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J. Phys. Chem. C 2007, 111, 1716-1724
Hydrolysis Studies of Polycrystalline Lithium Hydride Carol L. Haertling,* Robert J. Hanrahan, Jr., and Joseph R. Tesmer Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: June 26, 2006; In Final Form: August 23, 2006
Polycrystalline LiH was reacted with decarbonated H2O to determine reaction products, rates, mechanisms, and the effects of experimental parameters. Rutherford backscattering analyses for measurement of elemental concentrations showed that product O growth rates showed an initial rise and then increased linearly with H2O exposure time. Increasing H2O concentration increased O growth rate while both increasing temperature and pressure decreased O growth rates. A thin-layer, diffusion-controlled reaction rate is suggested to explain the results, and a growth process for LiOH is illustrated. Micrographs of polycrystalline LiH show a twophase bulk material and a surface hydrolysis layer with cracks.
1. Introduction Lithium hydride (LiH) has unique nuclear and chemical properties that have made it a popular material for study. Most remarkably, its combination of high hydrogen density and low weight density makes it attractive as a neutron shield and as a hydrogen fuel source. LiH is highly reactive with water, easily forming reaction products. The reaction products between LiH and H2O are LiOH and H2(g) for reactions at room temperature, ambient pressure, and H2O concentrations below 523 Pa. Li2O is another product that has been both experimentally observed1,2 and calculated to exist at these conditions.3 Li2O occurs as a layer at the interface of LiH and LiOH. A schematic of the layer structure is given in Figure 1. LiH hydrolysis has been studied previously, using widely varying materials and experimental conditions. Here, we have used depth-profiling capabilities offered by Rutherford backscattering spectroscopy (RBS), as well as other analytical techniques, to offer unique insights into the hydrolysis process. Our goal is to better understand the reactions and products that occur during hydrolysis of LiH, as well as to determine the mechanisms and kinetics of the reactions.
Figure 1. Schematic of LiH hydrolysis product layers that have been observed and proposed for LiH hydrolysis reactions at room temperature, ambient pressure (101.3 kPa), and with typical concentrations of H2O in air.1-3
2. Experimental Methods 2.1. Sample Preparation. The LiH used in our experiments comprises 6Li (primarily) and 1H. Here, we refer to the material as simply LiH, as the specific Li isotope is not important for hydrolysis studies. We formed LiH samples into cylindrical compacts of LiH particles that were pressed together and sintered into a monolithic structure at elevated temperatures. The LiH was machined to a size of ∼2 cm diameter × 5 mm thickness. Figure 2 is a photograph of a typical sample. Just prior to analysis, sample surfaces were ground with SiC paper to an ∼16-µm finish. Energy dispersive spectrometry showed Si to be present on the surface of the samples, indicating the presence of some contamination from grinding paper. Particle induced X-ray emission was used to determine impurities present in the samples. The major impurities were Si, Ca, and Cl. Smaller * Address correspondingence to this author. E-mail:
[email protected]. Phone: 505-665-9058. Fax: 505-667-5268.
Figure 2. Photograph of a typical pressed polycrystalline LiH sample.
quantity impurities included N and O. Carbon and elements lighter than C were not detectable by this technique. As LiH is highly reactive with environmental contaminants, extensive measures were used to minimize the incidence of unintended corrosion. A N2 glovebox with e10 ppmv H2O was used to prepare and load the samples into an Al sample holder. Samples were directly transferred to an environmental chamber through the glovebox antechamber and were never exposed to air. 2.2. Sample Exposures. The environmental chamber was used to transport samples between the N2 glovebox and the RBS chamber without corrosion, as well as to expose the samples to controlled environments. The environmental chamber contained ports for vacuum or for gas entry, as well as temperature and
10.1021/jp0639896 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007
Polycrystalline Lithium Hydride
Figure 3. Schematic of the sample environmental chamber (with exposure capabilities) and the RBS analysis chamber.
pressure control capabilities and monitors. When the environmental chamber was attached to the RBS chamber, samples were passed through a load-lock directly into the vacuum environment of the RBS chamber. A schematic of the environmental chamber is shown in Figure 3. RBS and exposures were completed sequentially to obtain discontinuous kinetic measurements, i.e., samples were moved into the RBS chamber and analyzed, moved into the environmental chamber and exposed, then moved back into the RBS chamber for analysis, and so on. Samples were exposed to ultrahigh purity Ar at 200 ( 5 mL/ min containing 540-3300 ppmv H2O (flow rate and H2O levels will be combined so that discussion uses units of mmol/min for H2O concentrations). A dew-point generator and gas manifold were used to create and flow moist gas streams. H2O levels were monitored just prior to entry into the environmental chamber; gas flow rates were verified with a digital flow meter. A chilled mirror hygrometer was used to determine the H2O concentrations of the experiments, but for routine H2O monitoring, a capacitance-based hygrometer was used that had less accuracy and precision (precision (2 °C dewpoint) than the chilled mirror instrument. Carbonic acid was removed from the H2O by bubbling with Ar. The lowered C levels were indirectly confirmed by pH measurements, which indicated pH levels of ∼8-9. Samples were exposed up to several thousand minutes (sum of all exposures). Although many experiments took place at ambient temperature and pressures, others used slightly elevated sample temperatures and pressures. LiH samples were radiantly heated during H2O exposure, using a halogen light bulb contained in the environmental chamber. Sample temperature was determined from a thermocouple that was in contact with an Al sample holder containing the LiH samples. Sample temperatures ranged from room temperature (∼24-26 °C) to 80 °C . Elevated temperature experiments were electronically controlled to (5 °C. Sample pressures were controlled with a pressure relief valve at the gas outlet of the environmental chamber; chamber pressures were monitored by a pressure transducer. Outlet pressures ranged from atmospheric (∼80 kPa) to 152 kPa. 2.3. Rutherford Backscattering Spectroscopy. A. Technique. RBS is a generally nondestructive technique in which a particle beam collides with target atoms and scatters back at energies indicative of both the target elements and element depth. A single measurement identifies the elements and their relative depth in a sample without destructively removing surface layers. Further, RBS data may be modeled for determination of diffusion coefficients and determination of compounds. While other techniques are available to perform these studies, RBS offers two specific advantages: (1) both hydrolysis products and their thicknesses can be identified in a single
J. Phys. Chem. C, Vol. 111, No. 4, 2007 1717 measurement and (2) the depth penetration and spot size are well suited for our LiH material. Our LiH grains can be many microns in diameter; RBS will show results for corrosion layers up to ∼25 µm thick, depending upon the measurement conditions. B. Measurements and Analysis. An NEC 3 MV 9SDH-2 Tandem Accelerator was used to perform RBS measurements. As H loss can occur from interaction of the beam with LiH, an unused spot on the sample was used for each measurement. A comparison of calculated O concentration values at six locations from each of three samples (all machined from the same larger piece) showed that the percentage ratio of O concentration standard deviation to average was ∼10%. Therefore, variations in bulk element concentrations may be largely attributed to sample inhomogeneity. Measurements used a scattering angle of 167° (Cornell geometry) and a beam size of ∼1 mm × 1 mm. The RBS technique has an absolute accuracy of ∼5 atom %. Measurements that compare relative amounts of elements give better results, ∼1-2 atom % accuracy. A 2.3 MeV H+ (proton) beam was used for determination of O concentrations. The H+ spectrum has relatively greater peak separation, so that elements measured with this beam and energy show spectra with relatively greater energy separation between peaks for different elements. Hence, O can be measured to a relatively greater distance into the sample. A 5.7 MeV He2+ (R) beam was also used for RBS measurements. This beam obtained results with an enhanced signal (resonance) for O and C. The spectrum and element peaks have greater peak size at this beam/energy combination, thus allowing greater precision in determining elemental concentrations. The He2+ beam was used to determine O diffusion coefficients. The computer program RUMP (version 2.0, Computer Graphic Service, Ltd.) was used to analyze the data. O and C peaks were integrated to determine relative concentrations of the elements. Simulations of spectrums were performed with SIM, a subset of RUMP. The simulations give indirect information on stoichiometry (phases) and hydrolysis layer thicknesses. 2.4. Microscopy. Scanning electron microscopy (SEM) was completed with a JEOL JSM-6300 FXV field emission gun microscope at 5 kV. Samples were transferred from a glovebox to the microscope in a transfer device fabricated for transferring LiH samples without air exposure. The transfer device consists of a reservoir for containing a sample along with a lid that slides off the reservoir when the reservoir and sample are placed into the SEM chamber. When samples are loaded into the reservoir in a glovebox, the device eliminates atmospheric exposure during transfer. A Nikon Epiphot optical microscope was used for optical microscopy. A sample preparation procedure appropriate for LiH polycrystalline samples was developed to prepare samples for optical microscopy. The procedure is an update of a procedure developed by Wang et al.4 The preparation procedure was completed in air, but with hydrocarbon chemicals that do not significantly react with LiH (except for the final etch step). These chemicals also provided a barrier for the short-term reaction of LiH with air. Optical micrographs were taken in air, but within 15 min of sample preparation to minimize corrosion. Any corrosion product that formed did not appear to significantly affect the observed grain structure. 3. Results 3.1. Rutherford Backscattering Spectroscopy. A. Effect of H2O Exposure. Figure 4 shows a typical H+ RBS spectrum for unexposed polycrystalline LiH. The y-axis is atomic intensity
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Figure 4. Typical H+ RBS spectrum (2.3 MeV) for unexposed polycrystalline LiH showing the presence of Li, C, O, and a nuclear reaction (fission of 6Li). Surface positions for elements are marked.
in units of counts, or more accurately, atoms/(ion‚steradian). The x-axis is the energy of a backscattered particle, in channel number units. A peak present at a greater channel number indicates the presence of an element of greater mass. The various peaks have been labeled with their element association; the position of the label is the position where an element on the surface of the sample would appear. Thus, a peak that is wider (extends to lower channel numbers) shows the presence of the element extending deeper into the bulk. The height of a peak will depend not only on the concentration present in the sample, but also on the scattering cross section of the particular element at that energy. Therefore, peaks of a particular element should not be compared (other than for gross qualitative purposes) until further calculations are completed. For example, the diminished 6Li signal at low channel numbers is a reflection of diminishing cross sections. Signals for different elements are additive; so, for example, a C peak may be sitting on top of an O peak in a spectrum. Peaks for 6Li and 7Li, as well as O and C, are seen in Figure 4. The unexposed sample shows O and C peaks, indicating that a corrosion layer on the surface is already present; the layer was unavoidable for our experiments due to the highly reactive character of LiH. At channel numbers less than ∼660, the O peak reflects the bulk concentration of O in the sample. Examination of the spectrum further shows that a small bulk C concentration is present. These bulk concentrations are to be expected for polycrystalline LiH, which comprises compacted particles, each containing their own corrosion layer. The spectrum does not show a peak for H since it does not backscatter the beams used for measurement. A background is present at ∼840 channels and below, which we attribute to both a nuclear reaction between 6Li and H+, and to contaminants. The background was subtracted from all yield determinations. Figure 5 shows spectra for a sample that was exposed to 37 mmol/min flowing H2O in Ar for various amounts of time. Clearly, the O peak originally at the surface moved significantly into the bulk of the sample with increasing exposure. The Li peaks show a lower Li concentration at the surface with increasing exposure, as would be expected for Li that has become “diluted” with O. The C peak grew minimally, if at all. Figure 6 shows curves for measured product O concentrations (with bulk O concentrations omitted) vs H2O exposure time for
Haertling et al.
Figure 5. H+ RBS spectra (2.3 MeV) for polycrystalline LiH exposed to 37 mmol/min decarbonated H2O.
Figure 6. Oxygen concentration as a function of exposure time to various concentrations of flowing H2O at ambient temperature and pressure.
polycrystalline LiH at room temperature and several H2O concentrations. All curves show an initial rise followed by a linear slope. Slopes (reaction rates) have been determined for linear portions of the curves and are listed in Table 1. The rates are typically 1 × 1015 to 10 × 1015 atoms/(cm2‚mmol), which corresponds to ∼1-10 monolayers/min. The curves clearly show that greater H2O concentrations increase the reaction rate. The relationship is shown in Figure 7. The calculated reaction rates given in Table 1 show O concentrations as a function of time. The O concentrations reflect the LiOH concentration, from the overall reaction to form LiOH, i.e., LiH + H2O f LiOH + H2. However, the measured O concentration includes O species which are diffusing into the LiH bulk and that are not necessarily in the form of LiOH. Therefore, the absolute value of the O concentration is not strictly equivalent to the LiOH concentration; nonetheless, the LiOH layer growth rates are expected to be the same as the O layer growth rate. B. Effects of Temperature and Pressure. Figure 8 shows curves for calculated product O concentration vs H2O exposure time for polycrystalline LiH exposed to 37 mmol/min H2O concentration at ambient pressure and at several temperatures. Figure 9 is a similar plot for samples exposed to 37 mmol/min H2O concentration at room temperature and two pressures.
Polycrystalline Lithium Hydride
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TABLE 1: Reaction Rates for LiH Hydrolysis and Diffusion Coefficients for an O Species in LiH at Various Parametersa sample temp (°C)
pressure (kPa)
H2O concn (mmol/min)
final O growth rate (atoms/[cm2‚min])
standard deviation (n ) 2)
diffusion coeff for O in LiH (cm2/s)
rtb 40 40 rtb rtb rtb
80 80 80 80 80 152
37 37 37 6 24 37
1.64 × 1016 1.43 × 1016 2.26 × 1015 1.20 × 1015 5.41 × 1015 1.11 × 1016
1.02 × 1015 1.14 × 1015 9.74 × 1014 1.81 × 1014 6.89 × 1014 1.29 × 1015
4.0 × 10-7 5.0 × 10-7 8.0 × 10-7 4.5 × 10-7 4.5 × 10-7 8.0 × 10-7
a The O growth rate is determined from the linear (latter) portion of concentration vs time curves in Figure 6. Diffusion coefficients are not rate-determining values for LiH hydrolysis reactions. b Room temperature.
Figure 9. Oxygen concentration as a function of exposure time to 37 mmol/min flowing H2O at room temperature and two pressures.
Figure 7. Oxygen growth rates as a function of various parameters.
Figure 10. He2+ RBS spectra (5.7 MeV) and RUMP-SIM models (see the Supporting Information for full details) for polycrystalline LiH at ambient temperature and pressure that was either unexposed or exposed to 37 mmol/min decarbonated H2O for 120 min. Spectrum surface positions for several elements are shown.
Figure 8. Oxygen concentration as a function of exposure time to 37 mmol/min flowing H2O at ambient pressure and various temperatures.
Curve shapes are the same as seen previously, with an initial rise followed by a linear slope. Increased reaction temperatures as well as increased reaction pressures result in decreased rates of O growth (listed in Table 1). Figure 7 shows the relationship between product O growth rate and temperature or pressure. C. RUMP Simulations. Simulations of RBS spectra were
completed with the RUMP-SIM program. The software models spectra for compounds in layers. Simulations of He2+ spectra, along with measured data for both unexposed and exposed LiH, are shown in Figure 10. The parameters used for the RBS data simulations, such as hydrolysis layer composition and thickness, are given in Table 2 (full simulation details are given in the Supporting Information). Similar to H+ spectra, the He2+ spectra show peaks for O, C, 6Li, and 7Li at respectively lower energies. For the unexposed sample, the RUMP-SIM model consists of bulk LiH with a small (3 µm) surface corrosion layer that
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TABLE 2: RUMP Parameters for RBS Data Simulations of Unexposed LiH and Exposed LiH layer
thickness, primary µm composition
surface
3
surface
5
intermediate
7
bulk
infinite
additional elements
unexposed LiH 6 Li/7LiH exponential diffusing species: O 0.08 atom %; C 0.03 atom % diffusion coefficient ) 4 × 10-7 cm2/s exposed LiH Li/7LiOH exponential diffusing species: C 0.03 atom % 6 Li/7LiOH exponential diffusing species: O 0.1 atom % diffusion coefficient ) 4 × 10-7 cm2/s 6 Li/7LiH bulk O
