Decreasing Measurement Temperature Research Articles

Низкотемпературные мёссбауэровские исследования замещенного феррита стронция La-=SUB=-0.67-=/SUB=-Sr-=SUB=-0.33-=/SUB=-FeO-=SUB=-3-γ-=/SUB=-

The valence states of Fe atoms and the formation of oxygen vacancies in substituted La0.67Sr0.33FeO3−γ orthoferrite have been studied in detail by low-temperature Mössbauer spectroscopy under oxygen removal. It has been shown that the averaged valence state of Fe atoms is not revealed with a decreasing measurement temperature. This makes it possible to reveal Fe4+ ions. The Analysis of the obtained data allows us to conclude that the presence of several Zeeman sextets associated with Fe3+ ions is related to the appearance of oxygen vacancies and Fe4+ ions in the nearest ionic surrounding of Fe ions. Using the Mössbauer data, the number of oxygen vacancies and oxygen ions has been determined for all the studied samples depending on the vacuum annealing temperature.

Low-temperature Mossbauer studies of substituted La-=SUB=-0.67-=/SUB=-Sr-=SUB=-0.33-=/SUB=-FeO-=SUB=-3-γ-=/SUB=- lanthanum ferrite

The valence states of Fe atoms and the formation of oxygen vacancies in substituted La0.67Sr0.33FeO3-γ orthoferrite have been studied in detail by low-temperature Mossbauer spectroscopy under oxygen removal. It has been shown that the averaged valence state of Fe atoms is not revealed with a decreasing measurement temperature. This makes it possible to reveal Fe4+ ions. The Analysis of the obtained data allows us to conclude that the presence of several Zeeman sextets associated with Fe3+ ions is related to the appearance of oxygen vacancies and Fe4+ ions in the nearest ionic surrounding of Fe ions. Using the Mossbauer data, the number of oxygen vacancies and oxygen ions has been determined for all the studied samples depending on the vacuum annealing temperature. Keywords: perovskites, substituted lanthanum ferrites, X-ray diffraction analysis, Mossbauer spectroscopy, Fe valence states, oxygen vacancies.

Temperature dependent magnetic studies of Ba0.5Sr0.5Co0.2Fe11.8O19 prepared by solid state reaction method

Ba0.5Sr0.5Co0.2Fe11.8O19 was synthesized using solid state reaction method. Lattice parameters ‘a’ and ‘c’ were found to be 5.886 Å and 23.110 Å respectively. Scanning electron microscopy studies showed platelet type structures in the sample and the elemental mapping showed the uniform distribution of cations throughout the sample. The spontaneous magnetization (Ms) of 62 emu/g with coercivity (Hc) of 1.22 kOe was observed at 300K. The Ms value increased whereas the Hc decreased with the decreasing measurement temperature. The highest Ms value of 88 emu/g was observed at 60K. Curie temperature of 819 K was observed for the sample. The anisotropy constant (K1) and anisotropy field (Ha) were found to be increased with the decrease in measurement temperature. The observed magnetic behaviour can be explained on the basis of distribution of cations in the lattice sites and shape anisotropy of the prepared sample.

Study of Interaction Mechanism between Positrons and Ag Clusters in Dilute Al-Ag Alloys at Low Temperature.

The microstructural evolution of dilute Al–Ag alloys in its early aging stage and at low temperatures ranging from 15 K to 300 K was studied by the combined use of Positron annihilation lifetime spectroscopy (PALS), high resolution transmission electron microscopy (HRTEM), and positron annihilation Coincidence Doppler broadening (CDB) techniques. It is shown that at low temperatures below 200 K, an Ag–vacancy complex is formed in the quenched alloy, and above 200 K, it decomposes into Ag clusters and monovacancies. Experimental and calculation results indicate that Ag clusters in Al–Ag alloys can act as shallow trapping sites, and the positron trapping rate is considerably enhanced by a decreasing measurement temperature.

Viscosity of Freeze-Concentrated Solution Confined in Micro/Nanospace Surrounded by Ice

An aqueous solution separates into ice and a freeze-concentrated solution (FCS) when frozen at temperatures above the eutectic point. The FCS acts as important reaction media in natural environment and industrial processes. The viscosities of the FCS in frozen glycerol/water solutions are evaluated by two spectrometric methods with different principles: (1) the reaction rate of the diffusion-controlled emission quenching and (2) fluorescence correlation spectroscopy. Thermodynamics indicates that the concentration of glycerol in the FCS is constant at a constant temperature regardless of the glycerol concentration in the original solution before freezing (cglyini). However, the viscosity of the FCS measured at a given temperature increases with decreasing cglyini, and this trend becomes more pronounced with decreasing measurement temperature. Further, the viscosity of the FCS in a rapidly frozen solution is higher than that in a slowly frozen solution. These results suggest that the viscosity of the FCS d...

Electrically modulated lateral photovoltage in μc-SiOx:H/a-Si:H/c-Si p-i-n structure at low temperatures

The lateral photovoltaic effect (LPE) in μc-SiOx:H/a-Si:H/c-Si p-i-n structure is studied at different temperatures under illumination of different lasers ranging from visible to infrared. The LPE improves with increasing laser wavelength with the position sensitivity nearly linearly dependent on the wavelength in the whole temperature range. Though the position sensitivity decreased gradually with decreasing measurement temperature from 296K to 80K, the nonlinearity improved a little, which can be ascribed to the decreased resistivity of conductive layer. Considering that the LPE was mainly determined by the Schottky barrier (SB), an external bias voltage was added to enhance the built-in field, it was found that the LPE improved dramatically with position sensitivity linearly proportional to the laser power. More importantly, both the position sensitivity and the nonlinearity were independent of temperature again in the whole laser power range. Our research sheds new light on the bias voltage-modulated LPE at low temperatures.

Effect of the degree of doping on the size and magnetic properties of nanocrystals La1 – x Zn x FeO3 synthesized by the sol–gel method

Nanopowders La1–x Zn x FeO3 (nominal degree of doping x nom = 0, 0.05, 0.075, 0.1, 0.15, 0.2, 0.3, and 0.4) were synthesized by the sol–gel method using aqueous ammonia as a precipitator and were then annealed at 950°C for 60 min. From the data of X-ray powder diffraction analysis and local electron probe microanalysis, the maximum actual limit of doping of lanthanum ferrite with zinc was determined: x = 0.072. The dependence of the particle size on the Zn2+ content was found to be nonmonotonic. The magnetic characteristics (specific magnetization J, coercivity H c, and magnetic susceptibility χ) of samples at temperatures of 300 and 100 K in fields of up to 1300 kA/m. It was shown that, with increasing degree of doping, J increases from 0.188 A m2/kg (at x = 0) to 0.245 A m2/kg (at x = 0.072), and χ varies nonmonotonically from 11.5 × 10–6 (at x = 0) to 15.3 × 10–6 (at x = 0.072) (at 300 K). With decreasing measurement temperature to 100 K, the magnetization and susceptibility monotonically increase.

Characteristic diode parameters in thermally annealed Ni/p-InP contacts

The Ni/p-InP Schottky diodes (SDs) have been prepared by DC magnetron sputtering deposition. After the diode fabrication, they have been thermally annealed at 700 °C for 1 min in N2 atmosphere. Then, the current–voltage characteristics of the annealed and non-annealed (as-deposited) SDs have been measured in the measurement temperature range of 60–400 K with steps of 20 K under dark conditions. After 700 °C annealing, an improvement in the ideality factor value has been observed from 60 to 200 K and the barrier height (BH) value approximately has remained unchanged in the measurement temperature range of 200–400 K. The BH of the annealed diode has decreased obeying the double-Gaussian distribution (GD) of the BHs with decreasing measurement temperature from 200 to 60 K. The BH for the as-deposited diode has decreased with decreasing temperature obeying the single-GD over the whole measurement temperature range. An effective Richardson constant value of 54.21 A/cm2K2 for the as-deposited SD has been obtained from the modified Richardson plot by the single-GD plot, which is in very close agreement with the value of 60 A/K2cm2 for p-type InP. The series resistance value of the annealed SD is lower than that of the non-annealed SD at each temperature and approximately has remained unchanged from 140 to 240 K. Thus, it can be said that an improvement in the diode parameters has been observed due to the thermal annealing at 700 °C for 1 min in N2 atmosphere.

The influence of the heating temperature on the yield stress and pour point of waxy crude oils

Abstract The yield stress and pour point are two important parameters related to flow assurance problems in waxy crude oil pipelines. The yield stresses at temperatures below the pour points of five waxy crude oils after heating to different temperatures have been investigated in the present paper. Both the experimental results in this work and data from the literature show that the yield stress values increase exponentially with the decreasing measurement temperature. The experimental results show that the heating temperature has an obvious influence on the yield stress and the pour point. However, it was noted that the yield stress values at the gel point temperature, defined as the highest temperature at which the movement of the sample cannot be observed during the quiescent pour point testing process, are approximately the same for a waxy crude oil after heating to different temperatures. An exponential relationship exists between the yield stress and the difference between the measurement temperature and gel point that is independent of the heating temperature. To explore this phenomenon, the relative magnitudes of the gravitational stress and the structural strength of the wax crystal network during the gel point test has been analyzed, and it was found that the yield stress value at the gel point has a favorable linear relationship with the oil density.

Estimate of the cation distribution for Ti-doped Ni0.68Fe2.32O4spinel ferrites by fitting magnetic moments at 10 K

Ti-doped Ni0.68Fe2.32O4 spinel ferrite samples with nominal composition Ni0.68−0.8xTixFe2.32−0.2xO4 (0 ≤ x ≤ 0.312) were prepared using conventional ceramic methods. The samples exhibited a single-phase cubic spinel structure. When the doping level had values x > 0.15, a transition temperature, TN, was found, below which the magnetization of the samples decreased with decreasing measurement temperature. This phenomenon indicates that an additional antiferromagnetic structure arises in the traditional spinel phase of ferrites resulted from Ti doping. Therefore, Ti ions appear in the form of Ti3+ and Ti2+ cations that have magnetic moments, rather than as Ti4+ cations without magnetic moment as assumed by many researchers. The dependence of the magnetic moments on the Ti doping level x at 10 K was fitted successfully using our quantum-mechanical potential barrier model proposed earlier. In the fitting process, we assumed that the magnetic moments of the Ti cations were opposite to the direction of the Fe and Ni moments. According to the fitted results, the Ti2+ cations at the [B] sites constitute about 81% of the total Ti content x for all samples, which is close to the content of the Ni2+ cations at the [B] sites (77%).

Magnetic moments of Ti cations in Ti-doped Ni0.68Fe2.32O4 spinel ferrites

Two series of Ti doped Ni0.68Fe2.32O4 ferrite samples with a single cubic spinel phase were prepared using conventional ceramic methods. A transition temperature, TN, was found when the Ti doping level was x>0.15, below which temperature the magnetization of the samples abruptly decreased with decreasing measurement temperature rather than rising or remaining nearly constant as in samples with less doping. It was found that TN increased with increasing applied magnetic field. These phenomena indicated that with Ti doping an additional antiferromagnetic structure arises in the traditional spinel phase of ferrites and therefore that Ti ions have magnetic moments rather than as Ti4+ cations which has zero moment.

Quantitative measurement of carbon content in Fe–C binary alloys by atom probe tomography

The carbon content in high-purity Fe–C binary alloys was measured by 3-D atom probe microscopy. The apparent carbon content increased with decreasing measurement temperature and greatly exceeded the nominal value. This anomaly is caused by the loss of some Fe ions during detection, resulting in an overestimation of the carbon content. A simple method was proposed to correct for this effect. The corrected carbon content agreed well with the nominal value to within 0.5 at.% in most cases.

Electrical properties of polycrystalline GaInAs thin films

Polycrystalline Ga x In 1 − x As films with x ranging from 0 to 1 were deposited on glass substrates by molecular-beam deposition at 240 or 350 °C. Room temperature Hall-effect measurements showed that the Ga x In 1 − x As films deposited at either temperature exhibit high electron concentrations in the range of 10 18 cm − 3 for x ≤ 0.21 while the electron concentration decreases with increasing Ga content for x ≥ 0.29 to be < 10 15 cm − 3 at x = 0.64. Even at the low deposition temperature of 240 °C, the electron mobility remains > 400 cm 2/(V s) at x ~ 0.2 and then decreases with Ga content to be ~ 40 cm 2/(V s) at x = 0.64. Temperature-varying Hall-effect measurements in the range of 100–390 K revealed that both the electron concentration and mobility of the samples with x ≤ 0.21 are almost independent of the measurement temperature, while those of the samples with x ≥ 0.30 decrease with decreasing measurement temperature. The concentrations and ionization energies of donor levels were deduced from the temperature dependence of the electron concentration with the non-parabolicity of the conduction band taken into account. The temperature dependences of electron mobility in the samples with x ≥ 0.30 are well explained in terms of thermionic electron emission across the grain-boundary barriers assuming fluctuation in potential barrier height, while the almost temperature-independent high electron mobilities in the samples with x ≤ 0.21 are attributed to the absence of potential barrier at the grain boundaries.

High-Temperature Characteristics of Ti/Al/Ni/Au Ohmic Contacts to n-GaN

We present the high-temperature characteristics of Ti/Al/Ni/Au(15 nm/220 nm/40 nm/50nm) multiplayer contacts to n-type GaN (Nd = 3.7 × 1017 cm−3, Nd = 3.0 × 1018 cmT−3). The contact resistivity increases with the measurement temperature. Furthermore, the increasing tendency is related to doping concentration. The higher the doped, the slower the contact resistivity with decreasing measurement temperature. Ti/Al/Ni/Au ohmic contact to heavy doping n-GaN takes on better high temperature reliability. According to the analyses of XRD and AES for the n-GaN/Ti/Al/Ni/Au, the Au atoms permeate through the Ni layer which is not thick enough into the Al layer even the Ti layer.

Evidence of a second acceptor state of theEcenter inSi1−xGex

We have found evidence of a second acceptor state of the $E$ center in ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ by using positron annihilation spectroscopy. To achieve this, we studied proton irradiated $n$-type ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ with a Ge content of 10%--30% and a P dopant concentration of ${10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, in which the number of Ge atoms around irradiation induced $E$ centers was increased by annealing. When measuring the Doppler broadening of the annihilation line, the shape parameter $S$ starts to decreases at $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with decreasing measurement temperature. This indicates that a charge transition in the upper half of the ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ band gap, above the well known (0/\ensuremath{-}) level, takes place. Hence, we suggest that the increased concentration of germanium around the $E$ center pulls down the localized second acceptor state into the ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ band gap, making the Ge decorated $E$ center a more effective trap for conduction electrons.

Rectifying characteristics of La1－xSrxMnO3/TiO2(x=0.2, 0.15, 0.04) pn heterojunctions

La1-xSrxMnO3(LSMO)/TiO2 pn heterojunctions have been successfully synthesized by RF magnetron sputtering and they exhibit good rectifying properties. The diffusion potentials increase with increasing Sr concentration, which may be attributed to the increased carrier concentration. And the diffusion potential increases with decreasing measurement temperature, which may have resulted from modification of the interface electronic structure of La1-xSrxMnO3/TiO2 pn heterojunction with changing temperatures. It is worth noting that the metal-insulator (M-I) transition of LSMO can also be observed in pn heterojunctions and the increased sheet-resistance of pn heterojunctions at low temperature is related to the introduction of TiO2.

Controlled Growth and Properties of One‐Dimensional ZnO Nanostructures with Ce as Activator/Dopant

AbstractA large amount of one‐dimensional (1D) Ce‐doped ZnO nanostructures with different morphologies has been successfully synthesized by annealing a polymeric precursor at various temperatures. The evolution of the morphologies and microstructures was investigated by field‐emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), and high‐resolution TEM (HRTEM). The results show that the morphologies vary drastically with increasing synthesis temperature and the photoluminescence (PL) of the products depends on both the synthesis and measurement temperatures. The CeO layer forms first and becomes a catalytic center for the ZnO growth. At a synthesis temperature lower than the boiling point of Zn, Zn and O atoms can stack epitaxially along the CeO catalytic layer and form a bicrystal nanobelt‐like structure with a trapezoid‐like end and a concave growth fault center. At a synthesis temperature higher than the boiling point of Zn, however, nanowires with an incommensurately modulated superstructure are obtained due to the high reaction rate and the formation of a periodic separation of the CeO layer. As for the room‐temperature PL of ZnO, the incorporation of donor Ce leads to the disappearance of the green band and the appearance of a purplish‐blue emission peak, whose position shifts towards the red and whose intensity decreases with increasing synthesis temperature. Analysis of this temperature‐dependent luminescence indicates that the purplish‐blue emission of nanobelts prepared at 850 °C originates from a donor‐bound exciton emission, and, contrary to the nanowires, it undergoes a change from an emission of the electron–hole plasma (EHP) to an emission of the donor‐bound exciton with decreasing measurement temperature.

Source of anomalies in the temperature dependence of ferromagnetic resonance and susceptibility data in the FeAg heterosystem

Nanometer size Fe particles in an Ag matrix were prepared by pulsed laser deposition and were characterized from 4 to 300 K using ferromagnetic resonance (FMR) at 9.46 GHz and superconducting quantum interference device magnetometry. The magnetic behavior changes systematically from superparamagnetic to ferromagnetic with increasing Fe concentration, increasing deposition or postdeposition annealing temperature, or decreasing measurement temperature; FMR linewidths and anisotropy fields also increase. At low temperatures a dramatic further increase in linewidth occurs, accompanied by a strong downward shift in the resonance field for all orientations, and zero field cooled susceptibility decreases sharply. These phenomena indicate the development of much larger anisotropy than in bulk Fe. For samples annealed in H gas above 600 K, this anisotropy is largely absent, indicating that the anomalous behavior is induced by a surface oxide layer on the Fe, as in the exchange bias effect, despite the absence of oxygen in the nominal composition.

Synthesis of Fe3O4 nanoparticles from emulsions

A unique oil-in-water emulsion route has been devised to synthesize nanosized magnetite (Fe3O4) particles using a small amount of cyclohexane as the oil phase, NP5 + NP9 as the surfactant phase, and a Fe(II)/Fe(III) salt solution as the aqueous phase. The Fe3O4 powder thus derived from the emulsion containing 88 wt% 0.3 M FeSO4 + Fe(NO3)3 in the aqueous phase possesses an equiaxial morphology and an average particle size of <10 nm. Studies on the electrical conductivity of the emulsions as a function of Fe2+/Fe3+ concentration in the aqueous phase revealed the complexation effect of the NP5 + NP9 surfactant towards Fe2+/Fe3+ ions. The resulting particle size and morphology are therefore dependent on the amounts of surfactant and aqueous phase in the emulsion. A strongly alkaline aqueous phase favors the formation of Fe3O4, while a low alkalinity favors the formation of α-FeOOH. Magnetic property measurements and Mossbauer spectroscopic studies indicate that the emulsion-derived iron oxide powder is superparamagnetic, which becomes ferrimagnetic with decreasing measurement temperature.

Temperature dependence of the photoelectron emission from intentionally oxidized copper

The characteristics of the photoelectron emission (PE), measured by varying the wavelength of incident light, form copper subjected to oxidation treatment in air have been investigated as a function of the measurement temperature between 25 and 300°C using a gas-flow counter. The intensity of PE at a certain measurement temperature decreased with increasing oxidation temperature (100 to 800°C) and the intensity of PE for a given oxidation temperature increased with decreasing measurement temperature. The PE behavior was correlated to the chemical and electrical nature of the surfaces analyzed by X-ray photoelectron spectroscopy and surface potential measurement. A mechanism is proposed for the PE of the untreated sample which exhibited a completely different PE behavior.