Superconductivity of Bi Confined in an Opal Host

Superconductivity is observed in a composite of rhombohedral crystalline bismuth nanoparticles imbedded in an insulating porous opal host via electrical transport and AC magnetic susceptibility. The onset of superconductivity in this system occurs in two steps, with upper transition temperature T _c,U =4.1 K and lower transition temperature of T _c,L =0.7 K, which we attribute to the granular nature of the composite. The transition at T _c,U is observed to split into two transitions with the application of a magnetic field, and these have upper critical fields extrapolated to T=0 K of H _c2,1(0)=0.7 T and H _c2,2(0)=1.0 T, corresponding to coherence lengths of ξ ₁(0)=21 nm and ξ ₂(0)=18 nm, respectively. We suggest that because of the lack of bulk-like states in the Bi nanoparticles due to confinement effects, superconductivity originates from surface states arising from Rashba spin-orbit scattering at the interface.     

Magneto-conductivity Analysis for GdBa2Cu3O7−δ Added with Nanosized Ferrite CoFe2O4

Superconducting samples of type (CoFe₂O₄) _x GdBa₂Cu₃O_7?δ , 0.0≤x≤0.1 wt.%, have been prepared by the conventional solid-state reaction technique and characterized using X-ray powder diffraction (XRD) and the scanning electron microscope (SEM). XRD analysis indicates that the orthorhombic structure of Gd-123 is not affected by the nanosized ferrite CoFe₂O₄ addition, whereas the volume fraction of Gd-123 increases with x up to 0.01 wt.%. Nanosized ferrite CoFe₂O₄ has been prepared by Co-precipitation method with grain size about 8 nm. The temperature dependence of electrical resistivity has been measured in zero and 0.44 T magnetic fields. Magneto-conductivity data has been analyzed in terms of Aslamazov–Larkin (AL) and Maki–Thompson (MT) models for layered superconductors, considering the orbital contribution. The superconducting parameters such as the coherence lengths along ab plane ξ _ab (0) and along c-direction ξ _c (0) at 0 K, anisotropic parameter Γ and phase breaking time τ _φ at 100 K have been determined as a function of nanosized ferrite CoFe₂O₄ contents. It is found that the low nanosized CoFe₂O₄ addition contents up to x=0.01 wt.% improves the physical properties of Gd-123, while for x>0.01 wt.% these properties are deteriorated.     

Magnetization Fluctuation Analysis and Superconducting Parameters of La1.5−x Ba1.5+x−y Ca y Cu3O z Superconductor

In this work we report analysis of experimental data of the magnetization of La_1.5?x Ba_1.5+x?y Ca _y Cu₃O _z superconducting system. The data are analyzed in terms of thermal fluctuations on the magnetization excess M(T) for different values of temperature in each of the samples. We describe a procedure for extracting the penetration depth in the ab plane (1537–1650 Å) and the coherence length in the ab plane (21–23 Å) parameters from the magnetization, as a function of the applied magnetic field. This procedure was performed for polycrystalline samples using the theory of Bulaevskii, Ledvij and Kogan, which analyzes the vortex fluctuation in superconducting materials within the Lawrence-Doniach framework. These data allowed one to determine the characteristic temperature value T ^? (53–73 K) in the magnetization curves for several magnetic fields. We calculated the data of magnetization excess from the curves of the magnetization as a function of the logarithm of the applied field. We notice that the values for these superconducting parameters are in agreement with reports for high-temperature superconductors. The value obtained of the superconducting volumetric fraction is compared with the value obtained through the measure of the Meissner effect.     

Correlation Between Ferromagnetism and Superconductivity at Interfaces of La2/3Ca1/3MnO3/YBa2Cu3O7−δ /La2/3Ca1/3MnO3 Trilayers Grown by dc Sputtering

In this work, we study the magnetoelectrical properties of La_2/3Ca_1/3MnO₃/YBa₂Cu₃O_7?δ /La_2/3Ca_1/3MnO₃ heterostructures grown by means of high oxygen-pressure dc sputtering onto (001)-oriented SrTiO₃ substrates. The test heterostructures are composed of ferromagnetic layers of constant thicknesses (～280 unit cells, ～110 nm) and superconducting interlayers with thicknesses ranging between 5 (～6 nm) and 15 unit cells (～17 nm). Transport measurements show a strong suppression of the superconducting properties when the thickness of the YBa₂Cu₃O_7?δ interlayer is reduced to a value below 10 unit cells. Magnetic measurements show superconducting transition in junctions with YBa₂Cu₃O_7?δ interlayers with thicknesses larger than ～15 unit cells. The observed difference in the values of the superconductivity onset when determined by electrical and magnetic measurements might be related with the presence of a spontaneous vortex phase in the temperature range around the superconducting transition.     

Resistive transition of superconducting wire networks. Influence of pinning and fluctuations

We have studied the resistive transition of several 2-D superconducting wire networks of various coupling strengths, which we characterize in terms of the Kosterlitz-Thouless transition temperature and the ratio /a of the coherence length to the array period. In the extreme strong coupling limit where the mesh size is of the order of the zero-temperature coherence length, the superconducting behavior is well described by the mean-field properties of the superconducting wave function. Extending to 2-D array, the 1-D phase slippage model explains the dissipative regime observed above the Ginzburg-Landau depairing critical current. On the other hand, when the coupling is weak, phase fluctuations below the Ginzburg-Landau transition and vortex depinning dominate the resistive behavior. An activated dissipation is observed even below the depairing critical current. Results obtained in this regime for critical temperature, magnetoresistance or critical current versus temperature, and magnetic field are shown; their periodic oscillations are discussed in terms of depinning of vortices on the array. A simple periodic pinning potential for a vortex in a wire network is calculated, and compared with the case of pinning in Josephson junction arrays. We show that this model explains qualitatively the experimental results observed for small /a.     

The role of Lu doping on microstructural and superconducting properties of Bi2Sr2CaLuxCu2Oy superconducting system

Lutetium (Lu) added Bi₂Sr₂CaLu_xCu₂O_y superconducting samples with x = 0, 0.1, 0.3, 0.5, 0.7 and 1.0 are prepared by solid-state reaction method and annealed at 840 °C for 50 h. The heating and cooling rates of the furnace are adjusted to be 10 and 3 °C/min, respectively. For the comparison, undoped sample is subjected to the same annealing conditions. The prepared samples are characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and dc resistivity (ρ?T) measurements. The volume fraction, grain size, texturing and lattice parameters are determined from the XRD measurements. The microstructure, surface morphology and element composition analysis of the samples produced are investigated by SEM and EDS measurements, respectively. Moreover, the resistivity (at room temperature), critical transition (onset and offset) temperature, variation of transition temperature and hole carrier concentration values of the samples studied are estimated from the dc resistivity measurements. According to the results obtained, the samples prepared exhibit the polycrystalline superconducting phase with less intensity of diffraction lines with the enhancement in the Lu addition due to the effect of the minor phase (Bi-2201). The lattice parameter c and volume fraction of Bi-2212 phase reduce systematically whereas the cell parameter a and minor phase fraction enhance with ascending the Lu content in the system, leading to the decrement in the superconducting properties. Scanning electron microscope measurements show that not only do the surface morphology and grain connectivity degrade but the grain sizes of the samples decrease with the increase of the Lu addition, as well. Energy dispersive spectroscopy results reveal that the Lu³⁺ ions might enter into the crystal structure by replacing Sr²⁺ ions, confirming why the superconducting properties of the pure sample are more superior to the samples doped. At the same time, dc resistivity results obtained show that the room temperature resistivity systematically increases with the enhancement of the Lu content as a result of the hole filling when the onset (T _c ^onset ) and offset (T _c ^offset ) temperatures determined from the resistivity curves decrease from 99.5 to 93.0 K and 85.0 to 60.0 K, respectively, illustrating not only the increment in the relative percentage of Bi-2201 phase formation and the reduction of the mobile carrier concentration but also the presence of impurities and weak links between the superconducting grains.     

Crossover from dirty to clean superconducting limit in dc magnetron-sputtered thin Nb films

High-quality Nb (110) thin films with residual resistance ratios up to 60 and critical temperatures T_c ≈ 9.27 K have been prepared by conventional dc-magnetron sputtering on α-Al₂O₃ by careful selection of the sputtering conditions. This allowed for a systematic study of the influence of the growth rate on the structural quality and the superconducting properties of the films. The optimized growth conditions were revealed at the substrate temperature T_s = 850 °C, Ar pressure P_s = 0.4 Pa, and the growth rate g ? 0.5 nm/s. The results of the films' structural characterization by X-ray diffraction, reflection high-energy electron diffraction, and atomic force microscopy are presented. In terms of the electron mean free path l and the superconducting coherence length ξ, deduced from the magneto-resistivity data, the clean superconducting limit (l > ξ) is realized in the high-purity films. For comparison, in impure Nb films sputtered at room temperature while keeping the rest of the sputtering parameters unvaried, the opposite dirty limit (ξ ? l) ensues. The merits of these findings are discussed in the context of the demands of present-day fluxonics devices regarding the normal-state and flux-flow properties of superconducting films they are made of.     

The Effect of the Pinning Center Size on the Vortex Pinning by?Embedded ZrO2 Nano-particles

The influence of pinning centers size on the superconducting properties was investigated. Through the addition of three batches of ZrO₂ nano-particles with mean size of D ₁=13 nm, D ₂=21 nm, and D ₃=85 nm, we have succeeded in incorporating effective artificial pinning centers within the YBCO matrix of the bulk superconductor. An enhancement in the flux pinning and an improvement in the critical current densities (transport critical current density J _ct and magnetic critical current density J _cm) were achieved. The results indicate that slight inclusions of ZrO₂ can greatly enhance the flux pinning capability of samples. Comparative analyses of the critical current densities and the resulting pinning force F _p for the three diameters have shown that pinning centers with finer size are much more efficient than those with a size larger than the coherence length ??.     

Role of diffusion-annealing time on the superconducting, microstructural and mechanical properties of Cu-diffused bulk MgB2 superconductor

In this study, the effect of various annealing time (0.5, 1, 1.5 and 2 h) on microstructural, mechanical and superconducting properties of the Cu-diffused bulk MgB₂ superconducting samples is investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers microhardness (H _v ) and dc resistivity measurements for the first time. The critical transition temperature, grain size, phase purity, lattice parameter, surface morphology, crystallinity and room temperature resistivity values of the bulk samples prepared are compared with each other. Electrical-resistivity measurements show that the sample (annealed at 850 °C for 1 h), exhibiting the highest room temperature resistivity, obtains the maximum zero resistivity transition temperature (T _c ). From the XRD results, all the samples contain MgB₂ as the main phase with a very small amount of Mg₂Cu phase. Moreover, SEM investigations conducted for the microstructural characterization illustrate that not only does the grain size of the samples studied enhance gradually, but the surface morphology and grain connectivity also improve with the increase in the diffusion-annealing time up to 1 h beyond which all the properties obtained start to degrade. Indeed, the worst surface morphology is observed for the Cu-diffused bulk MgB₂ superconductor exposed to 2 h annealing duration. At the same time, Vickers microhardness, elastic modulus, load independent hardness, yield strength, fracture toughness and brittleness index values are calculated separately for the pure and Cu-diffused samples. It is found that the microhardness values depend strongly on the diffusion-annealing time. Furthermore, the diffusion coefficient of the Cu ion in the bulk MgB₂ superconductor is obtained to change from 1.63 × 10^?7 to 2.58 × 10^?7 cm² s^?1. The maximum diffusion coefficient is observed for the sample prepared at 850 °C for 1 h whereas the minimum one is noted for the sample annealed at 850 °C for 2 h, confirming that the annealing-time of 1 h is the best ambient to improve the mechanical, microstructural and superconducting properties of the samples produced.     

On the Effect of Carbon Nano-Tubes Addition on Structure and Superconducting Properties of Bi(Pb)-2223 Phase

In this work, a series of ceramic samples of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O _y (Bi(Pb)-2223) added with different amounts (0, 0.2 and 0.4 wt%) of carbon nano-tubes (CNT) are prepared from commercial powders and characterized. The study shows that the volume fraction of the Bi(Pb)-2223 phase decreases with CNT content while the grain size of the samples increases. The obtained cell parameters as well as the onset critical transition temperature are independent of this kind of addition. Also, it has been concluded that CNT addition makes the grains of the samples more connected. The measured magnetization in FC and ZFC modes indicates that CNT addition makes the grains of the samples more connected even if the irreversibility line is decreased. The variation of the residual resistivity and metalicity with CNT content suggests that the addition introduces disorder and defects into the samples. Also, the added samples present broad transitions to the superconducting state when compared to the pure one; this result may be associated to the increase of the volume fraction of the secondary phases. The activation energies, upper critical fields H _c2(0) and coherence lengths ξ(0) are extracted from the magneto-resistivity curves and their evolutions with CNT content are discussed.     

The Influence of Postannealing on Structural and Superconducting Properties of Bi-2212 Ceramics

The effect of postannealing on the structural and superconducting properties of Bi-2212 sintered ceramic samples prepared by the solid state reaction method has been investigated. Postannealing times were varied from 0 h to 192 h at 700 ^°C. Electrical resistivity studies showed that postannealed samples at 96 h have the lowest room temperature values while critical transition temperature does not change significantly. XRD data have shown that the Bi-2212 phase content does not change noticeably, independently of the postannealing length. In addition, J _c values, calculated from the hysteresis loops using the Bean’s model, increased with increasing the postannealing time until 48 h and decrease for longer ones.     

Coexistence of Superconductivity and Ferromagnetic Phases in YBa2Cu3O7−δ Nanoparticles

High-temperature superconductor YBa₂Cu₃O_7?δ (YBCO) nanopowders were synthesized by the citrate-gel route, which is a modification of the sol-gel method. The fine powders were calcinated at 860 and 900 °C. They were of small size, in the range of 30–35 nm. X-ray diffraction (XRD) patterns verified production of the orthorhombic superconducting phase in all samples. Measuring the magnetic properties of these nanoparticles at room temperature, via a vibrating sample magnetometer (VSM), indicated ferromagnetism behavior in the YBa₂Cu₃O_7?δ nanoparticles. As the size of the nanoparticles decreased, the magnetic saturation of all samples increased. The development of the ferromagnetism effect was attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. Thus, in an innovative work, the produced samples were annealed at 700 °C for 5 h under 0.8–0.9 bar of air atmosphere. The results showed that a small increase in the nanoparticle size provided a dramatic increase of magnetic saturation in all samples. Thus, we can say that the annealing process at vacuum improves the ferromagnetic properties of YBCO nanoparticles.     

Metal-insulator transition in thin films of vanadium dioxide: The problem of dimensional effects

Vanadium dioxide has various potential applications in electronics due to the metal-insulator transition (MIT). It is known that oxide structures with nanometric dimensions exhibit properties different from bulk oxide materials because of the spatial confinement and the proximity of the substrate. However, in order to integrate VO₂ into the thriving nano-scale electronics, it is necessary to explore the MIT in this material in thin film nano-structures. In this work, it is shown that there is a fundamental dimensional restriction for the transition to occur even for pure epitaxial VO₂ nano-films and nano-wires. This is associated with the fact that any phase transition turns out to be impossible when the system size is decreased below a certain characteristic length d_c. This dimension is estimated to be d_c ∼ ξ (where ξ is the correlation length, ∼ 2 nm for VO₂), and, on the basis of the available experimental data, it is shown that the transition temperature falls as the characteristic size (film thickness or nano-wire radius) diminishes, though the predicted theoretical limit of 2 nm is not still experimentally achieved by now.Experimental results concerning the dependence of the threshold voltage on the film thickness at MIT-induced switching in VO₂ based sandwich structures are presented. Finally, the comparison of the authors' experimental data with the literature data, as well as with the analogous features of superconducting phase transitions, is carried out.     

Improvement of the Nature of Indentation Size Effect of Bi-2212 Superconducting Matrix by Doped Nd Inclusion and Theoretical Modeling of New Matrix

Neodmium (Nd) inclusions at different stoichiometric ratios (x=0.0, 0.001 %, 0.005 %, 0.01 %, 0.05 %, 0.1 %) are doped in the Bi-2212 superconducting samples and the samples obtained are subjected to the sintering process at 840 ^°C constant temperature for 72 hours. The effect of Nd doping on the structural and mechanical properties of prepared samples is investigated by the standard characterization measurements. XRD and SEM measurements are performed to obtain information about surface morphology, phase ratios, lattice parameters and particle size. Moreover, Vickers microhardness (H _V ) measurements are exerted to investigate the mechanical properties of the all samples in detail. It is found that all the properties given above retrogress with the increase of the Nd concentration in the Bi-2212 superconducting core. However, the ISE nature of the materials improves systematically. Additionally, the experimental results of microhardness measurements are analyzed using Meyer’s law, PSR, MPSR, EPD models and HK approach. The results show that Hays–Kendall approach is determined as the most successful model.     

Investigation of Lu effect on YBa2Cu3O7−δ superconducting compounds

This study reports the effect of Lu addition on the microstructural and superconducting properties of YBa₂Lu_xCu₃O_7?δ (Y123) superconducting samples with x = 0, 0.1, 0.3, 0.5 and 0.7 by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistivity and transport critical current density (J_c) measurements. The samples prepared by the liquid ammonium nitrate and derivatives are exposed to various annealing time (20, 40 and 60 h) and temperature (950, 960 and 970 °C), and the best ambient for the sample fabrication is determined to be 970 °C for 20 h. Zero resistivity transition temperatures (T_c), critical current densities (J_c), variation of transition temperatures, hole-carrier concentration, grain size, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the bulk superconducting samples prepared at 970 °C for 20 h are compared with each other. T_c and J_c values of the samples are inferred from the dc resistivity and the critical current measurements, respectively. The results show that the T_c value of the pure sample is about 90.6 K while the sample doped with 0.1 wt% Lu has the maximum T_c value (92.5 K). However, beyond x = 0.1, the T_c value is observed to decrease toward to 83.5 K with increment in the Lu addition. Similarly, the J_c values measured are found to reduce from 142 to 76 A/cm² with the addition. Moreover, XRD measurements show that both pure and Lu-doped samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain Y123 and Y211 phase, confirming the incorporation of Lu atoms into the crystalline structure of the samples studied. At the same time, comparing of the XRD patterns of samples, the intensity ratio of the characteristic (110) and (013) peaks on the sample doped with 0.1 wt% Lu is more than that on the other samples prepared. Additionally, SEM images display that the sample doped with 0.1 wt% Lu obtains the best crystallinity, grain connectivity and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.7). Further, EDX results demonstrate that the Lu atoms doped are successfully introduced into the microstructure of the Y123 samples studied and the maximum Cu element level is observed for the sample doped with 0.1 wt%, explaining that why this sample obtains the best superconducting properties compared to others. According to all the results obtained, it is concluded that the 0.1 wt% Lu addition into the Y123 system improves the microstructural and superconducting properties of the samples studied.     

Physical, electrical, transport and magnetic properties of Nd(Ba,Nd)2.1Cu3O7−δ system

In this study, the superconducting Nd(Ba,Nd)_2.1Cu₃O_7?δ system has been prepared using conventional solid-state reaction technique. Transport properties including structural/microstructural evolution, electrical, magnetic and critical current density properties were investigated. After high temperature heat treatments at over 1,000 °C, large and strongly connected grains were obtained but weakly connected and small in size granular formation were obtained for the low temperature heat treated samples at around 900 °C. The best T _c and T ₀ values were obtained as 93 and 89 K respectively for the sample prepared at 1,020 °C for 24 h, which is very close to peritectic temperature of YBCO material. Magnetization of the sample heat treated at 1,020 °C was investigated in detail. The magnetization hysteresis loops are expounded to be the product of Nd-123 grains and unscreened Nd³⁺ ions within intergranular boundaries and vortex cores. The peak effect on the magnetization curves was described by the extended critical state model. Temperature dependencies of the irreversibility field, the peak field and the full penetration field correlate and there is scaling behavior of the pinning force as well. Thermoelectric power data was analyzed by “Modified two band model with linear T-term for superconductors”. Temperature dependence of thermal conductivity of the samples showed small peak with broad maximum just below the T _c value. Thermal conductivity of samples prepared was calculated by using “The Modified Callaway Model and Wiedermann–Franz law” and results obtained discussed.     

Effect of Different Nanosized MgO on the Transport Critical Current Density of Bi1.6Pb0.4Sr2Ca2Cu3O10 Superconductor

Magnesium oxide powders with average particle size 20 and 40 nm were added into Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ (Bi,Pb-2223) with nominal composition Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀(MgO) _x (x=0–0.15 wt.%). The transport critical current density (J _c) and transition temperature were determined using the four-probe method. The structure and microstructure were examined by X-ray diffraction method and scanning electron microscopy, respectively. The transition temperature and J _c increased with nanosized MgO addition. The 20 nm MgO added samples showed a higher J _c compared with the 40 nm MgO added samples. J _c was higher as size of MgO was closer to the coherence length of the superconductor. These results showed that the size variation of flux pinning center at the nanoscale is important in enhancing J _c.     

Scanning Tunneling Spectroscopic Studies of the Low-Energy Quasiparticle Excitations in Cuprate Superconductors

We report scanning tunneling spectroscopic (STS) studies of the low-energy quasiparticle excitations of cuprate superconductors as a function of magnetic field and doping level. Our studies suggest that the origin of the pseudogap (PG) is associated with competing orders (COs), and that the occurrence (absence) of PG above the superconducting (SC) transition T _c is associated with a CO energy Δ _CO larger (smaller) than the SC gap Δ _SC. Moreover, the spatial homogeneity of Δ _SC and Δ _CO depends on the type of disorder in different cuprates: For optimally and under-doped YBa₂Cu₃O_7?δ (Y-123), we find that Δ _SC<Δ _CO and that both Δ _SC and Δ _CO exhibit long-range spatial homogeneity, in contrast to the highly inhomogeneous STS in Bi₂Sr₂CaCu₂O_8+x (Bi-2212). We attribute this contrast to the stoichiometric cations and ordered apical oxygen in Y-123, which differs from the non-stoichiometric Bi-to-Sr ratio in Bi-2212 with disordered Sr and apical oxygen in the SrO planes. For Ca-doped Y-123, the substitution of Y by Ca contributes to excess holes and disorder in the CuO₂ planes, giving rise to increasing inhomogeneity, decreasing Δ _SC and Δ _CO, and a suppressed vortex-solid phase. For electron-type cuprate Sr_0.9La_0.1CuO₂ (La-112), the homogeneous Δ _SC and Δ _CO distributions may be attributed to stoichiometric cations and the absence of apical oxygen, with Δ _CO<Δ _SC revealed only inside the vortex cores. Finally, the vortex-core radius (ξ _halo) in electron-type cuprates is comparable to the SC coherence length ξ _SC, whereas ξ _halo～10ξ _SC in hole-type cuprates, suggesting that ξ _halo may be correlated with the CO strength. The vortex-state irreversibility line in the magnetic field versus temperature phase diagram also reveals doping dependence, indicating the relevance of competing orders to vortex pinning.     

Fluctuation Induced Conductivity in (Cu0.5Tl0.5)Ba2Ca2Cu3O10?δ Superconductor Synthesized at Different Temperatures

The fluctuation induced conductivity (FIC) analysis of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu₃O_10?δ superconductor synthesized at different temperatures has been carried out in the frame work of Aslamazov-Larkin (AL) theory. Almost all the superconducting parameters studied in this research work have been improved with the increase of synthesis temperature up to 850°C, which is most likely due to (Cu_0.5Tl_0.5)Ba₂Ca₂Cu₃O_10?δ superconducting phase stability at this synthesis temperature. The parameters calculated from FIC analysis are cross-over temperature (T ₀), zero temperature c-axis coherence lengths ξ _c (0), interlayer coupling strength (J) and the exponents (λ _3D and λ _2D). The FIC analysis has shown an increase in T ₀ and the shift of three dimensional (3D) Aslamazov-Larkin (AL) regions to the higher temperature with the increase in synthesis temperature up to 850°C.