Sieverts-type apparatus

PCT and Kinetic Curves Determination by Volumetric Method in a Sieverts-Type Apparatus... 

The hydrogen desorption or absorption PCT curves of various hydrides can be evaluated using a Sieverts-type apparatus. The scheme of typical Sieverts-type apparatus is shown in Fig. 1.31. 

The Sieverts-type apparatus consists of a calibrated volume determined physically, a reactor whose temperature is controlled by the temperature control system and the cooling system, a vacuum system, a pressure monitoring system, valves, and source of hydrogen and argon delivery. The quantity of desorbed hydrogen (number of molls) is calculated using ideal gas flow ... 

Fig. 1.31 Scheme of Sieverts-type apparatus where T transducer, hydrogen cut-off valve, argon cut off valve, vacuum system cut-off valve, reactor cut-off valve, calibrated volume and its cut-off valve, V vent valve, R reactor... 

To determine PCT cnrve by volnmetric method at first we have to know mass of analyzed powder (hydride or pnre metal). The typical mass of powder used in volumetric method is in a range 50-500 mg and depends on (reactor volume with volume of connecting pipes, valves, and transdncer). After the mass measurement, the powder is loaded into specimen holder and then it is placed in the Sieverts apparatus reactor. To prevent any oxidation and for safety reason the system must be purged a few times by argon and then evacnated. However, one must be careful how much powder is appropriate for the absorption/desorption volume of a Sieverts-type apparatus. 

In our laboratory, absorption experiments were carried out on an unmiUed but activated ABCR MgHj powder. In the preliminary stage, the powder was heated up to 350°C in a volumetric Sieverts-type apparatus (Sect. 1.4.2) under a preventing pressure of 3.5 MPa (so as not to allow desorption), held for about 30-45 min to stabilize temperatnre, and then snbjected to the following desorption/absorption cycles ... 

Fig. 2.27 XRD patterns of MgH2 (Tego Magnan) powders milled continuously for 100 h under the IMP68 mode in argon and subsequently desorbed directly after milling in a Sieverts-type apparatus at various temperatures...

Fig. 2.53 Desorption kinetic curves at various temperatures obtained in a Sieverts-type apparatus under 0.1 MPa of hydrogen pressure of (a) ABCR MgH + n-Ni (SSA = 14.5 mVg) baU-miUed for 15 min and (b) Tego Magnan MgH + m-Ni ball-milled for 20 h...

Imamura et al. [137] studied absorption of Mg ball-milled with graphite and benzene as milling additives. In a Sieverts-type apparatus, the mixture after 20 h milling was able to absorb at 180°C. This in itself is nothing outstanding because milled, activated, and cycled MgH can also absorb at 200°C (Fig. 2.22a). Bouaricha et al. [138] also studied absorption of a Mg -i- graphite mixture at 300°C, which showed much better kinetics than that of just milled Mg. A number of researchers studied both absorption and desorption [25, 139-141] but unfortunately, all desorption studies were conducted in vacuum. However, it must be pointed out that even under vacuum conditions, desorption kinetics were no better than those obtained with a number of other additives discussed earlier in the text. The lowest desorption temperature applied was 290°C. 

Unfortunately, quite promising hydrogen desorption behavior in DSC as shown in Fig. 3.4 did not translate into desorption in a Sieverts-type apparatus as shown in Fig. 3.5. The powder milled sequentially for 270 h desorbed in a Sieverts-type apparatus at 250 and 290°C (Fig. 3.5) under primary vacuum only about 1.2 wt%Hj which is approximately a half of the hydrogen content obtained during DSC and TGA tests. No desorption of hydrogen was detected in a Sieverts-type apparatus at 250 and 290°C after 128 and 70 min, respectively, from the powder continuously milled for 270 h. The latter easily desorbed 3.13 and 2.83 wt%Hj in DSC and TGA... 

Fig. 3.5 Desorption curve at 290°C under primary vacuum obtained in a Sieverts-type apparatus from the 2Mg-Fe mixture reactively nrUled in a sequential mode for 270 h...

Fig. 3.8 (a) Desorbed hydrogen (wt%) from the NaAlH milled for 5 h as a function of heating time while heating to 225, 300, 400 and 425°C. Quasi-Temperature Programmed Desorption (TPD) carried out in a Sieverts-type apparatus, (b) Temperature profile change as a function of time while heating to 225, 300 and 400°C in a Sieverts apparatus... 

The effect of catalytic metal chloride additives on the kinetics of isothermal decomposition of LiAlH in a Sieverts-type apparatus has been studied by a few research groups and the results seem to be rather contradictory. 

Figure 2.6a shows typical kinetic curves of first desorption carried out in a Sieverts-type apparatus at the initial hydrogen pressure of 0.1 MPa (atmospheric pressure of 1 bar) for the as-received, nonmilled, and nonactivated Tego Magnan powder. For each temperature, a fresh load of sample was desorbed. At each temperature in Fig. 2.6a, the desorption process is complete with 100% of MgH2 des-... 

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