P.C. de Camargo

Hydrogen damage detection by ultrasonic spectral analysis

The non-destructive testing technique known as ultrasonic spectral analysis was applied to steel samples for the detection of minute cracks produced by hydrogen in a H2S environment. The cracks were characterized and quantified by metallographic analysis and compared with the ultrasonic results. The backwall echoes and the backscattered signals were analyzed in the frequency domain and the characteristics of these spectra were evaluated by second order moments, which emphasizes the higher frequency components. The second order moments showed a greater variability for hydrogen attacked materials than for non-attacked ones, denoting good sensitivity to this type of material degradation.

Magnetic behaviour in the spin-density-wave phase of ( x < 20% Mn) alloys

The magnetic susceptibilities of twelve samples in the alloy system , with x ranging from to 19.4% Mn, are measured in the temperature range , in applied fields in the range Oe, after cooling in zero field and in the measuring field. The field dependence of the magnetization M(H) is measured at T = 5 K in the range kOe. The magnetic behaviour is characteristic of a spin glass, with hysteresis of M(H) and also hysteresis between the field-cooled and zero-field-cooled states, a peak in for the zero-field-cooled state, and relaxation of M as the logarithm of time when H is changed at low temperature. Unlike a typical spin glass, however, the peak in in the zero-field-cooled state is at roughly the same temperature (between 25 and 40 K) for all concentrations, and Curie - Weiss paramagnetism is seen at higher temperatures only for Mn. The Curie constant increases by an order of magnitude between x = 11.2 and 13.5% Mn, corresponding perhaps to a change in the atomic short-range order.

Local moments in the paramagnetic phase of dilute CrV alloys

The magnetic susceptibility of Cr+0.2 at.% V and Cr+0.4 at.% V alloys (nominal concentrations) is found to show a sharp increase at the Neel temperature with increasing temperature and to obey a Curie-Weiss law in the paramagnetic phase up to 400 K, corresponding to a moment per V atom of about one Bohr magneton. The relation of these results to the behaviour of other physical properties of dilute CrV alloys is discussed.

Magnetic behavior of spin-density-wave (Cr + 2.7% Fe)1−x(V,Mn)x alloys

Abstract The magnetic susceptibility χ of the binary alloy Cr + 2.7% Fe and of nine V-doped and seven Mn-doped ternary alloys has been measured as a function of temperature T from 5 to 300 K, and as a function of field H in the range −55 H T = 5 K. The Neel temperature T N of the V-doped alloys drops with increasing V content x , and antiferromagnetism disappears for x ≥ 2.5% V. χ ( T ) exhibits a Curie-Weiss law in both the paramagnetic and antiferromagnetic phase. In the Mn-doped alloys, for x ≥ 1.5% Mn, an additional contribution to χ ( T ) appears at lower temperatures, which is hysteretic between the field-cooled and zero-field-cooled states. The onset or pinning temperature TP p increases only slowly with increasing x , which suggests that this component of χ ( T ) corresponds to the new type of spin glass seen in binary CrMn alloys.

A new type of spin glass in spin-density-wave CrMn and CrSiMn alloys

Spin-density-wave (SDW) binary Cr1-xMnx (x<<0.1-4.6% Mn) and ternary (Cr+1.3% Si)1-xMnx (x=0.17 and 0.6% Mn) alloys are found to exhibit spin-glass behaviour, which differs however from that of a conventional spin glass in that the magnetic susceptibility chi (T) is essentially independent of temperature T between the low-temperature maximum and the Neel temperature TN, and in that the temperature of the maximum is essentially independent of the Mn concentration. Comparison of binary and ternary alloys having the same Mn content shows that the nature of the maximum in chi (T) depends on whether or not the SDW is commensurate. A model is proposed that explains the increase in chi (T) at low temperature as resulting from pinning of the phase of the SDW to the Mn moments, which are frozen below TN, causing frustration on the host Cr moments on the surfaces between the resultant SDW phase domains.

Tuning protein GlnB-Hs surface interaction with silicon: FTIR-ATR, AFM and XPS study.

Herbaspirillum seropedicae GlnB (GlnB-Hs) is a signal transduction protein involved in the control of nitrogen, carbon and energetic metabolism. The adsorption of GlnB-Hs deposited by spin coating on hydrophilic and hydrophobic silicon forms a thin layer that was characterized using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared attenuated total reflectance spectroscopy (FTIR-ATR). AFM allowed the identification of globular, face-up donut like array of protein on hydrophilic silicon substrate, favoring deprotonated residues to contact the silicon oxide surface. Over hydrophobic silicon, GlnB-Hs adopts a side-on conformation forming a filament network, avoiding the contact of protonated residues with silicon surface. XPS allowed us to determine the protonated and non-protonated states of nitrogen 1s (N 1s). The FTIR-ATR measurements provided information about protein secondary structure and its conservation, after surface adsorption.

Coexistence of spin-glass and Curie-Weiss paramagnetism in spin-density-wave CrFeMn alloys

Abstract The temperature and field dependence of the magnetic susceptibility χ ( T ) in the antiferromagnetic alloys (Cr + 2.7% Fe) 1− x Mn x and (cr + 1.5% Fe) 1− x Mn x is reported. The anomalies in χ ( T ) below about 100 K, followed at higher temperatures by a Curie-Weiss law, show that, in these ternary alloys containing magnetic Fe impurities, a spin-glass state like that reported recently for binary CrMn alloys co-exists with the Curie-Weiss paramagnetism characteristic of the spin-density-wave phase of CrFe alloys. At low concentrations, x ⩽ 0.3% Mn reduces the effective moment of the Fe impurity suppresses the spin-glass behaviour, and conversely the Mn reduces the effective moment of the Fe atom.

Magnetic field dependence of the Curie–Weiss paramagnetism in CrV alloys

The determination of the magnetic properties of antiferromagnetic Cr alloys requires careful consideration of the influence of the applied magnetic field. In this work we show that alloys of Cr-x at. % V present a Curie–Weiss paramagnetism above the Neel temperature, which is suppressed by a characteristic field HL. Samples with x=0.1, 0.2, and 0.4 were investigated through measurements of the magnetic susceptibility as a function of temperature, for different values of the magnetic field. A magnetic phase diagram showing the characteristic line HL vs x at. % V is proposed.

Suppression of the Curie - Weiss paramagnetism seen above the Néel transition in dilute CrV alloys

The temperature dependence of the AC magnetic susceptibility of antiferromagnetic Cr - x% V (x = 0, 0.1, 0.67, 1.0, 1.5) has been investigated in the vicinity of the Neel transition. The behaviour of Cr - 0.1% V is similar to that reported previously for the alloys containing 0.2 and 0.4% V, in that the susceptibility above the Neel transition has a component with a temperature dependence that obeys the Curie - Weiss (CW) law. This CW paramagnetism was not however seen in the samples containing higher concentrations of V. The DC susceptibility of Cr - 0.2% V was also measured in fields of 2.0, 6.5 and 20.0 kOe, and the high field was found to suppress the CW paramagnetism.

Spin-flip transition and magnetic phase diagram of CrV alloys

Abstract The DC magnetic susceptibility of Cr-x at.%V (x = 0, 0.1, 0.2, 0.4 and 0.67) has been measured as a function of temperature. The boundary between the transverse and the longitudinal SDW was obtained, and an upperbound limit for the occurence of spin-flip transition was determined to be x = 0.67.