Stress development of metal hydride composites for high density hydrogen storage applications

Accepted Manuscript Stress development of metal hydride composites for high density hydrogen storage applications Felix Heubner, Sebastian Mauermann, Bernd Kieback, Lars Röntzsch PII:

S0925-8388(17)30551-0

DOI:

10.1016/j.jallcom.2017.02.113

Reference:

JALCOM 40841

To appear in:

Journal of Alloys and Compounds

Received Date: 19 December 2016 Revised Date:

30 January 2017

Accepted Date: 11 February 2017

Please cite this article as: F. Heubner, S. Mauermann, B. Kieback, L. Röntzsch, Stress development of metal hydride composites for high density hydrogen storage applications, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.02.113. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT REVISED MANUSCRIPT for publication in the

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Journal of Alloys and Compounds

Stress development of metal hydride composites for high density

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hydrogen storage applications

Felix Heubnera, Sebastian Mauermannb, Bernd Kiebacka,b and Lars Röntzschb,*

a) Technische

Universität Dresden, Institute of Materials Science,

b) Fraunhofer

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Helmholtzstraße 7, 01069 Dresden, Germany Institute for Manufacturing Technology and Advanced Materials IFAM,

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Branch Lab Dresden, Winterbergstraße 28, 01277 Dresden, Germany

*corresponding author: phone: +49-351-2537-411; fax: +49-351-2537-399;

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e-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract Strong mechanical forces can appear from gas absorbing solids if a volume expansion occurs during the chemical reaction. In particular for metal hydride reactors, the evolution of mechanical stresses on the reactor wall or internal assemblies over many hydrogen absorption/desorption cycles needs to be

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considered for reactor safety. In this work, we report on a recently developed in-situ measuring principle that allows the determination of mechanical stresses that originate from a metal hydride formation. For this purpose, a unique lab-scale reactor, equipped with a measuring cell, has been designed and tested

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with recently developed metal hydride composites (MHC). For spatially confined MHC we found that mechanical stresses were developed two and a half times higher than the hydrogen gas pressure

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applied for hydride formation.

Keywords: hydrogen storage

•

metal hydride composites

•

solid-gas reactor

•

volume expansion

•

stress development

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•

Highlights: • • •

Volume expansion of gas absorbing solids causes significant mechanical stresses

An in-situ measuring device for stress monitoring has been developed and tested Spatially confined metal hydrides develop stresses many times higher than the applied H2 pressure

•

Both gas pressure and mechanical stresses have to be considered for reactor design.

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ACCEPTED MANUSCRIPT Notations: area in axial direction measured axial force measured radial force

ℎ

temperature measuring points height of the MHC radius of the centered hole in the MHC radius of the MHC; radius of the measuring cell radius of the thermocouple holes segment

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s

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T1-T10

axial stress

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radial stress

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ACCEPTED MANUSCRIPT 1

Introduction

The volume expansion of solids in solid-gas reactors is a well-known phenomenon, in particular, for gas absorption reactions [1,2]. If gas-absorbing solids are spatially confined in between the reactor walls or

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internal assemblies, strong forces are appearing in the confinement during gas absorption that can trigger a deformation or even the destruction of the reaction vessel [3–5]. Hence, it is crucial to monitor the volume expansion and related stresses for a high reactor safety.

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The formation of a metal hydride (MH) through a metal-hydrogen reaction is a prominent example of a heterogeneous reaction with a significant volume change of the solid [6,7]. In state-of-the-art MH

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storage devices, loose metal powder beds with high internal porosities (> 50 vol.-%) and additional free expansion space (< 30 vol.-%) are used to avoid any stress development in the reaction vessel [8–10]. Particularly for MH-based hydrogen storage applications, a high packing density of the hydride-forming metal alloys is desired which demands for an optimized reaction bed design and a controlled volume

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expansion.

Besides controlled volume expansion, a fast heat transfer is important for MH reactors to avoid hot spots inside the reaction bed. The absorption of hydrogen is exothermic in most cases (20-80 kJ/mol-

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H2) [11]. In order to realize a fast heat exchange, tube bundle reactors with preferably small tube diameters are used in many cases [12,13]. Moreover, the thermal conductivity inside the reaction bed is

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typically improved by rigid auxiliary structures like metal fins or foams [14]. Recently, metal hydride composites (MHC) have been developed with improved heat and gas transport characteristics compared to loose MH powder beds. MHC consist of a hydrogen absorbing metal with minor contents of materials with highest thermal conductivity, e.g. expanded natural graphite, aluminum or copper [15– 17]. In addition, MHC have a residual internal porosity in the range between 15 and 30 vol.-%. By variation of the volume content of the secondary phase the thermal conductivity of MHC can be scaled typically between 1 and 25 Wm-1K-1. Due to the higher intrinsic thermal conductivity of MHC a very fast

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ACCEPTED MANUSCRIPT hydrogen absorption within a few minutes has been demonstrated for MH reactors [18]. The fabrication of MHC involves the densification of a powder mixture in a uniaxial compaction step that can lead to an anisotropic phase alignment inside the MHC [19,20]. Due to the volume expansion of the hydrogen absorbing phase, the whole MHC expands during

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hydrogen uptake. The degree of MHC expansion, however, depends crucially on MHC composition and its internal structure [21,22]. The internal porosity of MHC is able to take up the whole volume expansion of the hydrogen absorbing phase, if the MHC is confined in one or more spatial directions, for example,

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by an internal assembly or the reactor wall. In a confined state, a swelling deformation force can occur during hydrogen uptake that acts onto the MHC environment and the MHC itself. The monitoring of

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these stresses can become crucial for the mechanical integrity of the reaction vessel during intensive cycling.

Various authors described in the past 25 years different aspects of the expansion-related stress of metal powder beds during hydrogenation. The most commonly used method is the application of strain

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gauges on the surface of reactors walls [3,4,23–29]. The strain distribution delivered various information, for example about particle decrepitation and particle sedimentation for vertically aligned reactors. In the last five years new setups were built to measure the axial volume expansion with a

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displacement sensor [30], with optical methods [31] or with a capacitive measuring device [32]. In 2013, the first expansion of a cylinder filled with a MHC was measured and stresses were calculated to

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compare different MHC compositions [21]. Furthermore, the first stress measurements of a powder bed in radial and axial direction the same time were performed by Salque et al. in 2015 with a force sensor in axial direction and an optical analysis of the cylinder deformation in radial direction [33]. The described strongly anisotropic properties of MHC [17,19,34] and the high emerging stresses [22] confirm the need of a force measuring technique in radial and axial direction of cylindrical composites. On these grounds, we are reporting on a recently developed method to measure the forces in operando

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ACCEPTED MANUSCRIPT that originate from a spatially confined MHC during hydrogen absorption and desorption. Our results

2

Material and methods

2.1

Metal hydride test reactor

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suggest that this method is applicable to further gas-solid reactions with significant volume change.

According to tubular reactors used for hydrogen storage, a special MH test reactor with two different

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temperature zones and a working gas pressure of 200 bar H2 was designed and constructed (Figure 1). The reactor with an outer diameter of 76 mm and a wall thickness of 6 mm has been equipped with two

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weld neck flanges and two double jackets that are used for active heating and cooling of two distinct reactor zones A and B. All parts are made of 1.4571 stainless steel (AISI 316Ti). The flanges are sealed by spiral wound gaskets (graphite, stainless steel) and they contain different feed-throughs for hydrogen gas supply, thermocouples and electrical cables for process monitoring (Figure 1B). Reactor zone A can

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be actively heated by a thermal fluid with a maximum temperature of 350°C. The hydrogen absorbing material is located in zone A, thus, we are able to investigate a wide spectrum from room temperature to high temperature MHs. The measuring cell can hold up to three MHC (height ~ 15 mm) with or without

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additional expansion space/filling pieces. The force sensor is located in zone B whose operation

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temperature should not exceed 65°C. Therefore, a second double jacket adjusts the temperature in zone B to 25°C (Figure 1). The two double jackets are spatially separated to reduce thermal stresses in the reactor related to the temperature gradient between the hot and the cold zone. Thermocouples detect the temperatures at different positions inside the reactor: The in- and outlet temperatures of the thermal fluids of the two double jackets are monitored by one thermocouple each (T1-T4). Zone A is monitored by four (T5-T8) and zone B by two thermocouples (T9-T10). The MH reactor is integrated into a test rack, where signals from gas flow meters, gas pressure sensors, thermocouples and the force

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ACCEPTED MANUSCRIPT sensor are amplified, detected and analyzed using Labview ®. The test rack has been described in detail in a previous work [18].

In-situ force measurement and stress calculation

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2.2

The test reactor contains a cylindrical measuring cell for the in-situ detection of radial and axial forces related to the volume expansion of the hydrogen absorbing phase in the MHC. In zone A, the measuring

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cell contains a cylindrical MHC that is spatially confined in radial and axial direction. In zone B, force sensors are installed to measure the forces during hydrogen ab- and desorption (Figure 2A). The force

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sensors are based on strain gauges and each contains a Wheatstone bridge to prevent influences from the ambience (temperature, gas). The measuring cell is described in detail as follows:

Axial direction

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The axial confinement of cylindrical MHC is achieved by metal disks that are connected with inner metal sticks (Figure 2A) that also connects to the axial force sensor. The axial sensor is of ring-like shape and can detect forces by up to 10 kN (purchased from ME-Meßsysteme GmbH). To prevent overheating a

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thermal insulation is located in between MHC and sensor. The measured force acts on a defined area that is the base area of a cylinder reduced by the area for the holes of the centered metal stick (rc = 3

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mm) and four thermocouples (rt = 0.75 mm). The axial stress

can be calculated using the following

equation: (1)

=

=

∙(

.

)

The force sensor is slightly influenced by the hydrogen gas pressure. The signal deviates by 0.1 MPa at 40 bar H2. This deviation could be due to the potentially compressible casting compound of the strain gauges in the force sensor.

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ACCEPTED MANUSCRIPT Radial direction The radial confinement (Figure 2A) consists of two metal half shells with the MHC-diameter as inner diameter. A radial force sensor that measures partly the tensile forces is inserted on one end of the measuring cell. On the other end, a threaded metal shaft connects both half shells. The MHC is placed

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on a defined position between the two half shells. The developing force during hydrogen absorption is received by the force sensor (5 kN, purchased from ME-Meßsysteme GmbH) and the stick as a function of the inserted force on a specific location in the measuring cell. A torque balance around axis X-X and the homogeneous mechanical stress

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(Figure 2C) under consideration of the measured force

(2)

∙

(3)

=

= "∙

∙ ! #∙

∙ℎ∙2 ∙

,

.

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from the volume expansion on the inner half shell of the rigid clamp leads to the following equations:

Again, the force sensor is slightly influenced by the hydrogen gas pressure. The signal deviates by 0.3 MPa at 40 bar H2. The high stress leads only to a minor deformation of the shells and the stick. Comsol

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Multiphysics® has been used to calculate the displacement of the half shells (Figure 2B). For example, a MHC with a radius of 20 mm, a height of 20 mm and a developed radial stress of 10 MPa on the whole contact area on both shells has been considered. The maximum displacement for a 11 mm thick

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shell is 0.18 mm and is, therefore, negligible for the following calculation of the radial mechanical stress

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. Furthermore, the gap between the reactor wall and the measuring cell amounts to 2 mm. It ensures a free expansion of the half shells.

2.3

Metal hydride composite preparation

MHC consisting of a Ti-Mn hydrogen absorbing alloy (AB2-type) and expended natural graphite were prepared. The hydride-forming alloy Hydralloy C52 (51 wt.% Mn, 28 wt.% Ti,14 wt.% V, 3 wt.% Fe, 3 wt.% Zr) purchased as granules from GfE Gesellschaft für Elektrometallurgie mbH) and the resulting page 8 of 16 13. Feb. 2017, 13:21

ACCEPTED MANUSCRIPT MHC have been analyzed carefully with various materials analysis methods regarding microstructure and hydrogen absorption/desorption behavior [17,22,35]. In the first processing step, Ti-Mn granules with 2 to 10 mm in diameter were milled for 5 min to a particle size between 5 and 100 µm using a swing-disk mill. The received metal powder was then mixed

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with expanded natural graphite (purchased from SGL Carbon SE) and pressed to mechanically stable cylindrical bodies (so-called MHC) using a uniaxial press at a compaction pressure of 75 MPa. All processing steps were conducted under inert atmosphere to prevent any unwanted surface

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contamination. The whole processing chain is described in more detail in our previous work [17]. To calculate the porosity of the composites on the basis of the bulk materials densities a pycnometer

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(AccuPyc 1330) was used to estimate the bulk materials volume and mass [17].

For the present experiments, a MHC containing metal particles with an average size of 6.4 µm, ~10 vol.-% (5 wt.-%) expanded natural graphite and a residual porosity of ~30 vol.-% were produced (Figure 3A). Figure 3 depicts next to the photographs the polished cross-section view of as-pressed MHC and

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as-cycled MHC in the measuring cell. The MHC were infiltrated with epoxy resin, cut and the cross sections were polished. The darker areas are filled pores with epoxy resin, the grey veins are graphite and the bright areas are the metal phases.

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The MHC materials parameters in the as-compacted (uncycled) state are listed in Table 1. The cylindrical MHC was drilled in order to provide space for the four thermocouples prior the insertion into

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the measuring cell. The drilling produced a mass loss of around 1 g of the MHC (cf. Table 1). In Figure 2C, the genuine measuring cell with the inserted MHC is depicted. Here, the upper half shell of the cell has been removed in order to take the photograph.

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Results and discussion

In our previous work we examined the forces in axial direction of spatially confined metal hydride composites (MHC) to consider stresses for the tank design [22]. The Mn-Ti-Graphite based MHC used

graphite network inside the MHC may give rise to anisotropic stresses.

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here have strongly anisotropic properties regarding heat and mass transfer [17]. The radially aligned

For the in-situ measurement, the measuring cell has been equipped with one fully confined MHC to determine the most possible stresses of this MHC configuration. This full confinement is the worst case

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for the technical application. The cell was inserted into the test reactor that was sealed, closed,

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evacuated and flooded with hydrogen gas (purity 99.999%) up to a pressure of 40 bar. The stress development in the first hydrogen absorption/desorption cycle at room temperature is depicted in Figure 4. Immediately after hydrogen inflow, the exothermal absorption of hydrogen starts which is clearly seen by the increasing temperature inside the MHC. The thermocouples inside the MHC are located at different radii, i.e. temperature gradients can be deduced, too. The legend in Figure 4 indicates the

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temperature measuring points (e.g. r18-1 means thermocouple 1 inserted in a radius of 18mm). In contrast, the temperatures inside the MHC promptly drop below room temperature during the

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endothermic hydrogen desorption reaction at a hydrogen pressure of 2 bar. The highest temperature peaks were measured near the center of the MHC (cf. temperature r6 in Figure 4). From former studies

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it is known that MHC do react slowly with hydrogen in the first few cycles and therefore often do not completely absorb hydrogen within a given time frame [20,22,35]. During the first 5 cycles – the socalled activation treatment – the hydrogen absorption kinetics of MHC and thereby the degree of conversion increases steadily until a fully activated MHC with a degree of conversion of 100% is reached. Therefore, the temperature gradients inside the MHC remain rather low in the first cycle (~10 K during absorption and ~15 K during desorption) because the heat source/sink density is lower compared to the fully activated state.

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ACCEPTED MANUSCRIPT Interestingly, the radial and axial stresses increase steadily to 8.8 MPa in radial direction and to 10.6 MPa in axial direction over the whole absorption time span of 60 min. The homogeneous temperature signals over the last 40 min of the first absorption indicate that there is only a minor continuing hydrogen absorption in the MHC. The first desorption is finished in nearly 10 min and the radial and axial stresses

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decrease steadily to 1.8 MPa in radial direction and to 2.2 MPa in axial direction. Figure 5 contains the maximum stresses during the absorption of hydrogen and the minimum stresses for the final desorbed state at the end of each cycle. Only cycles with comparable process conditions are shown in the

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diagram. After the first cycle the maximum axial stress decreases slightly and, in contrast, the maximum radial stress increases to the fifth cycle (cf. Figure 5). After this activation procedure, both, the radial and

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the axial stress decrease. The steady state was present from cycle 17 (9.3 and 10.8 MPa respectively after 20 cycles; cf. Figure 4; cf. Figure 5). This significant change in stress development with an increasing cycle number is most likely due to reorientation and/or relaxation of phases inside the MHC or due to local decrepitation of the hydrogen absorbing phase inside the MHC [36–38]. Figure 3 depicts

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next to the MHC photograph the polished cross-section view of the MHC in the as-pressed state (A) and in the as-cycled state in the measuring cell without significant volume expansion (B). The coarse metal phases in (A) decrepitated to smaller ones with some visible cracks in (B). Obviously, microstructural

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changes inside the MHC lead to residual stresses in the dehydrogenated state (compare the final values of cycles 1, 2 and cycle 20 in Figure 4). The homogeneous stresses in the dehydrogenated state

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around 2 MPa from the 12th cycle confirm that there is no significant drift of the force sensors signal in the measuring cell. Again, Table 1 lists the materials properties of the as-cycled state after 43 absorption/desorption cycles. It is evident that the measuring cell prevents the MHC from expanding its volume. Only a minor swelling was observed (approximately 7 vol.-%) that can be explained by the swelling of the MHC into volume gaps that naturally are present between MHC in the as-compacted state and the confining parts of the measuring cell during test cell assembly. A second possible reason

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ACCEPTED MANUSCRIPT could be a spring-back effect after opening the measuring cell, which is widely known for the ejection of MHC after the compaction of the blend [17-19]. In our previous work an in-situ measuring cell for axial stresses was developed and tested with an comparable MHC (Mn-Ti-based alloy, 5 wt.-% Graphite, 75 MPa compaction pressure) [22]. The

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measured maximum axial stress of 8.9 MPa was measured in the second cycle, like in this work, but for a pellet with 11.36 mm in height. In comparison, the used samples were of the same outer diameter but different heights what leads to different stresses. The stress load normalized by the different sample

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heights in axial direction were of 0.71±0.05 MPa per millimeter pellet. This value is scalable for this height to diameter ratio. Both measurements proved that confined composites are highly stable and

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particle migration is strongly inhibited (cf. Figure 3).

These high mechanical stresses, which potentially occur if no or only little free volume is provided inside the reactor, have to be considered in the design and construction of the reaction vessel in order to assure a safe operation and to prevent a distortion or even destruction of the reaction vessel. Here,

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almost homogeneous stresses in axial and radial stresses were detected. They are the maximum stresses for the investigated MHC configuration. The high mechanical stresses are two and a half times higher than the hydrogen gas pressure applied for hydride formation. Thus, the cylinder wall has to be

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designed to withstand both hydrogen gas pressure and mechanical stress due to MHC hydrogenation. It is therefore useful to define axial and/or radial expansion spaces for the MHC to reduce the stresses on

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the reactor wall. The residual stresses can be measured with a measuring cell similar to the one presented here. Thus, further measurements are necessary to find the optimum between pellet stability and volumetric storage density of the tank system.

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Summary and Outlook

The developed MH test reactor and the genuine measuring setup enables the direct evaluation of stresses originating from hydrogen-metal reactions with significant volume change, i.e. metal hydride

be evaluated with the same or a similar experimental set-up.

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formation. We are convinced that other gas-solid reactions, in particular gas absorption reactions, can

The major conclusions of the developed measuring system read as follows: •

The MH test reactor can work with gas pressures by up to 200 bar at 350°C and is made of

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1.4571 stainless steel. The inserted measuring cell can detect forces in axial direction up to

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10 kN and in radial direction a minimum of 5 kN. It is possible to mount up to three composites with additional expansion space (length up to 80 mm). •

Thermocouples inside the reactor monitor heat generation or consumption during the gas-solid reaction and indicate internal temperature gradients.

The first measurement in the developed measuring cell leads to the following conclusions: The hydrogen storage composite material (Mn-Ti-based alloy, 10 vol.-% ENG, 29.1% porosity)

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•

under investigation develops radial and axial stresses up to approximately 10 MPa which is

•

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about 250 % of the hydrogenation gas pressure. The developed stresses increase in the first cycles and reflect the ongoing activation of the

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material. In the subsequent cycles the axial and radial stresses decreases what is due to reorientation and/or relaxation of phases inside the MHC or due to local decrepitation of the hydrogen absorbing phase inside the MHC.

•

The fully confined MHC are of highest stability after 44 cycles. Particle migration was prevented successfully.

Both gas pressure and mechanical stresses acting on the reactor wall have to be taken into account for the design and the construction of reactor vessels in order to operate MH reactors safely. Additional

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ACCEPTED MANUSCRIPT expansion space for the gas-absorbing solid can help reduce residual stresses. An optimum between the induced stresses on the reactor wall and the volumetric hydrogen capacity of the MH reactor has to be reached. Future experiments focus on the influence of the MHC particle size and MHC shape on volume

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expansion and developed stresses during hydrogenation. A comparison of the results with fully confined MHC in the force measuring cell with freely expanding MHC (in-situ neutron radio- and tomography) is in progress and will be published soon. Furthermore, future experiments will focus on several aspects

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module of a solid-state hydrogen storage system.

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like axial and radial expansion spaces or the arrangement of more than one MHC to represent an entire

Acknowledgements

This work was partly funded by the Friedrich-und-Elisabeth-Boysen-Stiftung (contract no. BOY-103) and by the German Federal Ministry of Education and Research (BMBF project funding reference number

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ACCEPTED MANUSCRIPT Table caption:

Table 1: MHC characteristics in the as-compacted and as-cycled state after the experiment.

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Figure captions:

Figure 1: (A) Schematic cross-section of the metal hydride test reactor for the hydrogenation of metals (zone A with a max. reaction temperature of 350°C) in combination with the in-situ stress measurement at 25°C in zone B (thermal fluids in

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the genuine setup with removed thermal isolation and process connections.

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double jackets for active heating (zone A) and cooling (zone B), T1-T10 temperature measuring points). (B) Photograph of

Figure 2: Axial and radial force measurement setup: (A) cross-section view (axial direction) of the measuring cell; (B) simulated displacement of the upper half shell of the rigid confinement (red line) for 10 MPa mechanical radial stresses (wall thickness = 11 mm, ri = 20 mm, h = 20 mm); (C) photograph of the genuine setup with removed upper half shell.

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Figure 3: Metal hydride composites (75 MPa compaction pressure, 5 wt.-% Graphite) (A) in the as-compacted state and (B) in the cycled state after 43 hydrogen absorption/desorption cycles in the measuring cell.

Figure 4: Stress development of a confined MHC (15.5 mm height, 19.8 mm radius, ~29 vol.-% porosity, ~10 vol.-% graphite)

EP

during the first, the second and the 20th (steady state) hydrogen absorption/desorption cycle.

AC C

Figure 5: Maximum stresses during cycling and minimum stresses on the end of each cycle (Cycle 1 and 2: 90 min cycle time, cycle 3-5 and 12-20: 60 min cycle time; absorption/desorption pressure: 40 bar/2 bar; room temperature).

page 16 of 16 13. Feb. 2017, 13:21

ACCEPTED MANUSCRIPT Table Table 1: MHC characteristics in the as-compacted and as-cycled state after the experiment.

MHC state

height

radius

mass

density

porosity graphite

as-compacted

[mm]

[mm]

[g]

[g/cm³]

[vol.-%]

[wt.-%]

[vol.-%]

15.74

19.8

73.92

3.96

29.1

5

0.0

16.35

19.9

72.13

3.63

35.0

5

6.4

M AN U

desorption cycles in

SC

(undrilled) after 43 absorption/

expansion

RI PT

content

volume

AC C

EP

TE D

the measuring cell

AC C

EP

TE

D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE

D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE

D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE

D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE

D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT