Dielectric study on the phase transitions in triammonium hydrogen disulfate (NH4)3H(SO4)2
TL;DR: In this paper, phase transitions in triammonium hydrogen disulfate (NH4)3H(SO4)2 were studied by means of dielectric measurements and differential thermal analysis.
Abstract: Phase transitions in triammonium hydrogen disulfate (NH4)3H(SO4)2 were studied by means of dielectric measurements and differential thermal analysis. Below room temperature three phase transitions were observed at −8, −132, and −140°C. The dielectric constant along the c*-direction shows breaks at the −8 and −132°C transitions, while it shows a discontinuous change accompanied with a thermal hysteresis at the −140°C transition. In addition to the above-mentioned transitions, a broad maximum of the dielectric constant was found at about −28°C along the c*-direction. However, no thermal anomalies were observed around −28°C.
Mit Hilfe dielektrischer Messungen und differentieller Thermoanalyse wurden die Phasenubergange in Triammoniumhydrogendisulfat (NH4)3H(SO4)2 untersucht. Unterhalb der Raumtemperatur wurden drei Phasenubergange bei −8, −132 und −140°C gefunden. Die entlang der c*-Richtung gemessene Dielektrizitatskonstante zeigt bei den −8 und −132°C-Ubergangen stufenformige Anderungen, beim −140°C-Ubergang andert sie sich, begleitet von thermischen Hystereseerscheinungen, diskontinuierlich. Zusatzlich zu den o. g. Ubergangen wurden bei Messung in c*-Richtung ein breites Maximum der Dielektrizitatskonstanten bei etwa −28°C, aber keine thermischen Anomalien gefunden.
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TL;DR: An overview of the different materials currently thought to be potential proton exchange membrane materials for fuel cells operated at medium temperatures (100-200°C) is given in this article.
184 citations
TL;DR: In this paper, the chemical and thermal stability of several solid acid compounds under fuel cell operating conditions has been investigated, primarily by thermogravimetric methods, and the overall decomposition process can be expressed as 2CsHSO_4 → Cs_2SO_4 + H 2O + SO_3 with Cs 2S_2O_7 appearing as an intermediate byproduct at slow heating rates.
Abstract: The chemical and thermal stability of several solid acid compounds under fuel cell operating conditions has been investigated, primarily by thermogravimetric methods. Thermal decomposition of CsHSO_4, a material which has shown promise as an alternative electrolyte for proton exchange membrane (PEM) fuel cells, initiates at ∼175°C under inert conditions. The overall decomposition process can be expressed as 2CsHSO_4 → Cs_2SO_4 + H_2O + SO_3 with Cs_2S_2O_7 appearing as an intermediate byproduct at slow heating rates. Under reducing conditions, chemical decomposition can occur via reaction with hydrogen according to 2CsHSO_4 + 4H_2 → Cs_2SO_4 + 4H_2O + H_2S. In the absence of fuel cell catalysts, this reduction reaction is slow; however, materials such as Pt, Pd, and WC are highly effective in catalyzing the reduction of sulfur and the generation of H_2S. In the case of M_3H(XO_4)_2 compounds, where M = Cs, NH_4, or Rb and X = S or Se, a similar reduction reaction occurs: 2M_3H(XO_4)_2 + 4H_2 → 3M_2XO_4 + 4H_2O + H_2X. In an operational fuel cell based on CsHSO_4, performance degraded with time, presumably as a result of H_2S poisoning of the anode catalyst. The performance loss was recoverable by exposure of the fuel cell to air at 160 °C.
59 citations
TL;DR: It is speculated that the formation of cubic ice within a highly viscous brine blocks the solvent mediated cubic to hexagonal phase transformation, thus stabilising the metastable cubic ice in the most concentrated solution droplets.
Abstract: Ice clouds in the Earth’s upper troposphere can form via homogeneous nucleation of ice in aqueous droplets. In this study we investigate the crystallisation, or lack of crystallisation, of the solute phase and ice in aqueous (NH4)3H(SO4)2/H2O and NH4HSO4/H2O droplets. This is done using in situ X-ray diffraction of emulsified solution droplets mounted on a cold stage. From the diffraction patterns we are able to identify the phases of crystalline solute and ice that form after homogeneous freezing in micrometer sized droplets. An important finding from this study is that crystallisation of the solute does not always occur, even when crystallisation is strongly thermodynamically favoured. The nucleation and growth of solute phase crystals becomes inhibited since the viscosity of the aqueous brine most likely increases dramatically as the brine concentration increases and temperature decreases. If ice nucleates below a threshold freezing temperature, the brine appears to rapidly become so viscous that solute crystallisation is inhibited. This threshold temperature is 192 K and 180 K, in (NH4)3H(SO4)2 and NH4HSO4, respectively. We also speculate that the formation of cubic ice within a highly viscous brine blocks the solvent mediated cubic to hexagonal phase transformation, thus stabilising the metastable cubic ice in the most concentrated solution droplets.
56 citations
TL;DR: In this paper, the dielectric properties of a single crystal (NH 4 ) 3 H(SeO 4 ) 2 were measured below room temperature and two phase transitions were found at around +2°C and at -92°C.
Abstract: Dielectric properties of single crystal (NH 4 ) 3 H(SeO 4 ) 2 were measured below room temperature Two phase transitions were found at around +2°C and at -92°C A λ-type peak of the dielectric constant along the c -direction is found at -92°C, and the crystal shows ferroelectricity below the transition The temperature dependence of the spontaneous polarization was measured by a pyroelectric method The Curie temperature linearly increases with increasing hydrostatic pressure up to 85 kbar with a slope of 23 deg kbar -1
47 citations
TL;DR: In this article, the Curie-Weiss constant and spontaneous polarization of the deuterated crystal are 3.7×10 3 K and 2.4 µC·cm -2 at -170^° C, respectively.
Abstract: Dielectric and dilatometric measurements were made for the deuterated crystal (ND 4 ) 3 D(SeO 4 ) 2 . For comparison the dilatometric measurement was also made for the undeuterated crystal (NH 4 ) 3 (SeO 4 ) 2 . Two phase transitions were found at +55^° C and at about +29^° C for (NH 4 ) 3 H(SeO 4 ) 2 besides the two phase transitions previously reported. Deuteration rises the Curie temperature by about 34°C, showing important role of the O–H–O bonds playing in the ferroelectric transition. The Curie-Weiss constant and spontaneous polarization of the deuterated crystal are 3.7×10 3 K and 2.4 µC·cm -2 at -170^° C , respectively.
44 citations
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TL;DR: In this paper, a determination of all theoretically possible species of ferroelastic crystals and the number of states and ferroelectricity or nonferroelctricity in each of these species was made.
Abstract: It is convenient to grasp crystals of a kind as “ferroelastic” crystals. They are defined analogously to “ferroelectric” crystals, mechanical strain and stress in the former corresponding to electric polarization and field in the latter respectively. The order of magnitude of the coercive stress for ferroelastic state-shifts which corresponds to the coercive field for ferroelectric state-shifts is expected, on the average, to be 10 2 newt·cm -2 . A determination is made of all theoretically possible species of ferroelastic crystals and the number of states and ferroelectricity or nonferroelctricity in each of these species. It is found that there are 94 of these ferroelastic species and that 42 of them are simultaneously ferroelectric. Incidentally, an alteration is made of the hitherto used symbols for the 88 species of ferroeletric crystals.
576 citations
TL;DR: In this paper, the behavior of the various dielectric properties, specific heat, and thermal expansion of the ferroelectric transitions in these crystals has been studied in detail under various experimental conditions, with particular reference to the Ferroelectric transition in these crystal.
Abstract: The behavior of the various dielectric properties, specific heat, and thermal expansion of ${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$S${\mathrm{O}}_{4}$ and ${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$Be${\mathrm{F}}_{4}$ has been studied in detail under various experimental conditions, with particular reference to the ferroelectric transitions in these crystals The curves of dielectric constant vs temperature do not obey the Curie-Weiss law The shift of the transition temperature as reported in the literature could not be detected even under extreme experimental conditions The transition energy and entropy change have been evaluated from a ${C}_{p}$ vs $T$ curve The variation of the lattice parameters and the unit cell volumes with temperature has been measured by the x-ray methodThe transition of ${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$S${\mathrm{O}}_{4}$ appears to be of first order, while that of ${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$Be${\mathrm{F}}_{4}$ is also of first but very close to second-order Deuteration of these crystals does not affect the transition of ${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$S${\mathrm{O}}_{4}$; but in the case of ${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$Be${\mathrm{F}}_{4}$ the transition temperature is raised by 3\ifmmode^\circ\else\textdegree\fi{}C by deuteration, and the transition energy is also increasedThe phase diagram of ${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$S${\mathrm{O}}_{4}$-${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$Be${\mathrm{F}}_{4}$ system is given
148 citations
105 citations
TL;DR: In this paper, a review of experimental studies of the critical phenomena in ferroelectric phase transitions of the order-disorder type is made, and the static critical indices obtained so far agree well with those given by the mean-field theory except in the extreme vicinity of a critical point.
Abstract: A review is made of experimental studies of the critical phenomena in ferroelectric phase transitions of the order-disorder type.The static critical indices obtained so far agree well with those given by the mean field theory except in the extreme vicinity of the critical point. Recent observations on the critical x-ray scattering, the critical slowing-down and the anomalies in transport processes are surveyed and their significance is discussed qualitatively.
36 citations
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