A. O. Guimarães

Magnetocaloric effect in GdGeSi compounds measured by the acoustic detection technique: Influence of composition and sample treatment

In this paper we explore the acoustic detection method applied to the investigation of the magnetocaloric effect in Gd and Gd5(Ge1−xSix)4 compounds, in the temperature range from 230 to 360 K and for magnetic fields up to 20 kOe. Measurements were performed in as-cast materials, both for powder and pellet samples, and in tree samples with compositions around Gd5Ge2Si2 that underwent different thermal treatments. Small differences were observed when comparing powder and pellet samples of Gd and Gd5(Ge1−xSix)4 compounds with 0.500<x≤1.00. For the alloys with composition around Gd5Ge2Si2, which exhibit giant magnetostriction and coexistence of distinct phases, expressive changes were observed when comparing powder and pellet samples. Based on these cases, it is easy to see that the acoustic method can distinguish a second-order phase transition from a first-order magnetic-crystallographic one, and that it presents good sensitivity to detect spurious material phase in small quantities.

Anisotropic magnetocaloric effect in gadolinium thin films: Magnetization measurements and acoustic detection

In this letter, it is demonstrated the ability of the magnetoacoustic technique in detecting the magnetocaloric effect in gadolinium thin films (1.0 μm and 3.0 μm thick), which is not accessible through conventional temperature sensors because of the reduced mass of the samples. The method, which detects the direct effect of the sample temperature variation, proved to be sensitive to the anisotropy of the films, making possible for the investigation of the anisotropic magnetocaloric effect. Magnetization measurements were also carried out, and from these measurements both the adiabatic temperature and the isothermal entropy variations were calculated. The acoustically detected magnetocaloric effect shows very good agreement with these calculations.

Determination of the entropy change using the acoustic detection technique in the investigation of the magnetocaloric effect

In this paper we demonstrate the use of the acoustic detection as an alternative way to determine the entropy variation, ΔST, a parameter normally used to characterize the magnetocaloric effect. The measurements were performed for a Gd sample in the 252–316 K temperature range for magnetic fields from zero up to 50 kOe. The reversible adiabatic curves were built in a T versus H diagram, and specific heat data obtained at zero-magnetic field were employed to assign the entropy values of each curve. Subsequently, the entropy was plotted as a function of temperature for fixed magnetic fields, and therefore the isothermal entropy variation, ΔST, was found as a function of the temperature for several magnetic field steps.

Assessment of the thermal expansion mismatch in lanthanum strontium cobalt ferrite-yttria stabilized zirconia two-layers systems using photoacoustic methodology

In this letter we investigate the thermo-elastic mismatch in lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.2Fe0.8O3−δ) (LSCF) films deposited onto yttria stabilized zirconia (YSZ) substrates, applicable to solid oxide fuel cells. We investigated composite LSCF+YSZ and pure LSCF films deposited onto commercial YSZ substrate. Photoacoustics was used to obtain the effective thermal diffusivity and thermal expansion coefficient of the two-layers samples. Based on a thermal–electrical analogy model, it was possible to get the thermo-elastic properties of the films and to confirm the desired reduction on the thermo-elastic mismatch between film and substrate when comparing composite LSCF+YSZ and pure LSCF films.

Large magnetocaloric effect and refrigerant capacity near room temperature in as-cast Gd5Ge2Si2−xSnx compounds

Large values of isothermal entropy change (ΔST) and refrigerant capacity have been found in Gd5Ge2Si2−xSnx compounds. Values of the order of 20 J kg−1 K−1 for −ΔST were obtained in as-cast samples when submitted to a magnetic field variation of 2 T. First-order-magneto-structural transition is induced by the substitution of silicon by tin and it is shifted to lower temperatures with the tin content. It means that the magnetocaloric effect on this series can be properly tuned to a specific practical thermodynamic cycle, including near room temperature range.

Influence of Environmental and Microclimate Factors on the Coffee Beans Quality (C. canephora): Correlation between Chemical Analysis and Stable Free Radicals

The present study reports a physicochemical comparison of shade-grown and sun-grown coffee beans, under unripe, rip and roasted-ripe conditions, using electrical conductivity measurements, electron paramagnetic resonance (EPR), infrared spectroscopy (FTIR), and high performance liquid chromatography (HPLC). Moreover, the assessed physicochemical parameters were compared with organoleptic evaluations based on the Coffee Quality Institute protocol. The values found for electrical conductivity, leached potassium, and stable free radicals were respectively 29%, 31%, and 350% higher for shade-grown coffee beans, whereas polyphenol oxidase enzymatic activity was 23% lower. By contrast, FTIR and HPLC measurements identified higher chlorogenic acid and lipid contents in sun-grown coffee beans. Importantly, the sensorial grade attributed to roasted-ripe grains was 12% higher for sun-grown coffee. Our findings suggest that shade-grown coffee beans have undergone microorganismal activity and undesired fermentation during cultivation, which resulted in lower coffee quality. A correlation between a set of selected physicochemical properties and organoleptic properties was robustly established and could be used in the development of future coffee bean quality control protocols.

Photoacoustic based technique for measuring the magnetocaloric effect

This paper presents a method for detecting the magnetocaloric effect (MCE), based on the acoustic detection. Small temperature oscillations, due to the application of a modulated magnetic field, are detected by a microphone in a closed cell. The continuous scanning of a superimposed dc magnetic field allows, by numerical calculation, the determination of large temperature variations caused by magnetic field steps from zero to tens of kOe. Measurements were performed in Gd and Gd5(SixGe1-x)4 compounds. The obtained results show the efficiency of the technique, which is suitable for the investigation of materials undergoing both purely magnetic phase transitions and magnetic-crystallographic first order ones.