3 (>99.9%) was also added to the Fe-Si powder mixture in the weight ratio of KNO3/(Fe + Si) = 0.2 to effectively activate ignition of the mixture by arc discharge at room temperature [3]. The powder mixture was compacted to pellets at 100MPa pressure by a uniaxial press. Washing, crushing and screening processes followed the combustion reaction. The produced powder (<325 mesh) was analysed by Xray diffractometer, and shown to be composed of eFeSi, o~-Fe2Si5 and a trace of unreacted Si. This powder was uniaxially pressed into pellets at 100MPa pressure together with the binder PVA, which was removed by heating at 500 °C for 2 h in air, and the pellets were sintered at 1155 °C for 3 h in Ar. X-ray diffraction analysis showed that the sintered samples consisted of a-Fe2Si5 and e-FeSi. The sintered specimens were annealed at 840 °C for 12 h in Ar to transform these two metallic phases to semiconducting ¢l-FeSi2 phase, and compared with those sintered and annealed under the same conditions using a FeSi 2 commercial powder (CERAC, USA). Fig. 1 shows X-ray diffraction patterns for the annealed samples. The main phase /3-FeSi2 can be observed for all samples and the intensity of the eFeSi peaks decreases with increasing Si content. It is considered that the composition of specimens changed to an Fe-richer type compared with the starting

Optical Characterization of EDTA-assisted CdS:Mn Nanoparticles Synthesized by Sonochemical Method

A study of the excitation spectra is of very great importance in understanding the nature of various emission bands. When light of a specific spectral composition is absorbed, individual emission bands may be excited; this offers the possibility of separating these out and controlling their behavior in relation to various external factors. By excitation spectrum we mean the relation between the active absorption (active in respect of a given form of luminescence) and the wavelength of the exciting light [77, 78], However, before studying the active part of the absorption, we shall review published data relating to the absorption of alkali halide salts.

Optical thermometric properties in Tb3+ and Eu3+-coactivated dual-emissive fluorophosphate phosphors

Light‐emitting diodes (LED) with vertical stacking geometry were fabricated and characterized, as reported by K. N. Hui et al. on p. S902. Red, green and blue LED chips are stacked on top of each other to form a color‐adjustable light emitter. Color‐tunable and white‐light emission have been demonstrated. The emission of different colors, ranging from green‐blue over pink‐purple to yellow‐orange as well as white light emission have been achieved by adjusting the relative intensity of two or three individual emission bands, which in turn are controlled by adjusting the voltage supply (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Light‐emitting diodes with vertical stacking geometry were fabricated and characterized. Color‐tunable emission and white‐light emission has been demonstrated in Stacked‐LED (SLED). Emissions of different colors, ranging from green‐blue, to pink‐purple, to yellow‐orange and white light emission have be achieved by adjusting the relative intensity of two or three individual emission bands, which in turn are controlled by adjusting voltage supply. Compared to conventional lateral side‐by‐side geometry, the shift of white color chromaticity with respect to viewing angles is less in the case of SLED device. The farthest shift of white CIE coordinate point away from the pure white CIE coordinate is (0.29, 0.29) in SLED compared to (0.37, 0.35) in the commerical available lateral packaged structure. This vertically‐stacked LED is poised to open new frontiers in high performance color‐tunable LEDs. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)