**2.2. The activation process and the cyclic hydrogenations**

correlate the storage capacity of the metal with sample oxidation. In the literature, several techniques are used to detect oxygen and hydrogen in materials; nevertheless, these methods are destructive and have limitations for the determination of concentrations and depth profiles. The ion beam analysis (IBA) techniques, such as Rutherford backscattering spectrometry (RBS) [6] and elastic recoil detection analysis (ERDA) [7], are non‐destructive nuclear techni‐ ques that allow performing a very accurate measurement of the concentration and depth

The physical principle of RBS consists of impinging a beam of collimated and monoenergetic particles (usually light ions such as hydrogen or helium) on the material to be analyzed. As the result of the interaction, part of the energy of the incident particle is transferred to the nucleus of the atom in the sample; so that the backscattered particle contains information of the target, because the reduction in energy of the incident particle depends on the mass of the

The ERDA technique is used to quantify the concentration of light elements, such as hydrogen and carbon. During an ERDA experiment, a beam of collimated and monoenergetic ions heavier than those who are to be detected, impinges on a material. The projectiles collide elastically with the nuclei of the sample, and as they are lighter than the projectiles, they acquire enough energy to leave the material. Simultaneously with the light ions, heavy projectiles could leave the sample and can reach the detector. For this reason, it is essential to place an absorber of Mylar or aluminum before the detector in order to stop the heavier projectiles and

In both techniques, the element that will be analyzed is identified by the energy and the type

In the case of oxygen measurements, an oxygen resonance energy must be used in order to quantify the element concentration. This is because the Rutherford cross section of the oxygen is quite low, causing its RBS signal to be veiled by the signal of other elements present in the material. Oxygen resonances are presented at different energies, being the most common the ones located at 3.045 and 6.585 MeV. These resonances increase the cross section of the element by several times, making possible to improve the oxygen signal and quantify its concentration.

of the emitted particles that comes from the RBS or the ERDA reactions.

profile of oxygen and hydrogen, respectively.

212 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 1.** SEM micrographs of (a) pure titanium and (b) Ti‐6Al‐4V alloy [8].

projectile and the nucleus.

only let to lighter recoils to reach it.

Since the metals or metal alloys used as hydrogen storage materials do not absorb hydrogen at room temperature, they must be activated. Several activation modes, including activation at high temperatures, are attempted in order to obtain a material capable to absorb hydrogen. After the thermal process, the number of paths for hydrogen to diffuse into the material increases, creating micro cracks at the grain boundaries that produces large amounts of clean surface, easing the hydrogen absorption. On the other hand, if the surface of the material is not completely clean or is covered with a surface passivation layer (combination of metal oxides and hydroxides), the material will not absorb until the contaminants or the passivation layer are removed. The activation process in this instance can also occur by increasing the temper‐ ature of the material.

The metals that were used as hydrogen storage materials in this chapter are pure titanium and the titanium alloy Ti‐6Al‐4V, which were manufactured by Goodfellow. The samples were polished and then were ultrasonically cleaned in acetone, followed by rinsing with deionized water. The later cleaning was used in order to remove any kind of impurity from the surface of the sample. Once the cleaning process was completed, the samples were hydrogenated in a 50% hydrogen and 50% argon atmosphere, at 1 atm pressure and a flux of 50 cc/min, during 2 h. The temperature of the hydrogenation process ranged from 150 to 650°C. When the hydride is formed during the first hydrogenation cycle, some micro‐cracks are revealed in both metals, as is shown in **Figure 2** [8]. This figure shows a SEM micrograph of a Ti‐6Al‐4V alloy sample taken after the activation process. These micro‐cracks are the result of the stress induced at the grain boundaries of both materials, since the density of the hydride is less than the metal. As a consequence of the hydrogenation reaction, the volume of the grains increases producing cracking in the grains and exposing new surfaces. This process is called the activation of the material. After activation, the particles have a higher ratio of surface area to volume than before the hydrogenation process, so the metals are able to react easily with hydrogen [9].

The activation process also reduces the metal oxides and the hydroxides deposited on the metal surface. This behavior is observed in **Figure 3**, where two RBS spectra of a Ti‐6Al‐4V alloy are shown before (**Figure 3a** [8]) and after (**Figure 3b**) activation. The RBS spectra show the normalized yield or number of counts of the elements present in the sample versus energy. During the RBS experiment a collimated 6.585 MeV alpha particle beam was used to measure the amount of oxygen in the samples. The 6.585 MeV oxygen resonance was used in order to improve the sensitivity of the element during the measurement. The resonances are regions where the scattering cross section is enhanced over the Rutherford cross section at the same energy and are very useful for measuring light elements, such as oxygen and carbon. After comparing the spectra of **Figure 3**, a high reduction in the oxygen concentration after activation is clearly observed. This result confirms that activation process plays an important role in cleaning the surface of the metals and preparing them for future hydrogenations.

**Figure 2.** SEM micrograph of a Ti‐6Al‐4V alloy sample after activation [8].

**Figure 3.** RBS spectra of Ti‐6Al‐4V alloy before (a) [8] and after (b) activation. The oxygen concentration is reduced considerably after the activation process.

Once the samples are activated, the hydrogen concentration is measured using the ERDA technique. In order to measure the hydrogen content, the samples were irradiated with a collimated 3 MeV alpha particle beam. The surface of the samples was placed at an angle of 15° with respect to the incoming beam; meanwhile a surface barrier detector was placed at an angle of 30° to the beam direction in order to detect the hydrogen recoils. A 12 µm Mylar foil was placed in front of the detector to stop elastically scattered ions heavier than the recoil hydrogen. Equation (1) was used to calculate the hydrogen concentration before and after the hydrogenation process. This equation relates the yield (*Yr*) of the recoil atoms detected at channel *Ed* with channel width *δEd*, with the atomic density of the recoil hydrogen atom (*Nr*) at the depth *x*.

$$Y\_r = \frac{QN\_r \sigma\_r \Omega \delta E\_d}{\cos \theta\_i \delta E\_d dE\_d / dx} \tag{1}$$

where *Q* is the incident projectile fluence, *σr* is the recoil differential scattering cross section, *Ω* is the detector solid angle, and *dx* is the increment of depth at *x* corresponding to an increment in energy *dEd* and *θ1* is the incident angle.

The scattering cross section is given by the equation:

**Figure 2.** SEM micrograph of a Ti‐6Al‐4V alloy sample after activation [8].

214 New Advances in Hydrogenation Processes - Fundamentals and Applications

considerably after the activation process.

at the depth *x*.

**Figure 3.** RBS spectra of Ti‐6Al‐4V alloy before (a) [8] and after (b) activation. The oxygen concentration is reduced

Once the samples are activated, the hydrogen concentration is measured using the ERDA technique. In order to measure the hydrogen content, the samples were irradiated with a collimated 3 MeV alpha particle beam. The surface of the samples was placed at an angle of 15° with respect to the incoming beam; meanwhile a surface barrier detector was placed at an angle of 30° to the beam direction in order to detect the hydrogen recoils. A 12 µm Mylar foil was placed in front of the detector to stop elastically scattered ions heavier than the recoil hydrogen. Equation (1) was used to calculate the hydrogen concentration before and after the hydrogenation process. This equation relates the yield (*Yr*) of the recoil atoms detected at channel *Ed* with channel width *δEd*, with the atomic density of the recoil hydrogen atom (*Nr*)

> s d

<sup>1</sup> / *rr d*

*d d*

*cos E dE dx* (1)

q d

*r*

<sup>W</sup> <sup>=</sup>

*QN E <sup>Y</sup>*

$$\sigma\_r = \frac{\left[Z\_1 Z\_2 e^2 \left(M\_1 + M\_2\right)\right]^2}{\left[2M\_2 E\_0\right]^2 \cos^3 \phi} \tag{2}$$

where *Zi* and *Mi* are the atomic number and the atomic mass of the *i* element, respectively, *E0* is the incident energy of the projectile, and *ϕ* is the recoil angle.

In order to study the behavior of the activation temperature in pure titanium and the titanium alloy Ti‐6Al‐4V, the hydrogen doping was performed at temperatures ranging from 150 to 650°C. According to Eq. (1), the hydrogen concentrations in pure titanium and the Ti‐6Al‐4V alloy are shown in **Figure 4** [2]. In both cases, the ERDA results show an increment in hydrogen concentration at temperatures higher than 550°C. The figure shows that hydrogen is not absorbed until a threshold temperature is reached, which is the activation temperature. In pure titanium, this threshold temperature is closed to 550°C, but in the alloy, it is located between 550 and 600°C. It is also observed in **Figure 4** that above this temperature, the hydrogen concentration reaches a value close to 3 × 1022 H atoms/cm3 in both materials, which is the maximum concentration that the material is able to get after activation. It will be noticed later that in both materials this hydrogen concentration can easily increase during cyclic hydroge‐ nations.

**Figure 4.** Hydrogen concentration versus hydrogenation temperature for pure Ti and the Ti‐6Al‐4V alloy [2].

Once the samples are activated, they can be cyclically hydrogenated in order to study the hydrogen absorption of the materials. **Figure 5** shows the ERDA spectra of a Ti‐6Al‐4V alloy sample hydrogenated three times after activation, where *"hydrogenation 1,"* in **Figure 5**, corresponds to the activation process. After the activation process, the samples were hydro‐ genated every 2 months and after that, their hydrogen concentration was measured, as it is shown in **Figure 5**. The curve labeled as "*reference sample"* corresponds to a Ti‐6Al‐4V alloy with any hydrogenation process. The tiny and superficial peak that it shows is due to the hydrogen contained in the material after preparation and before the hydrogenation process. This value was subtracted from the hydrogenated curves in order to obtain an accurate hydrogen concentration. The results in this figure also show that hydrogen content increases after each hydrogenation process, which make us assume that the Ti‐6Al‐4V alloy can be considered a good material for hydrogen storage purposes.

**Figure 5.** ERDA spectra of Ti‐6Al‐4V alloy after cyclic hydrogenations [8].

One way to compare the hydrogen storage of the materials is by means of its gravimetric storage capacity (wt%). This amount gives the percentage weight of hydrogen relative to the metal. The gravimetric storage capacity of Ti and Ti‐6Al‐4V alloy samples after each hydro‐ genation cycle was calculated by using Eq. (3).

$$wt = \frac{wt(\mathbf{H})}{wt(\mathbf{H} + \mathbf{M})} \times 100\tag{3}$$

where H and M represent the hydrogen atoms and all the metal atoms in the material, respectively.

**Table 1** shows the gravimetric storage capacity of Ti and Ti‐6Al‐4V alloy after cyclic hydro‐ genations [8]. One can appreciate that the storage capacity increases after each cycle and that after activation (first hydrogenation) the storage capacity of both materials increases consid‐ erably. After the second cycle, the increment of hydrogen in the metal and the alloy is lower, but it continues increasing. According to **Figure 2** and the titanium SEM micrographs not presented in this chapter, we can assure that after activation, both materials present micro‐ cracks that ease the hydrogen diffusion through the materials, increasing the storage capacity after each hydrogenation cycle.


**Table 1.** Gravimetric storage capacity of Ti and Ti‐6Al‐4V after cyclic hydrogenations [8].

genated every 2 months and after that, their hydrogen concentration was measured, as it is shown in **Figure 5**. The curve labeled as "*reference sample"* corresponds to a Ti‐6Al‐4V alloy with any hydrogenation process. The tiny and superficial peak that it shows is due to the hydrogen contained in the material after preparation and before the hydrogenation process. This value was subtracted from the hydrogenated curves in order to obtain an accurate hydrogen concentration. The results in this figure also show that hydrogen content increases after each hydrogenation process, which make us assume that the Ti‐6Al‐4V alloy can be

One way to compare the hydrogen storage of the materials is by means of its gravimetric storage capacity (wt%). This amount gives the percentage weight of hydrogen relative to the metal. The gravimetric storage capacity of Ti and Ti‐6Al‐4V alloy samples after each hydro‐

> = ´ (H) <sup>100</sup> (H+M)

where H and M represent the hydrogen atoms and all the metal atoms in the material,

**Table 1** shows the gravimetric storage capacity of Ti and Ti‐6Al‐4V alloy after cyclic hydro‐ genations [8]. One can appreciate that the storage capacity increases after each cycle and that after activation (first hydrogenation) the storage capacity of both materials increases consid‐ erably. After the second cycle, the increment of hydrogen in the metal and the alloy is lower, but it continues increasing. According to **Figure 2** and the titanium SEM micrographs not presented in this chapter, we can assure that after activation, both materials present micro‐ cracks that ease the hydrogen diffusion through the materials, increasing the storage capacity

*wt* (3)

*wt wt*

considered a good material for hydrogen storage purposes.

216 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 5.** ERDA spectra of Ti‐6Al‐4V alloy after cyclic hydrogenations [8].

genation cycle was calculated by using Eq. (3).

respectively.

after each hydrogenation cycle.

On the other hand, **Table 2** presents the gravimetric storage capacity of some other materials used as hydrogen storage media [10]. According to the data of **Table 2**, the storage capacity of our materials after the fourth hydrogenation, has similar values to those materials that are commercially used as hydrogen storage materials. This fact makes us think that pure titanium and the Ti‐6Al‐4V alloy are good candidates as storage materials.


**Table 2.** Gravimetric storage capacity of different materials used as hydrogen storage materials [10].

#### **2.3. The improvement of hydrogen absorption by using ion implantation**

Pure titanium is an element with a high affinity to hydrogen; however, it reacts with hydrogen at high temperatures, so in recent years some investigations have been carried out in order to improve the activation properties of this and other metals used as hydrogen storage materials. These methods include glow discharge in Al [11], ion mixing in TiFe [12], ion implantation in Ti [13], Pd [14], and MgH2 [15].

The implantation process goes back to the nineteenth century and has been continually refined ever since. In the late 1940s and 1950s Robert Van de Graaff was the pioneer of accelerator construction, and the high‐voltage technology, which was the base for building the first commercial ion implanters. One of the most common applications of ion implanters are the integrated circuit manufacturing, where doping or modifying silicon and other semiconductor wafers is the main goal.

Ion implantation is a process that involves generating an ion beam and impinging it into a substrate, so that the ions come to rest inside the material. During implantation, the ions interact with nucleus and electrons of the substrate producing physical, electrical, and chemical changes in the material by transferring their energy and momentum to the electrons and atomic nuclei of the target material. When an energetic ion collides with a solid, it loses energy by two processes: (1) by inelastic or electronic process in which the electrons of the material are excited and (2) by elastic or nuclear collisions with the target atoms, producing structural changes, such as interstitials and vacancies. During the nuclear collisions, an ion can transfer enough energy to a matrix atom, becoming a projectile inside the material, and producing collision cascades. After losing its energy, when the ion or the released atom does not find any vacant space in the lattice, they tend to occupy any space in the solid known as interstitial. When the ions are implanted into a metal or metal alloy, the structural change is mostly produced in the surface of the material, creating pathways by which hydrogen can migrate into the metal in an easier way.

In this section, the results of a study of hydrogenation of pure titanium after ion implantation are presented.

The material used to carry out the study of hydrogen storage was pure titanium, manufactured by Goodfellow with 99.6% purity. The samples consisted of slices cut from titanium rods, which were polished, cleaned in acetone, and rinsed with deionized water. After the cleaning process, the materials were implanted at room temperature with a Colutron ion gun. The titanium samples were implanted with 5 keV H ions at a fluence of 1 × 1014 ions/cm2 . With this process, we intentionally induced defect zones in the surface region that may accelerate the diffusion of hydrogen. After implantation, the samples were hydrogenated at 300, 450, and 600°C in a 50% hydrogen and 50% argon atmosphere, at 1 atm pressure, and a flux of 50 cc/min, during 2 h. We intentionally implanted hydrogen ions in order to avoid any element contamination in the material. The 5 keV energy was chosen in order to assure that the biggest damage and the highest number of vacancies produced during implantation, would be produced in surface.

**Figure 6** shows the hydrogen depth profile of the implanted and the nonimplanted samples hydrogenated at 450°C [13]. It can be observed in the figure that the hydrogen absorption of titanium without implantation is almost null, as can be verified in **Figure 4**; however, when a previous 5 keV H ions implantation is achieved to the metal, the scene is completely different and the hydrogen storage capacity of the metal is highly improved. Both curves in **Figure 6** show a small hydrogen peak at the surfaces of both titanium samples. These peaks are related to the superficial hydrogen in the nonimplanted sample (blue circles) and to the combination of the superficial hydrogen and the implanted hydrogen in the irradiated sample (red squares), where the hydrogen signal is slightly higher as a consequence of the implanted hydrogen. It can be also appreciated in **Figure 6** that the nonimplanted material does not absorb hydrogen beyond its surface. This can be confirmed in the ERDA graph as an almost null hydrogen signal beyond the 0.15 µm. In the case of the implanted materials, they show the same superficial hydrogen signal as the nonimplanted samples, but they also present a major hydrogen signal from 0.15 to 0.83 µm. In order to verify if the hydrogen continues beyond 0.83 µm into the bulk, ERDA measurements were performed from the back of the samples. The results showed that hydrogen is present through all the material.

interact with nucleus and electrons of the substrate producing physical, electrical, and chemical changes in the material by transferring their energy and momentum to the electrons and atomic nuclei of the target material. When an energetic ion collides with a solid, it loses energy by two processes: (1) by inelastic or electronic process in which the electrons of the material are excited and (2) by elastic or nuclear collisions with the target atoms, producing structural changes, such as interstitials and vacancies. During the nuclear collisions, an ion can transfer enough energy to a matrix atom, becoming a projectile inside the material, and producing collision cascades. After losing its energy, when the ion or the released atom does not find any vacant space in the lattice, they tend to occupy any space in the solid known as interstitial. When the ions are implanted into a metal or metal alloy, the structural change is mostly produced in the surface of the material, creating pathways by which hydrogen can migrate into the metal in

218 New Advances in Hydrogenation Processes - Fundamentals and Applications

In this section, the results of a study of hydrogenation of pure titanium after ion implantation

The material used to carry out the study of hydrogen storage was pure titanium, manufactured by Goodfellow with 99.6% purity. The samples consisted of slices cut from titanium rods, which were polished, cleaned in acetone, and rinsed with deionized water. After the cleaning process, the materials were implanted at room temperature with a Colutron ion gun. The titanium

we intentionally induced defect zones in the surface region that may accelerate the diffusion of hydrogen. After implantation, the samples were hydrogenated at 300, 450, and 600°C in a 50% hydrogen and 50% argon atmosphere, at 1 atm pressure, and a flux of 50 cc/min, during 2 h. We intentionally implanted hydrogen ions in order to avoid any element contamination in the material. The 5 keV energy was chosen in order to assure that the biggest damage and the highest number of vacancies produced during implantation, would be produced in surface.

**Figure 6** shows the hydrogen depth profile of the implanted and the nonimplanted samples hydrogenated at 450°C [13]. It can be observed in the figure that the hydrogen absorption of titanium without implantation is almost null, as can be verified in **Figure 4**; however, when a previous 5 keV H ions implantation is achieved to the metal, the scene is completely different and the hydrogen storage capacity of the metal is highly improved. Both curves in **Figure 6** show a small hydrogen peak at the surfaces of both titanium samples. These peaks are related to the superficial hydrogen in the nonimplanted sample (blue circles) and to the combination of the superficial hydrogen and the implanted hydrogen in the irradiated sample (red squares), where the hydrogen signal is slightly higher as a consequence of the implanted hydrogen. It can be also appreciated in **Figure 6** that the nonimplanted material does not absorb hydrogen beyond its surface. This can be confirmed in the ERDA graph as an almost null hydrogen signal beyond the 0.15 µm. In the case of the implanted materials, they show the same superficial hydrogen signal as the nonimplanted samples, but they also present a major hydrogen signal from 0.15 to 0.83 µm. In order to verify if the hydrogen continues beyond 0.83 µm into the bulk, ERDA measurements were performed from the back of the samples. The results showed that

. With this process,

samples were implanted with 5 keV H ions at a fluence of 1 × 1014 ions/cm2

hydrogen is present through all the material.

an easier way.

are presented.

**Figure 6.** Hydrogen depth profile of pure titanium hydrogenated at 450°, with (red squares) and without (blue circles) implantation [13].

The high continuum hydrogen signal that can be observed in the implanted samples can be explained by means that during implantation strains in the surface region change, as well as structural changes are produced in the surface of the metal, introducing defects and disloca‐ tions and creating pathways in which hydrogen atoms can migrate into the metal. These paths may accelerate the diffusion of hydrogen through the metal, improving its storage capacities.

In **Figure 7**, a comparative between hydrogen concentration of the implanted [13] and the nonimplanted (see **Figure 4**) titanium samples after being hydrogenated at different temper‐ atures is observed. These results were obtained directly by ERDA spectra using Eq. (1) and comparing with a reference sample of TiH2. For 300°C, there is no difference between the hydrogen absorbed by the implanted and the nonimplanted samples; however, for tempera‐ tures above 450°C, the behavior is completely different, showing an improvement in the hydrogen content for the titanium samples previously implanted. In this way, when hydrogen ions are implanted into the pure titanium material, its activation temperature is reduced.

**Table 3** shows the gravimetric storage capacity of pure titanium samples after being implanted and hydrogenated [13]. The results show that as well as the hydrogen concentration, the storage capacity was improved after the 5 keV H ion implantation. When comparing with **Table 1**, it can be noticed that the storage capacity increased from 3.02 to 3.77% in the sample hydrogenated at 600°C during the activation cycle. This confirms that implantation process does accelerate the activation process of pure titanium. We can assure that cyclic hydrogena‐ tions must increase considerably the hydrogen absorption in pure titanium, as well as in the Ti‐6Al‐4V alloy.

**Figure 7.** Hydrogen concentration of the hydrogenated samples, before and after implantation [13].


**Table 3.** Gravimetric storage capacity of titanium samples after implantation and hydrogenation [13].

In order to check if the hydrogen absorbed by the metal is forming hydrides, XRD analysis was conducted [13]. **Figure 8** shows the results of the XRD of the implanted and hydrogenated samples, as well as a pure titanium reference. The results show the diffractions of the titanium alpha phase. The titanium dihydride phase (TiH2) is also observed, showing the (100) and (200) lines, which correspond to the higher diffractions. The results also show a decrease in the intensity in the pure titanium (101) reflection as the temperature of hydrogenation increases. This behavior could be the consequence of a structural change or loss of crystallinity produced during implantation, hydrogenation, or both. This figure also shows that the relative intensities of the reflections (100) and (002) in the HCP titanium were modified with the presence of hydrogen. This indicates that during hydrogen absorption a change in crystal orientation was induced. An important result obtained from this figure is that the implanted hydrogen does not produce hydrides in the metal, so the hydride phase is formed during hydrogenation.

**Figure 8.** Comparative XRD patterns of five titanium samples: pure titanium (reference), titanium implanted with 5 keV H ions (Ti + H(5 keV)), and the three patterns in the top that correspond to samples implanted with 5 keV H ions and hydrogenated at different temperatures (Ti + H(5 keV) + TT) [13].
