Cr Single Crystals Research Articles

Cyclic plastic deformation of Fe–13.6 wt% Cr single crystals

The cyclic deformation behaviour in tension–compression of b.c.c. Fe-13.6 wt% Cr alloy crystals with single-slip orientation is investigated at room temperature and strain rates of 1.7 × 10−3 or 1.7 × 10−2 s−1. The cyclic deformation is found to occur in the intermediate regime of plastic deformation of pure b.c.c. metals and shows similarities with f.c.c. metals. The cyclic stress-strain curves exhibit only one plateau, which is connected with the occurrence of persistent slip bands. The corresponding dislocation arrangements are similar to those observed in f.c.c. metals, too. By contrast, the flow stress depends strongly on the strain-rate and the circular cross-section of specimens changes into an elliptical one during the cyclic deformation. The experimental results are compared with the cyclic deformation behaviour of FeSi alloy single crystals. The similarities and differences are discusssed considering the much larger solution hardening of iron produced by silicon in comparison to that produced by chromium. Mittelorientierte Einkristalle der k.r.z.-Legierung Fe-13,6 Gw% Cr werden bei Raumtemperatur mit Verformungsgeschwindigkeiten 1,7 × 10−3 oder 1,7 × 10−2 s−1 im Zug und Druck zyklisch verformt. Es wird gefunden, daß sich die Legierungskristalle mittels der gleichen Mechanismen verformen, wie man sie in reinen k.r.z.-Metallen oberhalb der kritischen Temperatur findet und daß die zyklische Verformung der k.r.z.-Legierung ähnlich verläuft wie bei k.f.z.-Kristallen. Die zyklischen Spannungs-Dehnungskurven lassen ein Plateau erkennen, das mit der Bildung persistenter Gleitbänder zusammenhängt. Auch die entsprechenden Versetzungsanordnungen sind den bei k.f.z.-Metallen beobachteten ähnlich. Im Gegensatz dazu hängt die Fließspannung jedoch stark von der Abgleitgeschwindigkeit ab und der ursprünglich kreisförmige Probenquerschnitt verwandelt sich während der zyklischen Verformung in eine Ellipse. Die experimentellen Ergebnisse werden mit dem zyklischen Verformungsverhalten von FeSi-Legierungseinkristallen verglichen. Die Ähnlichkeiten und Unterschiede werden in Anbetracht der vie1 größeren, durch Silizium im Vergleich zu Chrom verursachten, Mischkristallhärtung diskutiert.

Thermal decomposition of chromium oxide hydroxide: II. Texture change of αHCrO 2 through thermal decomposition under vacuum

The decomposition of α-HCrO 2 was studied under vacuum by comparatively using the techniques of thermal gravimetry, gas adsorption, IR spectroscopy, X-ray analysis, and high-resolution electron microscopy. The topotactic decomposition from hexagonal α-HCrO 2 to Cr 2O 3 was observed at 450∼500°C, which is higher than that in O 2 by about 50°C and lower than that in H 2O by about 70°C. However, during the course of the decomposition a particle was once destroyed into fine Cr 2O 3 crystallites, accompanied by micropore formation, and recrystallized to give a single crystal Cr 2O 3 particle having macropores inside. The decomposition mechanism was discussed on the basis of the experimental data and the crystal structures before and after the decomposition of the material, by which the shrinkage of the sample to the c-direction was explained.

Oxygen chemisorption and corrosion on Cr(100) and Cr(110) single crystal surfaces

The interaction of oxygen with the clean (1 × 1) surfaces of Cr(100) and Cr(100) single crystals has been examined by LEED, XPS, AES and Δφ measurements. At low exposures, oxygen on both surfaces is found to form metastable overlayers at 300 K which slowly undergo incorporation into the underlying metal with a characteristic half-life of several minutes. At higher exposures (10–100 L), formation of Cr 2O 3 films takes place; the rate of corrosion is demonstrated to be structure sensitive, film thicknesses of 0.4 and 1.3 nm being observed after 100 L O 2 exposures on Cr(100) and Cr(100) respectively. The oxide on Cr(100) was found to adopt a well-ordered c(2 × 4) structure, whereas Cr(110) exhibited oxygen-induced faceting. The results are discussed in the light of previously reported studies of these chemisorption systems.

Experiments on the high-field thermoelectric and thermal properties of chromium.

We take advantage of the high sensitivity of thermopower measurements to investigate the de Haas\char21{}Shubnikov oscillations in a Cr single crystal at fields up to 8 T. The increased resolution, compared to that offered by magnetoresistance measurements, allows more detailed information to be obtained on a major branch of frequencies and reveals new branches. By analyzing the peaks in the Fourier transforms, rough estimates are made of the breakdown fields for the various orbits; typical values are probably in the range of 0\char21{}40 T with most being in the region of 30 T. Measurements on the monotonic components of the electrical and thermal conductivities allow the lattice conductivity to be extracted. The result is compatible with those of the isoelectronic and isostructural elements Mo and W.

Work-hardening in Fe-13·6 wt. % Cr single crystals between 77 and 800 K

Single crystals of Fe-13·6 wt. % Cr oriented for single slip were deformed in tension at temperatures between 77 and 800 K, at a nominal strain rate of 5·5×10−5s−1. The stress-strain curves were measured, stress relaxations were performed and the slip line structure was studied. The experimental results are qualitatively similar to those of pure iron and iron-silicon alloys studied earlier. The quantitative differences are due to the different effect of solute atoms on dislocation behaviour in iron alloys.

Time characteristics of the photoluminescence in CdIn 2S 4:Cr single crystals

The kinetics of the photoluminescence spectra in CdIn 2S 4:Cr single crystals has been investigated in the range of doped impurity concentration N Cr = 0.1 − 5 atom per cent. It has been appreciated, that the amplitude of longwave spectrum component with the maximum at λ = 960 nm increased together with N Cr. The longtime relaxation with characteristic times of increasing τ (i) ≈ 1 μs and decay τ (d) ≈ 100 μs of spectrum band due to ligand-field transition of Cr +3 ions has been found out. A model of resonance energy transmission between the stages of intrinsic structure defects and the subsystem of Cr +3 ions, which permits to understand the most of the observed experimental results is suggested.

Internal friction of chromium single crystal after neutron irradiation at low temperature

Abstract Internal friction and elastic modulus measurements of Cr single crystal after fast neutron irradiation at 5–7 K are made as a function of temperature. The recovery stages are also studied by electrical resistivity measurements. Several internal friction peaks and elastic modulus changes are observed. The peaks are discussed by the relaxation processes of interstitial-impurity complexes or di-interstitials and interstitial clusters. The ratios of the relaxation strength of the stress direction ⟨111⟩ to the stress direction ⟨100⟩ and the activation energies for these peaks are given. The migration of interstitial is considered to occur at 35 K since a modulus change due to dislocation pinning is observed at this temperature.

Electroabsorption in single crystal indium phosphide

The electroabsorption properties of InP: Fe and InP: Cr single crystals in the long wavelength region of the fundamental absorption edge have been investigated. By means of several supporting experiments and improved instrumentation we have eliminated or compensated for several possible sources of error including photoconductivity and space charge. Using simple one electron theory we have thus obtained accurate values of the reduced effective mass in this important semiconductor.

Thermoreflectance investigation of the antiferromagnetic and paramagnetic phases of Cr

Thermoreflectance measurements have been performed on Cr single crystals at several temperatures above and below the N\'eel temperature. We observe dramatic changes induced by the magnetic phase transition. In contrast, static optical data fail to show appreciable differences in the (0.5-5.0)-eV photon-energy range. Magnetic ordering gives rise to the disappearance of transitions involving specific regions of the Fermi surface. New critical-point absorptions appear at the boundaries of the new Brillouin zone in antiferromagnetic Cr. Most of the observed experimental features have been identified by comparison with recent band-structure calculations.

Temperature variation of the elastic constant C44 of VCr alloys

The elastic constant C 44 has been measured for eight VCr single crystals from 4.2 K to room temperature. The observed temperature variation is totally similar to the 4 d and 5 d systems.

The mechanism of electrical threshold switching in VO2crystals

I-V characteristics of doped VO2 (0.8% Ti, 1.0% Al and 0.8% Cr) single crystals have been measured using a constant current source in the ambient temperature region 230 to 310K. The temperature of the crystal surface has also been measured. At switching the temperature of the crystal surface increases by only 3 to 6K above the ambient for different ambient temperatures. This suggests the formation of a current channel before switching. It is observed that the threshold voltage (Vth) increases but the current at switching (Ith) decreases with decreasing ambient temperature, giving a temperature-independent threshold power. The average critical power density at switching is 4.33*10-3 W mm-3 for different doped and undoped VO2 crystals. I-V measurements have also been made by placing electrodes on the crystal surface. It is observed that the threshold voltage increases with increasing electrode separation d but for d to 0 Vth gives a finite value. The ideal thermal models fail to give even qualitative agreement with the observed I-V and switching characteristics.

Magnetic Properties of a New Magnetic Phase in Fine bcc Chromium Particles Prepared by Gas Evaporation Method

Magnetization of single crystal Cr particles ranging from 380 to 750 A in the average diameter was measured without exposure to the air in the magnetic field up to 17 kOe and in the temperature range from 77 to 870 K. The particles with an A-15 type structure (δ-Cr) showed temperature-independent paramagnetism. The particles with a bcc structure showed a new magnetic transition with an ordering temperature about 800 K. An anomaly was found at 310 K, which coincides with the Neel temperature of the bulk SDW transition, in the temperature-dependence of the magnetization of the particles. The new magnetic phase below 800 K is interpreted as the magnetic order localized near the surface. The features in the experimental results are discussed on the basis of the localized-order model.

Net Optical Quenching Spectrum and Disappearance of Stationary High-Field Domain in GaAs: Cr single Crystal

Net Optical Quenching Spectrum and Disappearance of Stationary High-Field Domain in GaAs: Cr single Crystal

Experimental verification of Cr 2+ models of photoluminescent transitions in GaAs:Cr and Al xGa 1− xAs:Cr single crystals

LPE layers of Al x Ga 1− x As:Cr:Sn single crystals were grown and the photoluminescence band due to Cr 2+ was observed over the range 0≤ x≤0.42 when measured at 77 K. Preliminary spectra which reveal the characteristic zero phonon line were obtained at 4.2 K for x=0.23. The experimental results support the model in which the 0.84 eV photoluminescence band in GaAs:Cr is attributed to intraimpurity transitions at Cr 2+.

The transverse magnetoresistance of chromium in the temperature range 15–80°C

Electrical measurements have been carried out on a 4N5 Cr single crystal in multi-Q and single-Q domains. It was found that the Néel temperature depends neither on the direction of current, nor on the existence of a magnetic field. In the multi-Q domains, the transverse magnetoresistance G changes according to G = AHn. The magnetoresistance decreases with increasing temperature showing an anomaly at 40°C. For the single-Q domain specimen, measurements with H ⊥ Q showed in the antiferromagnetic state the existence of saturation in the curve G = G(H). In the paramagnetic state, the transverse magnetoresistance is an increasing function of the magnetic field. Measurements with H || Q showed that G increases according to G = AHn, and this is ascribed to the existence of open orbits along the direction of polarization.

ESR of chromium(+1) in sulfur-deprived zinc sulfide

ZnS : Cr single crystals were grown under conditions which favored the formation of sulfur vacancies. In some cases a vacancy paired with a Cr + ion at a nearest-neighbor Zn site to form a complex defect. Identification of the defect was made by use of ESR and heat treatments of the crystals.

Q-Switch and Polarization Domains in Antiferromagnetic Chromium Observed with Neutron-Diffraction Topography

The $Q$-switch and polarization domain configurations in a nearly perfect Cr single crystal has been directly observed by neutron-diffraction topography in the A${\mathrm{F}}_{1}$ and A${\mathrm{F}}_{2}$ phases. The size of the domains proves to be as large as ${10}^{\ensuremath{-}3}$ to ${10}^{\ensuremath{-}2}$ ${\mathrm{cm}}^{3}$ in either phase. As far as the present observation is concerned, each domain in the A${\mathrm{F}}_{2}$ phase splits into domains in the A${\mathrm{F}}_{1}$ phase, with two possible polarizations of the transverse spin-density wave.

First-Order Transition in Chromium at the Néel Temperature

A latent heat, hysteresis, and superheating and supercooling have been observed in a Cr single crystal by means of an extension of the ac calorimetric method.Received 13 October 1971DOI:https://doi.org/10.1103/PhysRevLett.27.1523©1971 American Physical Society

Electron Tunneling into Chromium Single Crystals

The conductance of Cr (single crystal)-Cr oxide-Pb tunnel junctions was measured as a function of bias voltage for (100) and (110) crystalline directions. Although our results are complicated by the presence of a large background conductance, the data support the existence of an antiferromagnetic energy gap in Cr at 1°K.

Spin Directions in Pure Chromium

We have carried out a triple-axis polarized-neutron-beam experiment with polarization analysis of the final beam and magnetic fields to 15 kG applied to a pure Cr single crystal. The purpose was to determine whether the spin axis in the transversely polarized spin-density wave state (122°K–38.5°C) is confined to the cube edges or whether in sufficient fields it can be made to lie in an arbitrary direction (perpendicular to the wave vector). The experiments show unambiguously that the latter is so. At 25°C, it is slightly more difficult to confine the spins to a single 110 axis than it is to a single 100 axis. At lower temperatures this anisotropy is enhanced. These results along with our previous results for the field dependence of the cube-edge components using unpolarized neutrons have been analyzed in terms of two different models. Both models have the spins in all directions perpendicular to the wave vector of the spin-density wave. One is the model of thermal activation of small domains. The other considers a domain structure with wall motion. In both models ansiotropy and magnetic field influence the net number of spins in any given direction.