Bending setup. Two cells for controlled hydrogen loading

Two cells for controlled hydrogen loading were designed and constructed. The first cell is schematically shown in Fig. 2.1. A sample, contacted for electrochemical hydrogen loading via a glued conductor, has to be clamped on one side.

___2. Experimental techniques_____________________________________________________

For the observation of buckle-formation, the electrochemical loading cell including the sample was mounted on an optical microscope stage. The film morphology development during hydrogen loading was monitored in situ with a CCD camera. The advantage of this cell is the possibility to monitor the surface of a sample during hydrogen loading, enabling the desired optical determination of the onset of buckling. This critical point is linked to the critical hydrogen concentration and the critical stress in the hydrogen absorbing layer (see chapter 1.2.4).

The monitoring of a sample during hydrogen loading allows the in-situ observation of the buckles’ morphology, buckled areas, which indicate places with a poor adhesion, etc. The disadvantage of such construction is the effect of buoyancy of the electrolyte onto the sample.

The density of the electrolyte is about 1.4 g/cm³. Therefore the buoyancy force on a sample with 30x7x1 mm³ dimension is about 2.9 mN calculated by using of Archimedes principle. But this force is comparatively small to the forces acting on the metal layer during hydrogen loading.

A highly viscous electrolyte was prepared by mixing two volume parts of glycerine with one volume part of phosphoric acid (85%). This electrolyte is hygroscopic and has to be renewed after several measurements. The high viscosity of the electrolyte decreases the mobility of molecular oxygen in the electrolyte and thereby reduces hydrogen desorption from the film surface. Additionally, the electrolyte was bubbled with argon before using to keep the amount of oxygen in the electrolyte small. Before loading, the Nb-samples were unloaded with a constant voltage of 0.4 V for the time interval t = 10 h. The discharge voltage should not be too large, since otherwise oxidation of the sample might occur and the electrolyte might dissociate [Lau98].

The stresses evolving in the metal layer during hydrogen loading were measured by determination of the deflection of the substrate due to film expansion. Therefore, the substrate was clamped at one side. The bending of the substrate was measured by using of a strain gauge mounted at the other side of the substrate. During hydrogen loading, the expanding film bends the substrate, as it is shown in Fig. 2.1. The vertical movement of the end of the sample during hydrogen loading is measured by an inductive sensor. The sensor consists of a coil which is at the same time a part of a resonant circuit. If such sensor is fixed close to a moving surface of a metal, e.g. a plate, then the induction changes as a function of the distance to the object. In the metal plate eddy currents are formed as a result from magnetic induction. According to the Lenz rule the eddy currents in the metal plate are directed in the way, so that they try to weaken the alternating current in the coil. The size of the eddy currents induced in the metal plate depends on the distance between coil and metal plate providing a contactless inductive displacement measurement. The sensor can be calibrated by using different metal plates. Substrate thickness and length have been optimized in order to be able to measure with the inductive sensor. In our

___2. Experimental techniques_____________________________________________________

niobium films with 50-200 nm thickness on polycarbonate substrate with 30 mm length, 7 mm width and 0.25-2mm thickness were chosen. Such geometry of the samples has the proper length-to-width ratio for using Stoney’s formula (see below). It has an appropriate area for sputter deposition and its bending during hydrogen loading is not larger than the measuring range of the inductive sensor.

The movement of the sample during “bending up” and “bending down” is shown in Fig. 2.2.

Figure 2.2: Schematical picture of sample bending. a) Bending down, this results in the negative slope in a stress-hydrogen concentration curve. b) Bending up, the slope is positive.

The sample can move upwards and downwards, while for each moving direction definite physical processes are responsible, that have been identified in this work. The calibration curve for the sensor I-W-A-/A4 (Amos company) against a Pd- plate is shown in Fig. 2.3.

Figure 2.3: Calibration curve of the sensor I-W-A-/A4 against a Pd-plate.

The X-scale in this plot is a distance between the sensor and a Pd-plate. The linear change of the

___2. Experimental techniques_____________________________________________________

Figure 2.4: The second cell for controlled hydrogen loading, prepared for exhibition during Hanover Trade Fair 2006. The blue wire is for the contact with a sample, the red wire is contacted with the counter electrode.

The second cell was constructed for presentation during an exhibition in Hannover and it was made from acrylic glass. This cell allows samples to be mounted and removed quickly. With a special clamping construction it is possible to contact a sample to electrical supply by pressing a needle onto it, which is connected with a wire and sealed with a rubber for protection from electrolyte. The sample can be moved to the inductive sensor in order to be in the measurement range of the inductive sensor. The disadvantage for this cell is that it is impossible to watch the surface of the sample using the light microscope during the measurement. Because of that the first cell was mainly used.

2.4. Buckles observation and determination of their shape for adhesion