Low Characteristic Temperature Research Articles

Enhancing thermal properties and surface quality of lappato glazed porcelain tiles through milling time optimization

Increasing the surface quality of porcelain tiles, preferred in public, commercial, and living centers, where human density is high, is one of the crucial goals. Surface applications can be applied on the tiles produced as glazed and unglazed tiles in different versions. In lappato glazed tiles, staining-resistance is of great importance besides glossy-surface and wear-resistance. In this study, technical properties such as staining, chemical agent-resistance, color-parameters, thermal-behavior (heat microscopy), roughness, abrasion resistance, and microstructure of glazes (SEM/EDS) depending on the optimizing grinding time were examined with compared to standard lappato-glaze under the industrial conditions. It was observed that with the increase of the glaze grinding, the whiteness value and the increasing smoothness, and higher staining chemicals resistance and the glaze which has lower characteristic temperatures than the standard. As the surface roughness decreased due to grinding, the number and size of pores in the microstructure decreased. The glassy phase exhibited homogeneous distribution due to the dissolution of anorthite crystals.

Hot bubbles of planetary nebulae with hydrogen-deficient winds. III. Formation and evolution in comparison with hydrogen-rich bubbles

We seek to understand the evolution of Wolf-Rayet central stars by comparing the diffuse X-ray emission from their wind-blown bubbles with that from their hydrogen-rich counterparts with predictions from hydrodynamical models. We simulate the dynamical evolution of heat-conducting wind-blown bubbles using our 1D radiation-hydrodynamics code NEBEL/CORONA . We use a post-AGB-model of 0.595 but allow for variations of its evolutionary timescale and wind power. We follow the evolution of the circumstellar structures for different post-AGB wind prescriptions: for O-type central stars and for Wolf-Rayet central stars where the wind is hydrogen-poor, more dense, and slower. We use the CHIANTI software to compute the X-ray properties of bubble models along the evolutionary paths. We explicitly allow for non-equilibrium ionisation of key chemical elements. A sample of 12 planetary nebulae with diffuse X-ray emission ---seven harbouring an O-type and five a Wolf-Rayet nucleus--- is used to test the bubble models. The properties of most hydrogen-rich bubbles (X-ray temperature, X-ray luminosity, size) and their central stars (photon and wind luminosity) are fairly well represented by bubble models of our 0.595 AGB remnant. The bubble evolution of Wolf-Rayet objects is different, thanks to the high radiation cooling of their carbon- and oxygen-rich winds. The bubble formation is delayed, and the early evolution is dominated by condensation instead of evaporation. Eventually, evaporation begins and leads to chemically stratified bubbles. The bubbles of the youngest Wolf-Rayet objects appear chemically uniform, and their X-ray properties can be explained by faster-evolving nuclei. The bubbles of the evolved Wolf-rayet objects have excessively low characteristic temperatures that cannot be explained by our modelling. The formation of nebulae with O-type nuclei follows mainly a single path, but the formation pathways leading to the Wolf-Rayet-type objects appear diverse. Bubbles with a pure Wolf-Rayet composition can exist for some time after their formation despite the presence of heat conduction.

Shock-wave structure in non-polar diatomic and polyatomic dense gases under rotation and vibration

This study investigates the effect of rotation and vibration on the structure of shock waves in moderately dense diatomic and polyatomic non-polar gases using the one-temperature Navier–Stokes–Fourier approach. The modified Enskog equation of state of the gas is taken to include the denseness and shielding effects. The specific heat at constant volume has been taken to be temperature-dependent. The shear viscosity, the bulk viscosity, and the thermal conductivity have been assumed to follow the temperature-dependent power-law model. Nitrogen and oxygen gas have been taken as the test cases for diatomic gases while carbon dioxide was taken for the polyatomic gases. The implicit system of equations is derived and solved numerically for density and temperature. The inclusion of denseness, rotational, and vibrational modes of molecular motion have a significant effect on the density and temperature profiles, the inverse shock thickness, the bulk to shear viscosity ratio, and the molar specific heat at constant pressure. The gas having a low characteristic vibrational temperature has been found to have a high value of inverse shock thickness. The inverse shock thickness, the bulk to shear viscosity ratio, and the molar specific heat at constant pressure for nitrogen and carbon dioxide are found to be in good agreement with the experimental values.

Effect of thermal nonequilibrium on the shock interaction mechanism for carbon dioxide mixtures on double-wedge geometries

The effect of thermal nonequilibrium on shock interactions of carbon dioxide (CO2) hypersonic flows is investigated. Given the relatively low characteristic vibrational temperature of the CO2 molecule, it is expected that excited vibrational modes play a significant role in the physics of shock/shock and shock/boundary layer interactions. The shock interference mechanism resulting from a CO2-dominated flow over different double-wedge geometries is investigated by numerically solving the Navier–Stokes equations within the framework of a two-temperature model that considers translational energy–vibrational energy transfer. To assess the impact of vibrational relaxation, a comparative assessment of the patterns obtained with three thermo-physical models is presented, with the two-temperature model flow pattern being compared to thermally perfect and perfect ideal gas ones. Results obtained with the two-temperature model show that increasing the aft angle significantly enlarges the separated region in the compression corner and generates numerous secondary shock waves and shear layers. Peaks of heat flux and pressure occur along the surface due to boundary layer reattachment downstream of the compression corner, except for the case of the higher angle, which results in the largest peaks due to shock impingement. Different assumptions on the excitation of vibrational modes are shown to largely influence the size of the recirculation bubble in the compression corner, shock interaction mechanism, and surface loads. The more energy transferred to the vibrational mode, the lower post-shock temperatures are obtained, which tends to reduce the post-shock density, leading to weaker shock interactions characterized by delayed onsets of separation, reduced separation regions, and smaller standoff distances.

Effect of different P2O5/SnF2 ratios on the structure and properties of phosphate glass

Phosphate glass is usually used as a low-temperature sealing material due to its lower sealing temperature, lead-free, and environmentally friendly characteristics. In this experiment, xP2O5-(70-x) SnF2-15SnO-10WO3-5B2O3(x=40,35,30,25,20) glasses were prepared by traditional melting quenching method. The glass structure, properties and chemical stability were studied. According to the analysis of Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), the reduction of P−O−P broken by fluorine led to the transformation of glass structural unit Q2 to Q1 and Q0 and the reduction of glass average chain length. Sn2+ ions participated in the network structure as glass formers, which increased the size of the glass network. Since the large number of short chain groups in the glass network, it had lower characteristic temperature and larger thermal expansion coefficient, but also showed better chemical stability, which made it have broad application prospects in the field of low temperature sealing.

Comparative Analyses between Raw and Preoxidized Pulverized Coals: Combustion Behaviors and Thermokinetic and Microcharacteristics.

Investigating the difference in the combustion performance and microcharacteristics of oxidized and raw pulverized coal (PC) can contribute to effectively prevent and control the spontaneous combustion of deposited coal dust in high-temperature environment and further help guarantee the safe operation of coal-fired boiler. In this study, the combustion performance and thermokinetic and microcharacteristics of three raw coal samples and their preoxidized forms were explored by a thermogravimetric analyzer (TGA) and Fourier transform infrared spectroscopy (FTIR). According to the characteristic temperatures and variations of the mass loss rate during heating, the entire combustion process of PC was divided into four periods. For each type of coal, the preoxidized PC had relatively lower characteristic temperatures than the corresponding raw PC. The preoxidized samples had larger values of ignition index (Cig) and comprehensive combustibility index (S), but lower values of burnout index (Cb) than raw samples. The values of apparent activation energy (E) for the preoxidized PC were below that of the corresponding raw PC at the same conversion rate (α), which suggested the preoxidized PC required relatively less energy to react and was more prone to spontaneous combustion. In addition, although parts of −OH, C=O, and aliphatic hydrocarbon groups were consumed after the preoxidation treatment, the increase in C–O and −COO– bonds compensated for the adverse effect of the reduction of the aforementioned groups on coal combustion.

Effect of Acid Concentration on Thermal Stability of nitrocellulose (NC) for Civil Use

The effect of acid concentration in nitrocellulose (NC) on thermal stability of NC for civil use was studied using differential scanning calorimeter (DSC). The thermal explosion temperature (Tb) and kinetic parameters of NC thermal decomposition were calculated using different methods based on the thermal decomposition behaviors of NC in high pressure-gold plated stainless steel crucible at different heating rates. The results show that acid content in NC reduces the stability of NC remarkably, including reducing the characteristic decomposition temperature such as peak temperature (Tp), onset temperature (Tonset), initial temperature (T0) and critical temperature of thermal explosion (Tb). The big difference between NC with different acid content is that the relative quantity of activation energy and the change trend of activation energy. The most interesting thing is that NC with more acid content has the lower characteristic temperature but higher calculated activation energy.

Kondo scenario of the \u03b3\u2013\u03b1 phase transition in single crystalline cerium thin films

The physical mechanism driving the γ–α phase transition of face-centre-cubic (fcc) cerium (Ce) remains controversial until now. In this work, high-quality single crystalline fcc–Ce thin films were grown on Graphene/6H-SiC(0001) substrate, and explored by XRD and ARPES measurement. XRD spectra showed a clear γ–α phase transition at Tγ−α ≈ 50 K, which is retarded by strain effect from substrate comparing with Tγ−α (about 140 K) of the bulk Ce metal. However, APRES spectra did not show any signature of α-phase emerging in the surface-layer from 300 to 17 K, which implied that α-phase might form at the bulk-layer of our Ce thin films. Besides, an evident Kondo dip near Fermi energy was observed in the APRES spectrum at 80 K, indicting the formation of Kondo singlet states in γ–Ce. Furthermore, the DFT + DMFT calculations were performed to simulate the electronic structures and the theoretical spectral functions agreed well with the experimental ARPES spectra. In γ–Ce, the behavior of the self-energy’s imaginary part at low frequency not only confirmed that the Kondo singlet states emerged at TKS ≥ 80 K, but also implied that they became coherent states at a lower characteristic temperature (Tcoh ~40 K) due to the indirect RKKY interaction among f–f electrons. Besides, Tcoh from the theoretical simulation was close to Tγ−α from the XRD spectra. These issues suggested that the Kondo scenario might play an important role in the γ–α phase transition of cerium thin films.

Influence of pressure and CO2 in fluidized bed gasification of waste biomasses

An autothermal fluidized bed reactor was used to research the influence of pressure (0–2 barg) on the gasification process of different types of biomasses. The tested feedstocks were bark and lignin while softwood pellet was used as a reference fuel. A mixture of O2/CO2/H2O was used as a gasification agent. The impact of the application of CO2 on the yield of H2 in product gas was determined. Resulting product gas was characterized by a high content of CO which makes its use for applications based on chemical synthesis very difficult without extensive upgrading or supply of H2 from external sources. CO2 proved to improve carbon conversion efficiency (CCE) of the gasification process and to be an option for its chemical sequestration (negative carbon footprint). A slight modification of conventional indices used to evaluate efficiencies of gasification systems (CCE and water/carbon ratio) was proposed, to take into account the impact of the additional source of carbon fed into the reactor. The increase of system pressure led to changes in the composition of the product gas in line with predictions of Le Chatelier’s principle. The influence was predominantly visible in higher yields of CH4 and lower overall production of product gas. For higher hydrocarbons (CxHy), the trend was unclear. A set of stable gasification parameters were achieved for each pressure level and a standard gasification temperature of 850 °C, except for gasification of lignin performed at 2 barg. A proposed explanation for the problem is the combined effect of the increasing concentration of ash in the fluidized bed and its low characteristic melting temperatures. Due to the obtained experimental findings, a new ash agglomeration index was formulated.

Impact of lubricating base oil on diesel soot oxidation reactivity

In this study, the surface functional groups types and concentrations, graphitization degree, and oxidation reactivity of exhaust soot samples generated by a heavy-duty engine when employing neat diesel and four base oil dosed mixtures as the fuel was compared. Fourier transform infrared (FTIR), Raman spectroscopy and Thermal gravimetric analyzer (TGA) were used to characterize particle physicochemical properties. Four lubricating base oils were blended in diesel at a mass ratio of 0.5% and 1.0% for combustion. It was found that although the chemical compositions of these four base oils were different, the aliphatic CH functional groups content, graphitization degree and oxidation reactivity of exhaust soot exhibited similar trends. The combustion of base oil was found to change the graphitization degree of emitted soot and lead to soot with a more disordered nanostructure. The soot particles derived from base oil blended fuels had higher aliphatic CH groups concentration than those from neat diesel. Soot oxidation reactivity analyses showed that soot generated from base oil blended fuels had lower characteristic temperatures and lower activation energy than that from neat diesel. Besides, there was a clear correlation between the soot graphitization degree, relative concentration of aliphatic CH groups and its oxidation reactivity.

Phonon-induced giant linear-in- T resistivity in magic angle twisted bilayer graphene: Ordinary strangeness and exotic superconductivity

We study the effect of electron-acoustic phonon interactions in twisted bilayer graphene on resistivity in the high-temperature transport and superconductivity in the low-temperature phase diagram. We theoretically show that twisted bilayer graphene should have an enhanced and strongly twist-angle dependent linear-in-temperature resistivity in the metallic regime with the resistivity magnitude increasing as the twist angle approaches the magic angle. The slope of the resistivity versus temperature could approach one hundred ohms per kelvin with a strong angle dependence, but with a rather weak dependence on the carrier density. This higher-temperature density-independent linear-in-$T$ resistivity crosses over to a $T^4$ dependence at a low density-dependent characteristic temperature, becoming unimportant at low temperatures. This angle-tuned resistivity enhancement arises from the huge increase in the effective electron-acoustic phonon coupling in the system due to the suppression of graphene Fermi velocity induced by the flatband condition in the moir\'e superlattice system. Our calculated temperature dependence is reminiscent of the so-called `strange metal' transport behavior except that it is arising from the ordinary electron-phonon coupling in a rather unusual parameter space due to the generic moir\'e flatband structure of twisted bilayer graphene. We also show that the same enhanced electron-acoustic phonon coupling also mediates effective attractive interactions in $s$, $p$, $d$ and $f$ pairing channels with a theoretical superconducting transition temperature on the order of $\sim$5 K near magic angle. The fact that ordinary acoustic phonons can produce exotic non-$s$-wave superconducting pairing arises from the unusual symmetries of the system.

Phosphotellurite glass and glass-ceramics with high TeO2 contents: thermal, structural and optical properties.

Phosphotellurite based glasses have interesting features such as low characteristic temperatures, high glass forming ability, high thermal stability against crystallization and a broad transparency window from ultraviolet (UV) to near-infrared (NIR), which makes them promising materials for photonic applications. In this work, phosphotellurite binary glasses, having a composition (100 - x)TeO2 - xBa(PO3)2 with x varying from 1 to 20 mol%, were synthesized by the conventional melt-quenching method in covered gold crucibles under air. Optical, physical and structural properties of the new glass samples were investigated by differential scanning calorimetry, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, linear optical absorption from UV to NIR, IR transmittance, and optical limiting experiments. Transparent glass-ceramics in the visible range were obtained for phosphotellurite samples containing 2, 4 and 6 mol% of Ba(PO3)2 and the phase crystallization was investigated through Rietveld analysis and transmission electron microscopy. The incorporation of Ba(PO3)2 into the TeO2 network drastically increases the thermal stability against devitrification and helps to shift the infrared multiphonon absorption edge to longer wavelengths. Nonlinear measurements performed with a picosecond laser at 532 nm indicate large effective nonlinear absorption coefficients for all samples. In summary, the dependence of the spectroscopic properties on the compositions of the samples revealed promising transparent glass and glass-ceramics for photonic applications.

Thermokinetic Behavior and Microcharacterization during the Spontaneous Combustion of 1/3 Coking Coal

ABSTRACTThermal analysis was used to investigate the oxidation and heat-release characteristics of four coal samples from the Huainan mining area in China. Temperature and the evolution of heat and gaseous products were studied to identify each sample’s ignition temperature and inform fire-prevention advice to avoid the spontaneous combustion of the coals studied. Thermogravimetry–differential scanning calorimetry (TG–DSC) tests demonstrated lower heating rates resulted in lower characteristic temperatures, corresponding to a greater propensity for spontaneous combustion. Using the Flynn–Wall–Ozawa method, spontaneous combustion was predicted from approximately 300°C, which agreed well with the TG–DSC analysis. The apparent activation energies for combustion were also obtained and gas products that were released throughout the entire oxidation process were monitored. Simultaneous thermal analysis–Fourier transform infrared spectroscopy was used to explain the changes in the evolution of CO and CO2, the results of which agreed well with those of TG.

Analysis of Photoluminescence Thermal Quenching: Guidance for the Design of Highly Effective p-type Doping of Nitrides.

A contact-free diagnostic technique for examining position of the impurity energy level of p-type dopants in nitride semiconductors was proposed based on photoluminescence thermal quenching. The Mg ionization energy was extracted by the phenomenological rate-equation model we developed. The diagnostic technique and analysis model reported here are priorities for the design of highly effective p-doping of nitrides and could also be used to explain the abnormal and seldom analyzed low characteristic temperature T0 (about 100 K) of thermal quenching in p-type nitrides systems. An In-Mg co-doped GaN system is given as an example to prove the validity of our methods. Furthermore, a hole concentration as high as 1.94 × 1018 cm−3 was achieved through In-Mg co-doping, which is nearly one order of magnitude higher than typically obtained in our lab.

Development in Phosphor Glass Ceramic Composites

The Bi doped borosilicate glass systems with low characteristic temperature and high refractive index were fabricated. Through microstructure control by Bi and Zn doping, a low softening point around 476oC of the glass was achieved. The YAG phosphor was successfully embedded and dispersed into the softened glass substrate under heat treatment. The emission light of GCP sample can be controlled from blue to white and yellow under LED blue chip package through heat treatment.

Scattering assisted injection based injectorless mid infrared quantum cascade laser

An injectorless five-well mid infrared quantum cascade laser is analyzed which relies on phonon scattering injection in contrast to resonant tunneling injection, which has been previously used for injectorless designs. A Monte Carlo based self-consistent electron and photon transport simulator is used to analyze the performance of the analyzed design and compare it to existing injectorless designs. The simulation results show that the analyzed design could greatly enhance the optical gain and the characteristic temperatures of injectorless quantum cascade lasers (QCLs) which have typically been hindered by low characteristic temperatures and significant temperature related performance degradation. Simulations of the analyzed device predict threshold current densities of 0.85 kA/cm2 and 1.95 kA/cm2 at 77 K and 300 K, respectively, which are comparable to the threshold current densities of conventional injector based QCLs.

Development in Phosphor Glass Ceramics Composites

Glass ceramics containing phosphor, GCP, has become an advanced material in high power LED lighting application recently due to its superior optical and thermal performance and elongated lifetime. Glass systems which have lower characteristic temperatures , glass transition temperature(Tg) and glass softening point(Td)…, as well as excellent mechanical strength and chemical stability, are proposed in microelectronics technology, infrared laser technology and sealing of materials. Borosilicate glass systems exhibit lower characteristic temperatures and quality optical properties. By controlling functional elements, further enhancement in thermal, mechanical, and optical properties can deduced. In this study, the YAG phosphor powders embed into designed glasses under controlled atmosphere to from glass ceramics phosphor composites. The thermal, optical properties and structure of borosilicate glass host are discussed. Heat treatment which affects the YAG phosphor incorporation into the glass is also considered. The glass transition temperatures(Tg) and softening points(Td) are 447~483oC, and 476~524oC respectively. A high refractive index of 1.85~1.94 can be reached. The results show that the developed borosilicate glass hosts are suitable for GCP matrix to contain phosphor powders. The YAG phosphor powders are well contained and dispersed in the glasses.Key words: LED, Glass ceramics phosphor composites(GCPC), Low characteristic temperature, Heat treatment

Quasi‐One Dimensional in‐Plane Conductivity in Filamentary Films of PEDOT:PSS

AbstractThe mechanism and magnitude of the in‐plane conductivity of poly(3,4‐ethy‐lenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films is determined using temperature dependent conductivity measurements for various PEDOT:PSS weight ratios with and without a high boiling solvent (HBS). Without the HBS the in‐plane conductivity of PEDOT:PSS is lower and for all studied weight ratios well described by the relation $ \sigma = \sigma _0 {\rm exp}[- \left({{{{T_0}}\over{T}}} \right)^{0.5}] $ with T0 a characteristic temperature. The exponent 0.5 indicates quasi‐one dimensional (quasi‐1D) variable range hopping (VRH). The conductivity prefactor σ0 varies over three orders of magnitudes and follows a power law σ0∝c3.5PEDOT with cPEDOT the weight fraction of PEDOT in PEDOT:PSS. The field dependent conductivity is consistent with quasi‐1D VRH. Combined, these observations suggest that conductance takes place via a percolating network of quasi‐1D filaments. Using transmission electron microscopy (TEM) filamentary structures are observed in vitrified dispersions and dried films. For PEDOT:PSS films with HBS, the conductivity also exhibits quasi‐1D VRH behavior when the temperature is less than 200 K. The low characteristic temperature T0 indicates that HBS‐treated films are close to the critical regime between a metal and an insulator. In this case, the conductivity prefactor scales linearly with cPEDOT, indicating the conduction is no longer limited by a percolation of filaments. The lack of observable changes in TEM upon processing with the HBS suggests that the changes in conductivity are due to a smaller spread in the conductivities of individual filaments, or a higher probability for neighboring filaments to be connected rather than being caused by major morphological modification of the material.

Thermoelastic and structural properties of ionically conducting cerate perovskites: (I) BaCeO3 at low temperature in the Pbnm phase

The thermoelastic and structural properties of BaCeO3 perovskite in the Pbnm phase field have been studied using high resolution neutron diffractometry at 37 temperatures between 2K and 350K. From a simultaneous fit of the isochoric heat capacity and unit cell volume, the vibrational density of states in the Pbnm phase is shown to be approximated by a two-term modified Debye model with characteristic temperatures 176(1)K and 543(2)K. Vibrational Debye temperatures, determined from the temperature-variation of the atomic displacement parameters, suggest the cations to be more associated with the lower characteristic temperature, whilst those for the anions, are closer to the higher characteristic temperature. The weighted average of the Grüneisen constants associated with the two characteristic temperatures is in good agreement with the experimentally determined thermodynamic Grüneisen parameter. Structural parameters are presented as the amplitudes of the seven symmetry-adapted basis-vectors of the aristotype phase, and a structural basis for the temperature-dependence of the bond lengths is outlined. The critical exponent associated with the temperature variation of the primary order parameter in the phase transition from Pbnm to Ibnn at 563K is consistent with this transition being tricritical in nature.

Low temperature thermoelastic and structural properties of LaGaO3 perovskite in the Pbnm phase

The thermoelastic and structural properties of LaGaO3 perovskite have been studied using high resolution neutron diffractometry at 158 temperatures between 11K and 548K. Data collected in 2K intervals between 410K and 430K show no evidence for an intermediate phase between the low temperature phase in space group Pbnm and the high temperature phase in space group F3¯2/n. From a simultaneous fit of the unit cell volume and the isochoric heat capacity, the phonon density of states in the Pbnm phase is shown to be approximated by a two-term Debye model, with characteristic temperatures 294(1)K and 831(1)K. Vibrational Debye temperatures, determined from fitting the temperature variation of the atomic displacement parameters, show the cations to be more associated with the lower characteristic temperature, whilst that for the anions, is closer to the higher characteristic temperature. Structural parameters are presented as the amplitudes of the seven symmetry-adapted basis-vectors of the aristotype phase, and a structural basis for the temperature-dependence of the bond lengths is outlined. The phase transition in both temperature and pressure arises when a non-bonded La–O distance approaches the La coordination sphere.