Tech Articles
Volumetric
The following summary of the experimental considerations and error sources in the measurement of hydrogen absorption by hydrogen-absorbing materials using volumetric, or manometric, apparatus (the Sieverts Method) applies to measurements made in the temperature range from ambient to high temperatures in the region of 673 K. An upper pressure range is not stated; however, particular attention must be paid to the validity of the equation of state for hydrogen throughout the pressure range of interest at the measurement temperature.
Error Source / Experimental Consideration | Guidelines |
| Calibration | Volume, temperature sensor and pressure measuring device calibration are essential. The sample cell dead space volume must be determined to sufficient accuracy. |
| Temperature Monitoring and Control | The temperature of the calibrated dosing volumes (the gas delivery system) should be thermostatted and/or room temperature fluctuations monitored carefully. Sample environment (for example, a water bath or furnace) temperature should be controlled to an acceptable level. Cold or hot spots in the dosing volumes and cold spots in the sample cell should be eliminated. The region of apparatus containing the temperature gradient must be minimized and accounted for in the calculation of the absorbed quantity. |
| Sample Temperature Measurement | The temperature sensor can be in direct contact with sample, and should be monitored continually throughout the measurement. |
| Thermal Effects from the Sample | Thermal effects from the sample may be severe, depending partially on bed size. The sample temperature should therefore be monitored carefully through each sorption step to ensure that it has returned to equilibrium, once a sufficient amount of hydrogen has been absorbed or desorbed by the sample. |
| Approach to Equilibrium | The pressure relaxation should be monitored at each absorption or desorption step to ensure that the sample has reached equilibrium, or at least an acceptable level of equilibrium. The temperature should also be monitored. |
| Sample Size Choice | Should be large enough to allow accurate mass determination, for example > 50 mg, but should ideally be matched to the size of the system volume and the pressure measurement accuracy. The sample size must not be too large as to allow significant thermal inhomogeneity (therefore bed sizes greater than a few grams should be avoided). The measurement of different sample sizes should not result in different wt.% uptakes; if this is the case it could indicate problems with the sample size choice and/or the sensitivity of the system. |
| Gas Purity | A minimum of 99.999 %, although the criterion may be higher (for example > 99.9995 %) for more sensitive materials. Filtration should be considered, in the case of more sensitive samples, particularly if gas delivery lines are present. If present, the filter system should be regenerated regularly. An oil-free pumping station is essential to eliminate additional contamination. |
| System Volume to Sample Size Ratio | For a given sample size, the system volume must be small enough for the pressure measuring device to detect the required or expected drop in pressure at a given absorption or desorption step, but not so small that the pressure drop required for a given sample size is too large, as this may lead to larger compressibility errors. Note that it is the system volume (the combined dosing and sample cell volume) that is important, not just the dosing volume. |
| Sample Degassing | Should be performed under vacuum conditions sufficient to remove environmental adsorbates that may inhibit the initial activation process. |
| Sample Pretreatment and History | Must be recorded as it can have significant effects on the measured uptake. Full sample details should be included to enable the comparison of data, including sample synthesis details, and sufficient microstructural characterisation of the starting material or compound. The activation procedure should be described (for example, degassing procedure, hydrogen pressures and temperatures, number of cycles, or the mechanical activation procedure, and so forth). If two measurements on the same material are to be compared, the degassing/activation procedures used must be comparable. |
| Pressure Measurement | The uncertainty in the pressure measurement should be substantially lower than the difference in pressures before and after an absorption or desorption step. |
| The Compressibility of Hydrogen | Should be represented accurately, using for example the 32-term modified Benedict-Webb-Rubin (BWR) equation (see the NIST Standard Reference Database 23) or the Hemmes et al EOS. |
| Thermal Transpiration Effects | If accurate measurements are to be performed at low pressures (for example < 100 Pa, depending on apparatus dimensions) corrections should be applied to pressure data using an accepted expression. |
| Accumulative Errors | Accumulative errors are a likely error source. Their presence can be tested by measuring isotherms with different step sizes, or comparing single step measurements with complete isotherms. However, material-related effects should also be taken into consideration. |
| Leakage | Thorough leak testing should be carried out with helium and/or hydrogen up to the maximum measurement pressure. |
Gravimetric
The following summary of the experimental considerations and error sources in the measurement of hydrogen absorption by hydrogen-absorbing materials using gravimetric apparatus applies to measurements made in the temperature range from ambient to high temperatures in the region of 673 K. An upper pressure range is not stated; however, in gravimetric measurement, particular attention must be paid to the application of buoyancy effect corrections as these increase significantly with increasing pressure (or hydrogen density). The sample density value used in the buoyancy effect corrections must be chosen with care. Attention should also be paid to the validity of the equation of state for hydrogen used throughout the pressure range of interest at the measurement temperature.
Error Source / Experimental Consideration | Guidelines |
| Calibration | Microbalance, temperature sensor and pressure measuring device calibration are essential. |
| Temperature Monitoring and Control | The temperature of the microbalance cabinet or enclosure should be well thermostatted. Sample environment (for example, a water bath or furnace) temperature should be controlled to an acceptable level. |
| Sample Temperature Measurement | The temperature sensor will be in the vicinity of the sample, and should be monitored continually. The temperature measured near to the sample will be affected by the hydrogen pressure in the chamber. Hydrogen has a relatively high thermal conductivity, which may increase the effects. Complementary measurements (for example, thermomagnetometry) can be used to check the correspondence of the sample and measurement temperatures. |
| Thermal Effects from the Sample | Thermal effects from the sample may be severe. The sample temperature should therefore be monitored through each sorption step to ensure that it has returned to equilibrium, once a sufficient amount of hydrogen has been absorbed or desorbed by the sample. |
| Approach to Equilibrium | The mass change should be monitored carefully at each absorption step to ensure that the sample has reached equilibrium, or at least an acceptable level of equilibrium. The temperature should also be monitored. |
| Sample Size Choice | This will be determined primarily by the balance capacity and sensitivity. Accurate sample mass determination is possible for very small masses but the sample size should not be reduced too far (for example < 50 mg, for a balance of < 1 μg resolution) due to the low mass of hydrogen, although this depends to a certain extent on the expected hydrogen uptake. |
| Gas Purity | A minimum of 99.999 %, although the criterion may be higher (for example > 99.9995 %) for more sensitive materials. Filtration should be considered, in the case of more sensitive samples, particularly if gas delivery lines are present. If present, the filter system should be regenerated regularly. An oil-free pumping station is essential to eliminate additional contamination. |
| Sample Degassing | Should be performed under vacuum conditions sufficient to remove environmental adsorbates that may inhibit the initial activation process. |
| Sample Pretreatment and History | Must be recorded as it can have significant effects on the measured uptake. Full sample details should be included to enable the comparison of data, including sample synthesis details, and sufficient microstructural characterisation of the starting material or compound. The activation procedure should be described (for example, degassing procedure, hydrogen pressures and temperatures, number of cycles, or the mechanical activation procedure, and so forth). If two measurements on the same material are to be compared, the degassing/activation procedures used must be comparable. |
| Pressure Measurement | The pressure measurement is not as crucial to the measurement of the absorbed quantity as it is in volumetric measurement, but must still be measured to sufficient accuracy. |
| The Compressibility of Hydrogen | Should be represented accurately, using for example the 32-term modified Benedict-Webb-Rubin (BWR) equation (see the NIST Standard Reference Database 23) or the Hemmes et al EOS. |
| Thermal Transpiration Effects | If accurate measurements are to be measured at low pressures (for example < 100 Pa, depending on apparatus dimensions) corrections should be applied to pressure data using an accepted expression. Thermomolecular flow disturbances of the balance should also be monitored and/or determined. |
| Buoyancy Effect Corrections | Buoyancy effect corrections must be applied to the measured data to take account of the displacement of the hydrogen by the sample, the sample holder and the balance hangdown, and any other components attached to the balance. The density of hydrogen at the measurement temperature and pressure must be represented accurately. |
| Leakage | Thorough leak testing should be carried out with helium and/or hydrogen up to the maximum measurement pressure. |
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