Influence of Accidental Impurities on the Spectroscopic and Luminescent Properties of ZnWO4 Crystal

Special techniques for deep purification of ZnO and WO₃ have been developed in this work. A ZnWO₄ single crystal has been grown by the Czochralski method using purified ZnO and WO₃ chemicals, along with the ZnWO₄ crystal-etalon, which has been grown at the same conditions using commercially available 5N ZnO and WO₃ chemicals. The actual accidental impurities compositions of both the initial chemicals and the grown crystals have been measured by inductively coupled plasma mass-spectrometry. A complex of comparative spectroscopic studies of the crystals has been performed, including optical absorption spectra, photo-, X-ray-, and cathodoluminescence spectra and decay kinetics, as well as the photoluminescence excitation spectra. The revealed differences in the measured properties of the crystals have been analyzed in terms of influence of the accidental impurities on these properties.

Nonequidiffusive premixed-flame propagation in obstructed channels with open, nonreflecting ends

Equidiffusive premixed combustion in obstructed channels with both open, nonreflecting ends exhibits various forms of flame propagation: oscillations, acceleration or a combination of both regimes. Given the limited practicality of equidiffusive premixed combustion, it is important to understand how these modes of combustion are altered at nonequidiffusive conditions, characterized by a nonunity Lewis number (thermal to mass diffusivity ratio) Le≠1. To achieve this, the impacts of Le on the flame dynamics and morphology are analyzed by means of the computational simulations of the reacting flow equations, with Arrhenius chemical kinetics, fully compressible hydrodynamics, and transport properties. In addition to varying Le, the parametric study includes various blockage ratios, channel widths, obstacle spacing and thermal expansion ratios. It is identified how these parameters influence the burning velocities as well as the scaled oscillation amplitude and frequency. Specifically, in the narrow channels with small blockage ratios, the amplitude and frequency of the oscillations vary with Le, with the frequency decreasing and the amplitude increasing as Le grows from 0.3 to 2. In other conditions, a transition from the flame oscillations to sudden flame acceleration or its propagation at a constant velocity is singularly influenced by Le, or by the interplay of Le with the geometric parameters of a channel. The delay time before the onset of flame acceleration, especially at Le<1, also varies as the channel width and the blockage ratio change. In all cases, Le has both quantitative and qualitative effects on flame propagation in obstructed channels with both open, nonreflecting ends.

Novel Cyan-Green-Emitting Bi3+-Doped BaScO2F, R+ (R = Na, K, Rb) Perovskite Used for Achieving Full-Visible-Spectrum LED Lighting

Cyan-emitting phosphors are important for near-ultraviolet (NUV) light-emitting diodes (LEDs) to gain high-quality white lighting. In the present work, a Bi³⁺-doped BaScO₂F, R⁺ (R = Na, K, Rb) perovskite, which emits 506 nm cyan-green light under 360 or 415 nm excitation, is obtained via a high-temperature solid-state method for the first time. The obtained perovskite shows improved photoluminescence and thermal stability due to the charge compensation of Na⁺, K⁺, and Rb⁺ co-doping. Its spectral broadening is attributed to two centers Bi (1) and Bi (2), which are caused by the zone-boundary octahedral tilting due to the substitution of Bi³⁺ for the larger Ba²⁺. Employing the blend phosphors of Ba_0.998ScO₂F:0.001Bi³⁺,0.001K⁺ and the commercial BAM:Eu²⁺, YAG:Ce³⁺, and CaAlSiN₃:Eu²⁺, a full-spectrum white LED device with R_a = 96 and CCT = 4434 K was fabricated with a 360 nm NUV chip. Interestingly, a novel strategy is proposed: the cyan-green Ba_0.998ScO₂F:0.001Bi³⁺,0.001K⁺ and orange Sr₃SiO₅:Eu²⁺ phosphors were packaged with a 415 nm NUV chip to produce the white LED with R_a = 85 and CCT = 4811 K.

Non-invasive monitoring of red beet development

Agricultural monitoring is required to enhance crop production, control plant stress, and predict pests and crop infection. Apart from monitoring the external influences, the state of the plant itself must be tracked. However, the modern methods for plant analysis are expensive and require plants processing often in a destructive way. Optical spectroscopy can be used for the non-invasive monitoring requiring no consumables, and little to none sample preparation. In this context, we found that the red beet growth can be monitored by Raman spectroscopy. Our analysis shows that, as plants age, the rate of betalain content increases. This increase makes betalain dominate the whole Raman spectra over other plant components. The dominance of betalain facilitates its use as a molecular marker for plant growth. This finding has implications in the understanding of plant physiology, particularly important for greenhouse growth and the optimization of external conditions such as artificial illumination.

Calibration-free scanned wavelength modulation spectroscopy--application to H(2)O and temperature sensing in flames

A calibration-free scanned wavelength modulation spectroscopy scheme requiring minimal laser characterization is presented. Species concentration and temperature are retrieved simultaneously from a single fit to a group of 2f/1f-WMS lineshapes acquired in one laser scan. The fitting algorithm includes a novel method to obtain the phase shift between laser intensity and wavelength modulation, and allows for a wavelength-dependent modulation amplitude. The scheme is demonstrated by detection of H(2)O concentration and temperature in atmospheric, premixed CH(4)/air flat flames using a sensor operating near 1.4 µm. The detection sensitivity for H(2)O at 2000 K was 4 × 10(-5) cm(-1) Hz(-1/2), and temperature was determined with a precision of 10 K and absolute accuracy of ~50 K. A parametric study of the dependence of H(2)O and temperature on distance to the burner and total fuel mass flow rate shows good agreement with 1D simulations.

Speedup of doping fronts in organic semiconductors through plasma instability

The dynamics of doping transformation fronts in organic semiconductor plasma is studied for application in light-emitting electrochemical cells. We show that new fundamental effects of the plasma dynamics can significantly improve the device performance. We obtain an electrodynamic instability, which distorts the doping fronts and increases the transformation rate considerably. We explain the physical mechanism of the instability, develop theory, provide experimental evidence, perform numerical simulations, and demonstrate how the instability strength may be amplified technologically. The electrodynamic plasma instability obtained also shows interesting similarity to the hydrodynamic Darrieus-Landau instability in combustion, laser ablation, and astrophysics.

Role of compressibility in moderating flame acceleration in tubes

The effect of gas compression on spontaneous flame acceleration leading to deflagration-to-detonation transition is studied theoretically for small Reynolds number flame propagation from the closed end of a tube. The theory assumes weak compressibility through expansion in small Mach number. Results show that the flame front accelerates exponentially during the initial stage of propagation when the Mach number is negligible. With continuous increase in the flame velocity with respect to the tube wall, the flame-generated compression waves subsequently moderate the acceleration process by affecting the flame shape and velocity, as well as the flow driven by the flame.

Growth rate and the cutoff wavelength of the Darrieus-Landau instability in laser ablation

The main characteristics of the linear Darrieus-Landau instability in the laser ablation flow are investigated. The dispersion relation of the instability is found numerically as a solution to an eigenvalue stability problem, taking into account the continuous structure of the flow. The results are compared to the classical Darrieus-Landau instability of a usual slow flame. The difference between the two cases is due to the specific features of laser ablation: sonic velocities of hot plasma and strong temperature dependence of thermal conduction. It is demonstrated that the Darrieus-Landau instability in laser ablation is much stronger than in the classical case. In particular, the maximum growth rate in the case of laser ablation is about three times larger than that for slow flames. The characteristic length scale of the Darrieus-Landau instability in the ablation flow is comparable to the total distance from the ablation zone to the critical zone of laser light absorption. The possibility of experimental observations of the Darrieus-Landau instability in laser ablation is discussed.

Physical mechanism of ultrafast flame acceleration

We explain the physical mechanism of ultrafast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the obstacles creates a powerful jetflow, driving the acceleration. This mechanism is much stronger than the classical Shelkin scenario of flame acceleration due to nonslip at the channel walls. The mechanism under study is independent of the Reynolds number, with turbulence playing only a supplementary role. The flame front accelerates exponentially; the analytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.