Pressure Composition Temperature Research Articles

Mg-Mg2Ni二相合金の共晶組織と水素化特性

We investigated the relation between hydrogenation property and eutectic structure of a Mg-Mg2Ni two-phase alloy that is made by melting Mg and Ni in a sealed steel crucible. Both the hydrogen content and the plateau pressure increase with the rise of the measurement temperature of PCT (Pressure-Composition-Temperature) curve for both the furnace cooled and water cooled specimens. In these specimens, the maximum hydrogen contents increase up to 5.6 mass% as the increase of the cycle number of PCT measurement at 400°C. On the other hand, the maximum hydrogen content of the heat treated specimen does not depend on the PCT cycle number, and maintains a higher value of about 5.6 mass%. The eutectic structures are fine for both the furnace cooled and water cooled specimens, and coarse for the heat-treated specimen. The hydrogenation behavior depends on the morphology of the eutectic structure, that is, the coarsening of the eutectic structure causes the acceleration of hydrogenation.

Comparison of hydrogen storage characteristics between as-cast and melt-spun TiCr1.1V0.5Fe0.1Mn0.1 alloys

Abstract Hydrogen storage characteristics of as-cast and melt-spun TiCr 1.1 V 0.5 Fe 0.1 Mn 0.1 alloys were investigated by pressure–composition–temperature (PCT) tests, X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD showed that both the as-cast and the melt-spun alloys were similarly consisted of V based BCC phase and Ti 1.07 Cr 1.93 phase. However, PCT tests showed that the melt-spun alloy, with larger hysteresis between hydrogen absorption and desorption, had higher hydrogen storage capacity than the as-cast alloy. More interestingly, the phase compositions of the two fully hydrogenated alloys were entirely different. The as-cast and the melt-spun alloys should have different atomic motion mechanisms during the course of hydrogenation.

Effect of ternary Al, Ni additions on hydrogen absorption behaviour of Ti–45Nb alloys

Abstract The present study investigates the hydrogen absorption behaviour of a binary Ti–45Nb alloy and the effect of ternary additions of Ni and Al on the pressure–composition–temperature (i.e. p – c – T ) behaviour of this alloy. It is found that at a given temperature and partial pressure, the Al-containing alloy has the lowest hydrogen content, followed by the Ni-containing alloy, and that the binary alloy exhibits maximum hydrogen absorption capacity. A comparison of the enthalpy of solution as a function of hydrogen content of the three alloys shows that the enthalpy minimum for the Al-containing alloy occurs at the lowest hydrogen content, followed by that for the binary alloy, and the enthalpy minimum for the Ni-containing alloy occurs at the highest hydrogen content. Enthalpy of solution at infinite dilution of hydrogen shows a linear dependence on the lattice parameter of the host alloys. X-ray analysis of the hydrogen containing samples reveals suppression of the α-phase and the Ti 2 Ni-phase in the presence of hydrogen.

Thermodynamic properties of the NdNi5Sn-H system

The hydrogen absorption and desorption properties of the NdNi5Sn intermetallic compound were determined by means of Pressure–Composition–Temperature (P–C–T) measurements over the temperature range 258–423K. At 258K and 2.5MPa H2 pressure hydrogen storage capacity equals to 1.2at. H/NdNi5Sn. Two hydride phases are formed in the system. For β-NdNi5SnH∼1.0 the plateau pressure is one order of magnitude higher than for the similar La-containing LaNi5Sn-based β-hydride. A marked reduction in the hysteresis value was observed going from NdNi5 to the NdNi5Sn. The partial molar thermodynamic values for the formation of the hydride of NdNi5Sn are ΔHH=−14.9±0.6kJ(molH)−1 and ΔSH=−51.9±2.1J(K·molH)−1. Hydrogen trapping in the area of dilute H solutions at temperatures below 273K is suggested to explain the marked exothermic deviation of ΔHH.

Hydrogen physisorption capacities of mechanically milled activated carbon powders in a H2 atmosphere using a gravimetric method

We investigated the hydrogen physisorption characteristics of mechanically milled activated carbon (AC) powders in a H2 atmosphere using thermal desorption spectroscopy (TDS) and the pressure–composition–temperature (PCT) method. TDS observations showed that AC milled for 10h started to desorb hydrogen molecules (mass number = 2) at about 100°C. This suggests there are hydrogen molecules that are easy to desorb. Although we were able to obtain the PCT curves of the hydrogenated milled AC using a gravimetric method, we found that the physisorption capacity of this sample was smaller than that of the host. One reason for this is that the specific surface area of the milled AC decreased. However, the adsorption and/or desorption behavior of hydrogen molecules trapped in milled AC could not be observed within the useful temperature range of 0–50°C. We considered that hydrogen molecules trapped in AC by mechanical milling were probably adsorbed by an interaction other than physisorption, e.g. they were trapped in pores destroyed by mechanical milling, and it might be difficult to achieve desorption solely by controlling the hydrogen pressure. Accordingly, we believe that it will be impossible to use hydrogenated milled AC as a hydrogen storage material that employs the physisorption phenomenon.

Hydrogen absorption–desorption characteristics of the LaNi 5Sn intermetallic compound

The pressure–composition–temperature (P–C–T) relations in the LaNi 5Sn–H system were measured volumetrically at temperatures between 258 and 423 K. Two hydride phases, β-LaNi 5SnH 2 and γ-LaNi 5SnH 3 are formed in this system in addition to the α-solid solution with a limiting H content of 0.3 H atoms per formula unit. The calculated work loss due to hysteresis is rather small, 112 J (mol H) −1 at 298 K. The partial molar enthalpy and entropy calculated for the formation of the β-LaNi 5SnH 2 are −18.5±0.8 kJ (mol H) −1 and −53.7±2.3 J (K mol H) −1, respectively. An advanced van der Waals lattice gas model was applied to fit the isotherms. The critical temperature of the LaNi 5SnH 2 equals T C=421±17 K.

Effect of carbon addition on activation and hydrogen sorption characteristics of TiMn 1.25Cr 0.25 alloy

Effect of carbon addition on activation and hydrogen sorption characteristics of TiMn 1.25Cr 0.25 alloy is investigated. With increasing carbon contents, the lattice constants and the unit cell volume decrease. The activation of TiMn 1.25Cr 0.25 alloy becomes easier with the increasing carbon content. Auger electron spectroscopy (AES) results show that carbon addition does not reduce the thickness of oxide layer formed on the surface of TiMn 1.25Cr 0.25 alloy, but makes the Ti/Mn ratio of alloy top surface become closer to the bulk components, which probably is one of the reasons for the improvement of alloy activation performance. Pressure–composition–temperature (PCT) results present that carbon addition decreases hydrogen absorption capacity and increases the hydrogen absorption–desorption plateau pressure. In addition, the cycle-life of hydrogen absorption–desorption increases with increasing carbon concentration.

The effect of ball milling and equal channel angular pressing on the hydrogen absorption/desorption properties of Mg–4.95 wt% Zn–0.71 wt% Zr (ZK60) alloy

The microstructure of commercial alloy ZK60 (Mg–4.95 wt% Zn–0.71 wt% Zr) was refined by equal channel angular pressing (ECAP), high energy ball milling (HEBM) and by a combination of both methods. The effect of microstructure refinement on the hydrogenation properties of ZK60 was studied. The alloy processed by a combination of ECAP and HEBM was found to exhibit the highest rate of hydrogen desorption at 300 °C and the final pressure of 2 atm in the desorption volume. Similar, but slightly slower, desorption kinetics was observed for the alloy processed by ECAP alone. The desorption times for 5.2 wt% of H2 were 20 and 26 min for the alloys processed by combined ECAP/HEBM and by ECAP alone, respectively. The hydrogen absorption capacity at 300 °C of the alloy processed by combined ECAP/HEBM was 6.2 wt% of H2. The pressure–composition–temperature diagrams of this alloy in the temperature range of 280–350 °C were determined. The enthalpy and entropy of hydrogen absorption/desorption were 76.8 kJ/mol and 141.8 J/mol K, respectively. No pressure hysteresis with regard to hydrogen absorption/desorption was found for the alloys processed by combined ECAP and HEBM. This hydrogenation behavior was correlated with the microstructure of bulk samples and the morphology of dehydrogenated powders observed by transmission electron microscopy and high resolution scanning electron microscopy.

Thermodynamic evaluation of the Ta–O system from pure tantalum to tantalum pentoxide

The partial thermodynamic functions of the tantalum–oxygen system (up to O/Ta=2.5) were evaluated from 873 to 3120 K. The pressure–composition–temperature ( p– C– T) relationship in the single-phase region of solid Ta was investigated. Relationships for other phases where no data are available in the literature were proposed based on thermodynamic analysis. The results are presented as oxygen isobars superimposed on a phase diagram.

TiCr1.2(V–Fe)0.6—a novel hydrogen storage alloy with high capacity

To develop a new alloy with high hydrogen storage capacity, partial Cr in TiCr1.8 was substituted by V—Fe alloy. X-ray diffraction and metallography observation proved that the substitution did not change the phase constitution of TiCr1.2(V–F)0.6. It still remained in its initial Laves phase related body centred cubic structure. Pressure–composition—temperature tests showed that the hydrogen storage capacity could reach up to 3.2 wt% and its reversible capacity to about 2.0 wt%. TG-DSC tests showed that there are two kinds of tetrahedral site for hydrogen atoms to locate.

Study of the structure and the electrochemical properties of Ml(NiCoMgAl) 5.1− xZn x hydrogen storage alloy

In this paper a new type of hydrogen storage alloy Ml( NiCoMgAl) 5.1−x Zn x (0⩽x⩽0.3) was obtained by vacuum inductive melting of La-rich mixed mischmetal elements with Ni and some other alloy elements such as Co, Mg, Al and Zn. The pressure–composition–temperature ( P– C– T) tests show that the highest hydrogen concentration can reach 1.58 wt% for Zn-free ( x=0) composition and then decreases from 1.44 wt% (x=0.2) to 1.19 wt% (x=0.3) , respectively, with the increase of Zn in the alloys. The electrochemical tests show that the electrochemical capacity of alloys reaches a very high level, 380 mAh/ g for x=0, and then decreases to 366 mAh/ g (x=0.1), 345 mAh/ g (x=0.2) and 271 mAh/ g (x=0.3) , respectively, with the increase of Zn in the alloys. The attenuation rate of capacity decreases from 16% ( x=0) to 4% ( x=0.3) after 100 charging/discharging cycles. Proper Zn content (0< x<0.2) influences the electrochemical capacity of alloys a little, but it improves the cycle stability and the discharge ability, especially the high rate discharge ability. Optic microscopy, SEM and EDX analyses indicate that the main part of alloys are Mg-free and Mg-contained LaNi 5-based solid solutions. The latter is the key to the high electrochemical capacity. The addition of Zn makes the secondary phase change from LaNi 2 to Co-rich phase in the alloys and also influences their quantity and shape.

Structural and thermodynamic properties of the pseudo-binary TiCr 2− xV x compounds with 0.0≤ x≤1.2

The TiCr 2− x V x compounds with 0.0≤ x≤1.2 series have been synthesised and characterised by X-ray powder diffraction. X-Ray qualitative and quantitative phase analysis has been carried out on the as-cast alloys using the Rietveld method. The refinements of the structure shows that the materials crystallise either in the hexagonal or in the cubic Laves phase type for low V contents. For x>0.6, the system is found of b.c.c.-type structure only. The pressure–composition–temperature ( P– C– T) isotherms measured at 298 K show that the as-cast alloys absorb large amounts of hydrogen, from 4 to 5.2 H/f.u. The P– C– T diagrams reveal also the presence of a relatively flat plateau, and a large hysterisis effect, and correspondingly the hydride cannot be completely dehydrogenated.

メカニカルアロイング法により作製したＴｉ‐Ｆｅ合金の構造と水素吸蔵特性

Elemental Ti and Fe powders were mechanically alloyed using a high-energy ball mill and a conventional ball mill in an argon atmosphere. The morphology, structure, and hydriding behavior of the resultant powders were compared. The powders milled for 100h using a conventional ball mill consisted of Ti and Fe phases together with TiFe phase. On the other hand, those milled for 100 h using a high-energy ball mill consisted mostly of the TiFe phase together with a small amount of Ti and amorphous phases. The mechanically alloyed powders were easily absorbed hydrogen after one step activation with heating at 723 K in vacuum and subsequent hydrogenation at room temperature. The characteristics of the pressure-composition-temperature (PCT) curve were deeply dependent on the amounts of Ti and TiFe phases in the mechanically alloyed powders. The powders milled for 100 h using a high-energy ball mill had a lower plateau pressure than those milled for 100 h using a conventional ball mill. These mechanically alloyed Ti-Fe powders had a lower plateau pressure than the fully activated Ti-Fe alloy counterparts.

Degradation behavior of LaNi 5− xSn xH z ( x=0.20–0.25) at elevated temperatures

Systematic studies of the hydriding behavior of LaNi 5− x Sn x alloys with tin contents in the range 0.20< x<0.25 have revealed changes in the pressure–composition–temperature ( P– C– T) isotherms measured after heating the hydrides above 450 K. Some loss in reversible capacity was observed along with reductions in the plateau pressures and hysteresis ratios while the slopes of the plateaus became greater. These changes are indications of degradation processes and increased disorder within the alloy structure. Additional experiments were performed for long periods (i.e. >1000 h) at elevated temperatures and hydrogen pressure to produce further degradation in the P– C– T isotherms. The impact of alloy composition on the isotherms has been determined. The crystal lattice properties of the alloys before and after hydrogen reactions have been studied using high-resolution X-ray powder diffraction with synchrotron radiation. Changes in these X-ray diffraction patterns are correlated to various structural modifications resulting from hydride formation and degradation.

Hydrogenation characteristics of Ti 2Ni and Ti 4Ni 2X (X=O, N, C)

Ternary Ti 4Ni 2X (X=O, N, C) compounds and a binary Ti 2Ni compound were compared based on their hydrogen pressure–composition–temperature (PCT) relations. It was demonstrated that the ternary compounds desorbed most of the absorbed hydrogen under moderate conditions such as room temperature and atmospheric pressure. The hydrogen desorption pressures of the Ti 4Ni 2X compounds were more than two orders of magnitude higher than the desorption pressure of the binary compound. In spite of the significant increase in the hydrogen desorption pressure, the slope of the PCT curve of each ternary compound was not large compared with that of the Ti 2Ni one. These four Ti 2Ni-based compounds and their corresponding hydrogen occlusion ones were discussed in terms of their relative thermal stability.

Assessment of Zr–V–Fe getter alloy for gas-gap heat switches

A commercial Zr–V–Fe alloy (i.e. SAES Getters trade name alloy St-172) has been assessed as reversible hydrogen storage material for use in actuators of gas-gap heat switches. For applications involving hydride compressors in closed-cycle Joule–Thomson sorption cryocoolers, the actuator need to produce a conducting (i.e. ON) state pressure above 670 Pa and an insulating (i.e. OFF) state pressure below 0.13 Pa in the gas-gap switch with switching times ∼200 s between the two states. Pressure–composition–temperature isotherms have been measured for the SAES St-172 material to define power efficient baseline performance at appropriate hydrogen concentration for these heat switch actuators. Two prototype actuators containing the SAES St-172 material were built and operated for several thousand cycles to evaluate performance of the metal hydride system under conditions simulating heat switch operation.

Hydrogen absorption–desorption isotherms of La (28.9)Ni (67.55)Si (3.55)

Pressure–composition–temperature ( P– C– T) isotherms of La (28.9)Ni (67.5)Si (3.55) which was fabricated by substitution of Ni by Si in LaNi 5 at different temperature has been studied in the present work. It was found that the activation of the alloy is sufficiently easy. The saturation value of H/ M of this alloy is about 1.0 for P– C– T at room temperature ( 293 K ). The H/ M values were found to be 0.9, 0.85, 0.8 for P– C– T isotherms at 313, 323, 333 K , respectively, and equilibrium pressures were 1.01×10 5, 1.41×10 5, 1.71×10 5, 2.12×10 5 Pa at these temperatures. The investigations show that the wt% of hydrogen in the alloy was about 1.75%. The maximum amount of hydrogen released by the alloy is in the temperature range 313– 343 K . For P– C– T curves the α+ β phase decrease at higher temperatures. This makes it a better alloy for hydrogen storage for technological applications than LaNi 5.

The effects of heat treatments on the microstructure and electrochemical properties of the ZrCr 0.7Ni 1.3 multiphase alloy

Qualitative and quantitative X-ray analysis using the Rietveld method of the as cast ZrCr 0.7Ni 1.3 alloy, shows an average phase abundance of 62.7% C15-Laves phase, 36.6% of Zr 7Ni 10 and less than 1% of chromium. The annealed alloy at 1373 K reveals a complete disappearance of pure Cr with an important reduction of the amount of the Zr 7Ni 10 phase based on EDS/SEM and XRD results. Pressure-Composition-Temperature (PCT) measurements show that the alloy behaves as a single phase with faster kinetics. Electrochemical measurements shows a relatively high discharge capacity of the cast alloy, c=320 mAhg −1 after 20 cycles. Contrary to that, the discharge capacity of the annealed alloy decreases by about a factor of 2 with c=156 mAhg −1.

Thermal properties of hydride fuel 45% U–ZrH 1.6

Hydride fuel of 45% U–ZrH 1.6 was fabricated and the measurement of its thermal properties was carried out. The thermal conductivity of 45% U–ZrH 1.6 in the range of room temperature −773 K was evaluated to be about three times higher than that of UO 2 and nearly independent of temperature. Such a high thermal conductivity is favorable when this material is used as nuclear fuel. The pressure–composition–temperature (P–C–T) characteristics of UZr alloy hydrogen system were different from that of pure Zr. A plateau of the hydrogen pressure was found for H concentration (relative to the Zr concentration) between 0.2 and 0.6 for all temperature below 1173 K. This plateau region was attributed to UZr hydride formation from the UZr alloy. Over the plateau region in H concentration from 1.0 to 1.4, the equilibrium hydrogen pressure was a little higher than that of pure Zr, which was attributed to the reduced activity of Zr in UZr alloy. The activity of Zr calculated from the equilibrium hydrogen pressure was 0.90 at 1073 K, and 0.83 at 973 K.

A computer-controlled apparatus for performing pressure–composition–temperature measurements on metal hydrides with protium, deuterium, and tritium

An apparatus for measuring protium, deuterium, and tritium pressure–composition–temperature (PCT) thermodynamic equilibrium behavior on metal hydrides has been designed, constructed, and operated with deuterium at the Weapons Engineering Tritium Facility at Los Alamos National Laboratory. The apparatus is capable of performing PCT measurements with the three hydrogen isotopes at a maximum pressure of 3.45 MPa, and sample temperatures in the range 25–300 °C. A personal computer that controls the system provides the capability for manual or automated operation. Two deuterium PCT isotherms on palladium at 51 and 70 °C were measured with the apparatus in the automated mode to demonstrate its operation. Comparison of the resulting isotherms with literature data showed a plateau pressure at 70 °C in close agreement with the literature value (57.06 kPa versus literature value of 57.33 kPa). The deuterium capacity at 70 °C in palladium at the high end of the isotherm was measured both by the PCT apparatus and by weighing the sample before and after the PCT measurement. The deuterium capacity determined by the PCT apparatus (D/M=0.624 at 144.52 kPa) agreed well with the value determined by the weight difference measurement (D/M=0.628, at 144.52 kPa). These values agreed closely with the literature value (D/M=0.629 at 144.52 kPa).