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1
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Entropy Change Calculations for Pure Gd and a Ni-Mn-Cu-Ga Heusler Alloy: Constant Field vs. Constant Temperature Experiment

100%
Ferenc J., Kowalczyk M., Wróblewski R., Cieślak G., Sielicki K., Leonowicz M., Kulik T.
Acta Physica Polonica A
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2015
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vol. 128
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issue 1
111-115
EN
The magnetocaloric effect may be assessed indirectly by expressing it as the change in magnetic entropy in varying magnetic field, H, as the function of temperature, T. Magnetization, M=f(T,H), may be experimentally acquired from a series of isothermal measurements with variable field, or from a series of constant field measurements with variable temperature. The accuracy of magnetic entropy calculation depends on the number of series in these experiments. The aim of this work is to determine how little data is sufficient to obtain accurate results of magnetic entropy change calculations, on the basis of real, magnetocaloric materials. Pure gadolinium and a Ni-Mn-Cu-Ga Heusler alloy were studied. For both materials, the magnetic entropy change and relative cooling power were calculated from both experiments, with the decreasing number of experimental data. For both materials, the constant field experiment with only 6 field values provided only a 5% error of calculations, as compared to the experiment with 100 field values. The Arrott plots were also drawn for constant field mode with 6 field values, easily indicating the order of transition. Comparison of the calculation results suggests that the constant field mode magnetization measurement may be more accurate and faster than isothermal mode.
2
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Liquid Crystalline Polymorphism of 4-Alkyloxybenzylidene-4'-Alkyloxyanilines and Entropic Effects of Their Phase Transitions

80%
Godzwon J., Sienkowska M., Galewski Z.
Acta Physica Polonica A
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2008
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vol. 113
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issue 4
1145-1154
EN
This paper summarizes liquid-crystalline polymorphism of the asymmetrical 4-alkyloxybenzylidene-4'-alkyloxyanilines. In this group, containing 121 compounds, 6 types of mesophases were detected: nematic, smectic A, smectic C, smectic B, smectic I, and smectic G. Influence of both alkyl chain length on the type of mesophases and entropic effects of phase transitions are discussed.
3
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Magnetic Properties of Dy_{11}Si_4In_6

80%
Duraj D., Baran S., Jaworska-Gołąb T., Kalychak Y., Przewoźnik J., Szytuła A., Tyvanchuk Y.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1118-1120
EN
Magnetic and specific heat measurements of Dy_{11}Si_4In_6 are reported. This compound crystallizes in the tetragonal Sm_{11}Ge_4In_6-type crystal structure (space group I4/mmm), in which Dy atoms occupy four different sites. The AC and DC magnetic measurements suggest complex magnetic properties. Below T_{c} = 52 K magnetic ordering has a ferromagnetic component, while below 20 K a change of the properties is observed. Near the Curie temperature the magnetocaloric effect with the magnetic entropy change ΔS_{m} equal to 16.5 J/(kg K) is observed. The specific heat data indicate only the phase transition at 52 K.
4
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Influence of Nonstoichiometry on Magnetocaloric Effect in (La_{0.7}Ca_{0.3})_{1 - x}Mn_{1+x}O_3

80%
Sazanovich A., Pietosa J., Pashchenko A., Zubov E., Dyakonov V., Szymczak H.
Acta Physica Polonica A
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2012
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vol. 122
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issue 1
162-166
EN
Magnetocaloric effect in (La_{0.7}Ca_{0.3})_{1 - x}Mn_{1 + x}O_3 manganites with x = 0, 0.1, 0.2 has been investigated. It is found a strong influence of nonstoichiometry caused by excessive manganese on the magnetic entropy change. The magnetocaloric effect was evaluated from the isothermal curves of spontaneous magnetization versus the applied magnetic field by using the well-known thermodynamical Maxwell relation. The maximum entropy value, | Δ S_{M}^{max} |, near the ferromagnetic-paramagnetic phase transition is shown to increase with the manganese content. The La_{0.56}Ca_{0.24}Mn_{1.2}O_3 manganite exhibits the largest | Δ S_{M}^{max} | value equal to 3.09 J kg^{-1} K^{-1} at 15 kOe near T_{C} = 267 K. The magnetocaloric effect values obtained allow to propose that the studied manganites are promising materials for future cooling application.
5
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Magnetic and Structural Studies into the Effect of Solvent Exchange Process in Metal-Organic Framework MOF-76(Gd)

80%
Almáši M., Zeleňák V., Zeleňáková A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
991-993
EN
Effect of solvent exchange/absence on magnetic and structural properties of microporous metal-organic framework MOF-76(Gd) has been performed. Three compounds have been prepared and characterized: as synthesized MOF-76(Gd)-DMF, which containing DMF (N,N'-dimethylformamide) and H₂O molecules in the cavity system, activated complex without solvents, MOF-76(Gd) and water exchanged sample MOF-76(Gd)-H₂O. The structural study showed that compounds undergo structural changes after activation/exchange processes. Changes were mainly observed in the magnetic properties and distances between Gd...Gd ions: 4.718 Å for MOF-76(Gd)-DMF, 4.326 Å for MOF-76(Gd)-H₂O and 4.296 Å for MOF-76(Gd). The magnetic properties of the samples were studied by magnetic susceptibility χ_{M}(T) and magnetization M(H). The low positive value of the Weiss constant, Θ = 1.54 K in compound MOF-76(Gd)-DMF shows on a very weak ferromagnetic interaction. On the contrary, the negative values of Θ in complexes MOF-76(Gd) (-5.58 K) and MOF-76(Gd)-H₂O (-6.23 K) confirm the existence of antiferromagnetic exchange interaction between Gd(III) ions.
6
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Anisotropy of the Magnetocaloric Effect in the Ni_{49.6}Mn_{27.6}Ga_{22.8} Single Crystal

80%
Wroblewski R., Sielicki K., Leonowicz M.
Acta Physica Polonica A
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2015
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vol. 128
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issue 1
54-55
EN
The magnetocaloric effect anisotropy in the Ni_{49.6}Mn_{27.6}Ga_{22.8} single crystal was investigated. In the examined alloy the structural phase transition and magnetic transition occur at room temperature and around 370 K, respectively. The magnetic entropy change, at those two temperatures, was determined on the basis of isothermal and isofield curves, recorded at fields up to 1200 kA/m (1.5 T) with temperature steps of 2.5 and 5 K. Although the calculated values of magnetic entropy change are relatively small, ≈0.7 J/(kg K), an anisotropy of the magnetocaloric effect is observed with a magnetic field applied along the main crystallographic directions of the single crystal. The magnetic entropy change at the structural phase transition depends on the orientation. The weakest magnetocaloric effect occurs when the field is applied along [1 0 0] direction whereas the highest magnetocaloric effect value is reached along [0 0 1] direction, which is an easy magnetization axis. Such behaviour can be explained with the high magnetocrystalline anisotropy of the martensitic phase. The magnetic entropy change value, at the structural phase transition, obtained for the polycrystalline specimen, is close to that for the [0 0 1] single crystal direction.
7
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Heat Capacity Studies of NdNi_4Si Compound

80%
Falkowski M., Reiffers M., Zapotoková M., Kowalczyk A., Toliński T., Gažo E.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
126-128
EN
The study of the heat capacity of the intermetallic compound NdNi_{4}Si including the effect of the magnetic field is reported. This compound crystallizes in the hexagonal CaCu_5-type structure, space group P6/mmm. NdNi_{4}Si is ferromagnetic with T_C = 8 K and the saturation magnetic moment of 1.5 μ_{B}/f.u. at 4.2 K (in H = 9 T). The heat capacity was analyzed considering the electronic contribution, the Schottky anomaly, and the lattice contributions in the frames of the Debye model. The scheme of the energy levels created by the crystal electric field split is determined from the Schottky contribution to the specific heat. NdNi_{4}Si was characterized by the electronic heat capacity coefficient γ = 85 mJ/(mol K^2) and the Debye temperature Θp_D = 325 K.
8
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Scaling Analysis of the Magnetocaloric Effect in Co/Au Nanoparticles

80%
Hrubovčák P., Zeleňáková A., Zeleňák V., Franco V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
795-797
EN
The system of superparamagnetic Co/Au bimetallic nanoparticles of average diameter 7 nm was investigated with respect to its magnetocaloric properties. DC magnetic measurements revealed the presence of field dependent zero field cooled M(T) maximum (6-8 K) and significant zero field cooled/field cooled irreversibility at low temperatures in the system. Documented thermal hysteresis disallow standard magnetic entropy change calculation from isothermal M(H) data, thus we attempted to employ zero field cooled M(T) data for this purpose. Magnetic entropy change was calculated employing the Maxwell relation. In maximal field variation of 1 T relative high magnetic entropy change for nanoparticles Δ S_M ≈ 0.7 J/(kg K) at T =9 K was observed. The data collapsed onto single universal curve after proper axis rescaling.
9
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Gas Sensing Properties of Carbon Nanotubes Modified with Calixarene Molecules Measured by QCM Techniques

61%
Mermer Ö., Okur S., Sümer F., Özbek C., Sayın S., Yılmaz M.
Acta Physica Polonica A
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2012
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vol. 121
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issue 1
240-242
EN
This study focuses on the characterization and optimization of calixarene modified carbon nanotube thin films for gas detection. Calixarene molecules were synthesized individually by considering their functional groups to attract the gas. Calixarene modified carbon nanotube based sensors were fabricated using drop-casting method on a quartz crystal microbalance gold electrode. Carbon monoxide, carbondioxide, oxygen and dry air were used as active gases for adsorption process, while high-purity nitrogen gas was used for desorption process. The selectivity and sensitivity of calixarene modified carbon nanotube are investigated in detail. Our experimental results show that functional calixarene modified carbon nanotube coated quartz crystal microbalance sensors are very sensitive and selective to gas of CO_2 at room temperature operation.
10
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Odor Adsorption Kinetics on Modified Textile Materials Using Quartz Microbalance Technique

61%
Sarıışık M., Okur S., Asma Ş.
Acta Physica Polonica A
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2012
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vol. 121
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issue 1
243-246
EN
In this study, a functional product with aromatic oil and vitamins for use at aromatherapy and spa centers or personal care has been designed to improve life quality of users by using multifunctional products with good odor, moisturizing, relaxation, anti-aging effects. For this purpose, commercial berry berry oil was capsulated in β-cyclodextrin, and then applied to 100% cotton towel fabric. Quartz crystal microbalance sensors were used to investigate odor adsorption kinetics on modified textile materials for the first time. The frequency responses vs. time plot shows clear linear step responses to the change in the concentration of odor molecules coming from the container filled with berry berry applied textile. The results also reveal that TiO_2 coated quartz crystal microbalance sensors can be used as e-nose to sense odor from textile coated with berry berry oil containing β-cyclodextrin capsules.
11
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Electrical Characterization of Interdigitated Humidity Sensors Based on CNT Modified Calixarene Molecules

61%
Özbek C., Culcular E., Okur S., Yılmaz M., Kurt M.
Acta Physica Polonica A
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2013
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vol. 123
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issue 2
461-463
EN
In this study, we report on the optimization and characterization of chloroform soluble calix[4]arene derivative as a humidity sensor based on electrical properties. Due to the fact that calix[4]arene molecules are nonconductive, we modified the calix[4]arene molecules with carbon nanotubes with electrostatic bond in order to increase the conductance of calix[4]arene molecule (5,11,17,23-tetra-ter-butyl-25,27-dehydrazinamidcarbonilmetoxy-26,28-dehydroxy-kalix[4]aren). Using photolithography technique, we fabricated interdigitated gold electrodes with 3 micrometers separation to investigate the electrical properties of carbon nanotubes modified calix[4]arene molecule where we used dropcast method to form a thin film of carbon nanotubes modified calix[4]arene molecule on the gold electrodes. Our reproducible experimental results indicated that the chloroform soluble carbon nanotubes modified calix[4]arene films have great potential for humidity sensing applications at room temperature operations.
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