Temperature-dependent Reflectivity Research Articles

Hofmann Clathrates: A "Blue Box" Approach to Modulate Spin-Crossover Properties.

Hofmann coordination polymers (CPs) with cationic ligands provide an innovative strategy for recognizing π-electron-rich aromatic molecules - similar to the "little blue box". In this study, we demonstrate that hydroquinone molecules can be incorporated into these coordination polymers when redox-active bipyridinium derivatives are used as axial ligands. The insertion leads to a significant structural modification, resulting in a shift of the spin transition by 150 K and an approximate 23 % increase in volume, caused by the strong donor-acceptor π-π stacking interaction formed between the ligands and the guest molecule. These findings have been confirmed through temperature-dependent single crystal X-ray diffraction, magnetic measurements and optical reflectivity measurements.

Temperature dependence of optical properties for thermoplastic composite prepreg during laser in-situ consolidation

The optical properties of the thermoplastic prepreg play a critical role in influencing heating efficiency during the in-situ consolidation process. This study investigates the temperature-dependent reflectance of the prepreg during laser heating. A reflectance testing device is developed based on an in-situ heating system, and the reflectance of carbon fiber/PEEK prepreg is studied from 20 to 500 °C. Through a comparative analysis, the reflectance of pure PEEK films and carbon fibers are individually examined within the same temperature range. The variation in reflectance at different temperatures is explained according to the classical electromagnetic theory. Furthermore, the effect of fiber density on the reflectance of thermoplastic prepreg is explored, and the results indicate that the reflectance of the prepreg decreases from 15.65 % to 11.80 % as the temperature rises from 20 °C to 500 °C, which can be primarily attributed to the decline in carbon fiber reflectance with increasing temperature.

Combined experimental and theoretical approach to improve measurement accuracy of temperature-dependent reflectance of copper for near-infrared lasers

AbstractThe low process stability of laser welding of copper with near-infrared lasers requires precise input data for process control and meaningful simulations. But meanwhile, available datasets of temperature-dependent reflectance or absorptance for near-infrared lasers on copper do not show good agreement between the different sources and often do not include the fusion process, which is of crucial importance for realistic laser welding simulations. Additionally, most of the datasets are only calculated. Therefore, in a previous study, temperature-dependent reflectance measurements were performed on electro tough-pitch copper using a near-infrared laser. The measurements revealed a reflectance drift, which was induced by the setup behavior during heating, and the time-dependence of chemical reactions like the redox-reaction as possible error sources. In this study, experiments on laser melting as the fundamental process of laser welding were performed, together with corresponding simulations using the measured reflectance values for oxide-reduced and for untreated copper from the previous study. Then, the simulations were compared with the experiments to estimate the accuracy of the reflectance measurements. To provide context, the same simulations were also conducted using reflectance datasets from other authors. In a second step, the reflectance data were corrected with respect to the reflectance drift and the effects of redox reactions were adapted to the conditions of the laser melting experiments. Using the resulting reflectance curves, an improved agreement of simulation results and the experiments was achieved over a range of different test cases, without the necessity of correction factors in the simulation model.

Unraveling Intrinsic Relationships of Thermal Properties in Thermoreflectance Experiments

Thermoreflectance techniques, particularly Frequency-Domain Thermoreflectance (FDTR), play a crucial role in measuring the thermal properties of bulk and thin-film materials. These methods precisely measure thermal conductivity, specific heat capacity, and interfacial thermal conductance by analyzing the temperature-dependent reflectivity changes in materials. However, the complex interplay among parameters presents challenges in data analysis, where single-variable analysis often fails to accurately capture intra-layer and inter-layer interactions. This paper uses FDTR as a case study and systematically explores the relationships between sensitivity coefficients of various parameters through Singular Value Decomposition (SVD). Specifically, the sensitivity matrix <b><i>S</i></b> of the system's parameters is subjected to SVD to identify smaller singular values and their corresponding right singular vectors, which are the basis vectors of the null space of matrix <b><i>S</i></b> . These vectors reveal the relationships among parameter sensitivities, and by contradiction, these relationships reveal the most fundamental combined parameters that determine the thermoreflectance signal. This approach not only clarifies the dependencies among variables but also identifies the maximum number of parameters that can be experimentally extracted, as well as the parameters that must be known beforehand. To demonstrate the practical value of these combined parameters, this study conducts a detailed analysis of FDTR signals from an aluminum/sapphire sample. Unlike traditional FDTR experiments, which typically fit only substrate thermal conductivity and interfacial thermal conductance, our sensitivity analysis reveals that it is possible to simultaneously determine the thermal conductivity of the metal film, substrate thermal conductivity, substrate specific heat capacity, and interfacial thermal conductance. The fitting results are consistent with reference values from the literature and measurements from other thermoreflectance techniques, validating the effectiveness and reliability of our method. This comprehensive analysis not only deepens the understanding of thermoreflectance phenomena but also provides robust support for advancements in thermal characterization technology and material research, showcasing the significant potential of applying SVD in complex multi-parameter systems.

Temperature-Dependent Reflectance of Copper with Different Surface Conditions Measured at 1064 nm

AbstractIn order to fully understand and to eventually overcome the instabilities in laser welding of copper using 1064 nm wavelength lasers, it is important to get a deeper understanding about the interaction of the incident beam with the copper surface during the process. One of the main parameters determining this behavior is the temperature-dependent absorptance. However, the existing datasets only show little consistency between the different sources, do often not include the important absorptance step at melt temperature, or they are only calculated. This article thus presents an experimental approach to measure the reflectance of electro tough-pitch copper (Cu-ETP) from room temperature until beyond the melting point with an integrating sphere setup. The setup was developed with particular attention to measurement accuracy up to high temperatures. The reflectance using a 1064 nm laser under an incident angle of 12° has been measured for four different sample groups, which are untreated rolled copper, mechanically polished copper, oxide reduced copper and resolidified copper. The measurement results reveal significant differences in the temperature-dependent reflectance between the sample types and, for some types, also between the samples of the same type. This study investigates the origins of the most significant features in the reflectance curves and their potential impact on processing copper using NIR lasers.

Analyzing Thermal Processes in Laser Welding of Multi- Component Heat-Resistant Alloy Thin-Walled Butt Joints: A Comprehensive Modeling Approach

Niobium-based alloys are vital in high-temperature environments due to their unique chemical composition, phase structure, high density, and oxidation resistance. These alloys find extensive applications in industries such as medicine, engine manufacturing, and space technology. This study delves into the complexities of modeling laser welding processes, necessitated by the multitude of dynamic parameters specific to each surface configuration. Finite element modeling was conducted using COMSOL Multiphysics to analyze the heat transfer in butt joints between Nb-15W-5Mo-1Zr alloy plates under 400 W laser radiation power. The research employed finite element modeling to analyze heat transfer in Nb-15W-5Mo-1Zr alloy plates, considering non-uniform movement at the start and end of the welding process, Gaussian heat flux distribution along the laser spot radius, and temperature-dependent reflection coefficient. The models accounted for non-uniform movement, Gaussian heat flux distribution, and temperature-dependent reflection coefficient. The study demonstrated that thin plates of Nb-15W-5Mo-1Zr alloy can be effectively welded using specific laser processing modes, leveraging their high initial cooling rate of 1.2-1.3 s. Successful welding of Nb-15W-5Mo-1Zr alloy plates requires careful consideration of laser processing parameters. While the alloy's thermodynamic properties allow for effective welding, ensuring weld quality mandates additional production processes aimed at preventing potential part failure.

Tunable ultrafast thermionic emission from femtosecond-laser hot spot on a metal surface by control of laser polarization and angle of incidence: A numerical investigation

Ultrafast laser induced thermionic emission from metal surfaces has several applications. Here, we investigate the role of laser polarization and angle of incidence on the ultrafast thermionic emission process from laser driven gold coated glass surface. The spatio-temporal evolution of electron and lattice temperatures are obtained using an improved three-dimensional (3D) two-temperature model (TTM) which takes into account the 3D laser pulse profile focused obliquely onto the surface. The associated thermionic emission features are described through the modified Richardson-Dushman equation, including dynamic space-charge effects and are included self-consistently in our numerical approach. We show that temperature-dependent reflectivity influences laser energy absorption. The resulting peak electron temperature on the metal surface monotonically increases with the angle of incidence for the P polarization, while for the S polarization it shows the opposite trend. We observe that thermionic emission duration shows a strong dependence on the angle of incidence and contrasting polarization dependent behavior. The duration of the thermionic current shows strong correlation to the intrinsic electron-lattice thermalization time, in a fluence regime well below the damage threshold of gold. The observations and insights have important consequences in designing ultrafast thermionic emitters using a metal based architecture.

Study of energy gaps and their temperature-dependent modulation in LaCrO3: A theoretical and experimental approach

In the present study, the origin of three energy gaps of lanthanum chromium oxide, one arises due to the charge-transfer gap between O-2p and Cr-3d and two arises from the d–d transitions, is analyzed in detail using the diffuse reflectance spectroscopy technique. The spin allowed transitions, such as 4T1(P)→4A2, 4T1(F)→4A2, and 4T2→4A2, and the spin and parity forbidden 2E →4A2 transitions are depicted using a Tanabe–Sugano (T–S) diagram and an absorption spectrum. The high crystal field strength of 3.27 obtained from the T–S diagram provides high directional emission and makes the system suitable for near infrared lasing applications. Moreover, the investigations into the variation of a charge-transfer gap with temperature will provide insights into the modification of numerous optical properties toward development of optoelectronic devices. Using this temperature-dependent diffuse reflectance spectroscopy studies, we have obtained the important optical parameters, such as Urbach energy (Eu) and Urbach focus. Furthermore, first-principles calculations are carried out in order to validate the experimental findings on LaCrO3. The experimental results are in consonance with the charge-transfer gap obtained from the theoretical calculations. Furthermore, the bandgap variation with temperature is fitted using Varshni's relation, and the Debye temperature is calculated.

Effect of surface modification by Ar+ ion irradiation on thermal hysteresis of VO2

Vanadium dioxide (VO2) undergoes a metal–insulator phase transition at ∼70 °C. As this is a first-order phase transition, VO2 exhibits thermal hysteresis. The reflectivity and electrical resistivity of VO2 drastically change at insulator-to-metal (TIMT) and metal-to-insulator (TMIT) transition temperatures during heating and cooling, respectively. For smart glass and thermal memory applications employing VO2, the origin and control factor of thermal hysteresis must be investigated. Additional elemental doping and nano-structuring of VO2 affect the thermal hysteresis width. However, the factors determining TIMT and TMIT remain unclear. TIMT and TMIT can be modified by irradiating Ar+ on the surface of VO2 nanostructures with varying Ar+ irradiation doses (nAr+) at 1 keV. The temperature-dependent reflectivity against IR light is evaluated. For VO2, TIMT decreases with nAr+ = 3.9 × 1014 cm−2; TMIT increases with nAr+ > 3.9 × 1015 cm−2. Ar+ irradiation decreases the thermal hysteresis width. Because the expected penetration depth of Ar+ at 1 keV into the VO2 surface is <6 nm, the VO2 chemical state at the outermost surface is investigated using x-ray absorption spectroscopy with soft x-ray irradiation. The V L-edge peak energy decreases with increasing nAr+ . Ar+ irradiation reduces V only at the outermost surface state. TIMT is more sensitive than TMIT to the reduction of V. The reduction of only a small fraction at the surface affects the phase transition of the entire VO2. These results are beneficial for understanding the cause of thermal hysteresis width and improving the performance of devices using VO2.

Ultraviolet-C AlGaN Resonant-Cavity Light-Emitting Diodes with Thermal Stability Pipe-AlGaN-Distributed Bragg Reflectors.

Ultraviolet-C AlGaN resonant-cavity light-emitting diodes with top and bottom pipe-AlGaN-distributed Bragg reflectors (DBRs) have been demonstrated. For the top/bottom DBR structures, 20 pairs of n+-AlGaN:Si/n-AlGaN:Si stack structures were transformed into the pipe-AlGaN:Si/n-AlGaN:Si DBRs through a doping-selective electrochemical wet etching process. The reflectivity of the pipe-AlGaN DBR structure was measured as 90% at 276.7 nm with a 20.9 nm flat stopband width. The anisotropic optical properties of the pipe-AlGaN DBR structure had been analyzed through the polarization-dependent reflectance spectra. For temperature-dependent reflectance spectra, the central wavelengths were slightly redshifted from 275 nm (100 K) to 281 nm (600 K) due to thermal expansion, refractive index increase, and partial strain release phenomena in the pipe-DBR structure. High photoluminescence emission intensity and line-width reducing phenomena were observed at 10 K in the UVC-LED with the resonant-cavity structure, which has potential for high-efficiency UV-C light source applications.

Fluorite-related iridate Pr3IrO7: crystal growth, structure, magnetism, thermodynamic, and optical properties

Spin–orbit coupling in heavy 5d metal oxides, in particular, iridates have received tremendous interest in recent years due to the realization of exotic electronic and magnetic phases. Here, we report the synthesis, structural, magnetic, thermodynamic, and optical properties of the ternary iridate Pr3IrO7. Single crystals of Pr3IrO7 have been grown by the KF flux method. Structural analysis shows that Pr3IrO7 crystallizes in an orthorhombic phase with Cmcm symmetry. The electron energy loss spectroscopy study indicates that Pr is in a 3+ valence state, which implies a 5+ oxidation state of Ir. Magnetization data measured at high and low magnetic fields do not exhibit any bifurcation between M ZFC and M FC , however, a weak hump in M(T) is observed at 10.4 K. The specific heat data reveal two maxima at ∼253 and ∼4.8 K. The optical conductivity spectrum shows 24 infrared-active phonon modes and reveals an insulating behavior with an optical gap of size ∼500 meV. During cooling down, the temperature-dependent reflectivity spectrum reveals eight extra phonon modes below the structural phase transition (∼253 K). An anomaly is observed at around in the temperature evolution of infrared-active mode frequencies suggesting the presence of significant spin–phonon coupling in the system.

Emergent Moiré Phonons Due to Zone Folding in WSe2-WS2 Van der Waals Heterostructures.

Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moiré superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moiré Raman modes from nearly aligned WSe2-WS2 vdWHs in the range of 240-260 cm-1, which are absent in both monolayers and homobilayers of WSe2 and WS2 and in largely misaligned WSe2-WS2 vdWHs. Using first-principles calculations and geometric arguments, we show that these moiré Raman modes are a consequence of the large moiré length scale, which results in zone-folded phonon modes that are Raman active. These modes are sensitive to changes in twist angle, but notably, they occur at identical frequencies for a given small twist angle away from either the 0-degree or 60-degree aligned heterostructure. Our measurements also show a strong Raman intensity modulation in the frequency range of interest, with near 0 and near 60-degree vdWHs exhibiting a markedly different dependence on excitation energy. In near 0-degree aligned WSe2-WS2 vdWHs, a nearly complete suppression of both the moiré Raman modes and the WSe2 A1g Raman mode (∼250 cm-1) is observed when exciting with a 532 nm CW laser at room temperature. Temperature-dependent reflectance contrast measurements demonstrate the significant Raman intensity modulation arises from resonant Raman effects.

Refractory Plasmonic Hafnium Nitride and Zirconium Nitride Thin Films as Alternatives to Silver for Solar Mirror Applications.

Harnessing solar energy by employing concentrated solar power (CSP) systems requires materials with high electrical conductivity and optical reflectivity. Silver, with its excellent optical reflectance, is traditionally used as a reflective layer in solar mirrors for CSP technologies. However, silver is soft and expensive, quickly tarnishes, and requires a protective layer of glass for practical applications. Moreover, supply-side constraints and high-temperature instability of silver have led to the search for alternative materials that exhibit high solar and infrared reflectance. Transition metal nitrides, such as titanium nitride, have emerged as alternative plasmonic materials to gold starting from a spectral range of ∼500 nm. However, to achieve high solar reflection (∼320-2500 nm), materials with epsilon-near-zero starting from the near-ultraviolet (UV) spectral region are required. Here, we show the development of refractory epitaxial hafnium nitride (HfN) and zirconium nitride (ZrN) thin films as excellent mirrors with a solar reflectivity of ∼90.3% and an infrared reflectivity of ∼95%. Low-loss and high-quality epsilon-near-zero resonance at near-UV (∼340-380 nm) spectral regions are achieved in HfN and ZrN by carefully controlling the stoichiometry, leading to a sharp increase in the reflection edge that is on par with silver. Temperature-dependent reflectivity and dielectric constants are further measured to demonstrate their high-temperature suitability. The development of refractory epitaxial HfN and ZrN thin films with high solar and infrared reflectance makes them excellent alternative plasmonic materials to silver and would pave their applications in CSP, daytime radiative cooling, and others.

Temperature-dependent diffuse reflectance measurements of ceramic powders in the near- and mid-infrared spectra

Radiative properties are critical to quantify radiative energy fluxes between surfaces and in participating media. However, there is limited experimental data on temperature-dependent radiative properties of materials. This work focuses on experimentally measuring temperature-dependent diffuse reflectance in the near- and mid-infrared spectra (1–20 μm) for ceramic particles with applications as heat-transfer and thermal-storage media in concentrated solar power (CSP) plants. Specifically, a commercially available sintered bauxite proppant, ACCUCAST ID80, and its primary chemical constituents—alumina (Al2O3) and silica (SiO2)—are measured in powder form using a Fourier transform infrared spectrometer (FTIR) coupled with a specialized diffuse reflectance accessory and a heated stage. Room-temperature diffuse reflectance measurements show increased absorption in tests with greater mass fractions of the ceramic samples. There is a strong correlation in the measured reflectance spectra of ACCUCAST with alumina and silica in the spectral range 2000–500 cm−1 (5–20 μm). Whereas, for shorter wavelengths (<5μm), the absorptance for ACCUCAST is greater than the absorptance for alumina and silica, indicating contributions from other chemical species present in the composite material. For the first time, temperature-dependent diffuse reflectance measurements are reported for ACCUCAST up to 1000 °C. These results are compared against those of alumina and silica through the calculation of a thermal emittance. All three materials exhibit a calculated emittance of ∼0.9 at room temperature. However, this value decreases to ∼0.6 for ACCUCAST and drops to less than 0.4 for alumina and silica at 1000 °C. Thermal cycling in air at 1000 °C resulted in a visible color change from dark grey to light orange for ACCUCAST and a subsequent larger increase in reflectance for wavelengths less than 5 μm as compared to ACCUCAST thermally cycled at 1000 °C in vacuum. Alumina and silica spectra proved to be largely unaffected by thermal cycling under atmospheric conditions. Overall, this study establishes a powerful technique for the characterization of radiative properties of particulate materials as a function of temperature and presents a detailed case study of ACCUCAST, a candidate for next-generation particle-based CSP.

Optical study of RbV3Sb5 : Multiple density-wave gaps and phonon anomalies

Temperature-dependent reflectivity studies on the nonmagnetic kagome metal ${\mathrm{RbV}}_{3}{\mathrm{Sb}}_{5}$ in a broad energy range ($50--20000{\mathrm{cm}}^{\ensuremath{-}1}$, equivalent to 6 meV--2.5 eV) down to 10 K are reported. Below ${T}_{\mathrm{CDW}}=102$ K, the optical spectra demonstrate a prominent spectral-weight transfer from low to higher energies as the fingerprint of the charge-density-wave (CDW) formation with the opening of a partial gap. A detailed analysis reveals two energy scales of respectively $\ensuremath{\sim}800 {\mathrm{cm}}^{\ensuremath{-}1}$ (100 meV) and $360{\mathrm{cm}}^{\ensuremath{-}1}$ (45 meV), the latter visible below 50 K only. Additionally, two modes at respectively $160{\mathrm{cm}}^{\ensuremath{-}1}$ (20 meV) and $430{\mathrm{cm}}^{\ensuremath{-}1}$ (53 meV) can be traced both above and below ${T}_{\mathrm{CDW}}$. They show strong anomalies already above ${T}_{\mathrm{CDW}}$ with a further renormalization across the transition, suggesting the importance of the electron-phonon coupling in ${\mathrm{RbV}}_{3}{\mathrm{Sb}}_{5}$ in both normal and CDW states. While the $160{\mathrm{cm}}^{\ensuremath{-}1}$ mode can be attributed to the ${E}_{1u}$ phonon, the $430{\mathrm{cm}}^{\ensuremath{-}1}$ mode could not be reproduced in our phonon calculations. The antiresonance nature of this mode suggests a nontrivial electron-phonon coupling in ${\mathrm{RbV}}_{3}{\mathrm{Sb}}_{5}$. A distinct localization peak observed at all temperatures signals damped electron dynamics, whereas the reduced Drude spectral weight manifests moderate deviations from the band picture in ${\mathrm{RbV}}_{3}{\mathrm{Sb}}_{5}$.

Temperature dependence of refractive indices of Al0.9Ga0.1As and In0.53Al0.1Ga0.37As in the telecommunication spectral range

In this work, we determine the temperature dependence of refractive indices of In0.53Al0.1Ga0.37As and Al0.9Ga0.1As semiconductor alloys at telecommunication wavelengths in the range from room temperature down to 10 K. For that, we measure the temperature-dependent reflectance of two structures: with an Al0.9Ga0.1As/GaAs distributed Bragg reflector (DBR) designed for 1.3 µm and with an In0.53Al0.1Ga0.37As/InP DBR designed for 1.55 µm. The obtained experimental results are compared to DBR reflectivity spectra calculated within the transfer matrix method to determine refractive index values. We further show that changes due to the thermal expansion of the DBR layers are negligible for our method.

Infrared study of the layered magnetic insulator Mn(Bi0.07Sb0.93)2Te4 at low temperatures

Topological insulators with intrinsic magnetic ordering, potentially hosting rare quantum effects, recently attracted extensive attention. MnBi$_2$Te$_4$ is the first established example. The Sb-doped variant Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$ shows a great variety of electronic properties depending on the Sb content~$x$, such as shifts in the Fermi level and the Neel temperature $T_{\mathrm N}$, and the change of the free charge carrier type from $n$- to $p$-type at high Sb substitution ratios. Here, we investigate the effect of magnetic ordering on the bulk electronic structure of Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$ with high Sb content $x=0.93$ by temperature-dependent reflectivity measurements over a broad frequency range. We observe anomalies in the optical response across $T_{\mathrm N}$ when the antiferromagnetic order sets, which suggests a coupling between the magnetic ordering and the electronic structure of the material.

Realization of a cost-effective thermochromic solar absorber with a high emittance change based on VO2 and an infrared transparent intermediate layer

In this study, we present a thermochromic solar absorber coating that reaches a high thermal emittance change by using a thin, optically switching VOx film located on an infrared transparent interlayer (spacer) of Si or Ge with an optical thickness of λ/4 (for λ = 7 μm). Using this so-called “lambda/4-concept,” temperature-dependent reflection measurements in the spectral range between 2500 and 50 000 nm from an absorber with a 450 nm Si spacer and a VOx film oxidized from a 30-nm-thick V display an overall increase in emittance from ε(25 °C) = 12.2% to ε(150 °C) = 55.1%, resulting in a change of Δε = 42.9%. In addition, using an absorber with a 400 nm Ge spacer in combination with a VOx film oxidized from 17.5-nm-thick V, an increase in emittance from ε(25 °C) = 8.2% to ε(150 °C) = 49.2% with a change of Δε = 41.0% was achieved. In addition, the optical properties of Ge and Si thin films over a wide spectral range of 250–38 000 nm were determined using spectroscopic ellipsometry. Using this optical data and a simple optical model of the VOx film, reflectance simulations could be performed by using the ellipsometry analysis software WVASE©. It was shown by x-ray diffraction measurements that the optically switching VOx films oxidized from V in a belt furnace consist of the VO2 and the V2O5 phases. Using scanning electron microscopy images, the surface morphology of optically switching VOx films was compared with over-oxidized VOx films. A correlation between the surface morphology and the crystalline phases was revealed and applied to search for the optimal furnace parameters. This method could significantly reduce the time cost to achieve optically switching VOx coatings using an oxidation process.

Direct comparison of the electrical, optical, and structural phase transitions of VO2 on ZnO nanostructures

We examined the temperature-dependent electrical, optical, and structural properties of VO2 on ZnO nanorods with different lengths in the temperature range from 30 to 100 °C. ZnO nanorods with a uniform length were grown on Al2O3 substrates using a metal organic chemical vapor deposition, and subsequently, VO2 was ex-situ deposited on ZnO nanorods/Al2O3 templates using a sputtering deposition. The optical properties of the VO2/ZnO nanorods were measured simultaneously with direct current (DC) resistance using the reflectivity of an infrared (IR) laser beam with a wavelength of 790 nm. The local structural properties around V atoms of VO2/ZnO nanorods were simultaneously measured with the DC resistance using x-ray absorption fine structure at the V K edge. Direct comparison of the temperature-dependent resistance, IR reflectivity, and local structure reveals that an optical phase transition first occurs, a structural phase transition follows, and an insulator-to-metal transition finally appears during heating.

In situ high temperature spectroscopic study of liquid inclusions and hydroxyl groups in high purity natural quartz

High purity natural based quartz is used as raw material for production of crucibles for solar-grade silicon production. The crucible properties are strongly related to presence of water and hydroxyl groups in the raw material. In this work, in situ temperature-dependent diffuse reflectance infrared (DRIFT) and Raman spectroscopy in the spectral range of the OH stretching modes, 3000–4000 cm−1, were used to investigate water inclusions and hydroxyl groups at temperatures up to 800 °C. DRIFT shows that a substantial amount of liquid water was removed at 700 °C whereas new silanol groups characterised by stretching modes at about 3650 cm−1 were formed above 200 °C. A surface OH mode was also identified by multivariante curve resolution (MCR) of Raman maps. These surface OH groups were interpreted as the sign of formation of a hydrated layer at the surface of inclusions. It was not possible to detect water signals in other types of structural defect like cracks. Images were acquired in an in situ cell showing different stages of the rupture of an inclusion with a diameter of ≈8 μm between 400 and 500 °C. The in situ Raman spectrum of the inclusion water showed the typical liquid water bands at room temperature with a maximum moving to higher wavenumbers with increasing temperature. At 400 °C, the single peak at 3645 cm−1 indicated a situation close to the critical point, triggering rupture. The rupture of such large inclusions was thus identified as the dominating channel for the removal of liquid water inclusions. A small inclusion with a diameter ≈4 μm, however, was stable even at 800 °C. These small inclusions contain supercritical water, as evidenced by the sharp Raman peak at 3600 cm−1. Via the peak position, the internal pressure in this stable inclusion was estimated to be 90 MPa. Stability may be facilitated by the formation of a hydrated interfacial layer; the different reactivities of the inclusion fluid at different temperatures leads to different interfacial structures at different temperatures. The presented evidence suggests that inclusions with diameter in the range of few μm are the main source of remaining water and hydroxyl groups in thermally treated high purity quartz sand.