Techniques

Experimental Techniques

 

Users of Razorbill Instruments uniaxial stress/strain cells carry out a wide variety of different experiments on a wide variety of different materials. Because each experiment will be unique, there will necessarily be a certain amount of trial-and-error in developing new procedures for mounting samples and applying your chosen probe. This page offers a collection of useful techniques, some which were first suggested by our customers. If you are looking to do your first experiment, and it is a standard electronic transport measurement on an ideally shaped sample, then our step-by-step guide may be a more convenient starting point.

The Main Goals in Sample Mounting

There are three goals in mounting you sample.

  • The sample is flat and straight.
  • There is a strong enough mechanical join at each end to transmit the force.
  • The glue lines are well controlled. This will mean one of two things depending on your sample:
    • If you sample is fragile it is helpful to have the glue to slightly wick out from the sample plate to allow a gentle transition from the glued part to the unglued part. This reduces the stress at the point where the end of the sample is held by the sample plate which is the highest strain area a most susceptible to breakages.
    • If your sample is robust, the glue lines should be kept as sharp as possible, so there it is unambiguous in your calculations what the length (L) of your strained sample is.

A sample mounted on a FC150 stress cell.

If your experiment involves an electrical probe you also require that:

  • The sample is electrically isolated from the sample plates.
  • There is no glue covering any surfaces that you need to make electrical contacts to.

All of the Razorbill Instruments products are similar with respect to sample mounting. All of them have two sample mounting screw holes. During operation these will move closer together or further away. Your sample will be glued to sample plates that will be anchored to these holes, so the movement of the sample mounting screw holes will cause a strain to be applied to the sample. The exact form of the sample plates can be relatively easy to change. As well as our standard sample plates, we provide flexure guided plates designed for small samples, plates designed so that the sample sits narrow edge upwards and sample plates that raise the sample up to give better access for optical, beam and scanning probe techniques.

 

 

These images show an exploded diagram of how our conventional sample plates are used along side photos of a sample mounted in this way and a particularly small sample mounted on our flexure sample plates.

Mounting a bulk sample to the stress/strain cell

Sample Preparation

Sample Shape

Most experiments using Razorbill Instruments strain cells will involve applying known forces or displacements to samples with long, thin aspect ratios. This ‘needle’ or ‘matchstick’ shape is preferred as it means that the spring constant of the sample will be as low as possible.
Before starting your experiment examine your sample and calculate the spring constant. This can be estimated relatively easily by multiplying the estimated cross-sectional area (A) by the Young’s modulus (E) and dividing by the strained length (L).

A diagram of a matchstick shaped sample with the relevant lengths labelled. It is important you have a reasonable estimate of the height (H) and width (W) before the sample is mounted.

The above picture shows a sample spanning the gap without epoxy. The microscope camera was used to measure L, the length of the sample suspended between the plates.

 

k=EA/L

The strained length, L, is the length of the sample that is actually strained during the measurement and will be equal to the total sample length minus the length of the sample that is embedded in glue at each end. You will probably need at least 100 um of glue at each end of the sample, and for stiff samples more than this.

The Young’s modulus can be found for many materials in the literature. If it is unknown, 100 GPa can be used as a reasonable estimate for most ceramics.

It is very important that the sample spring constant is within the allowed sample spring constants for the ‘CS’ strain cells. Using a sample stiffer than this could cause the cell to immediately break when a voltage is applied. The FC100 strain cells can tolerate any sample spring constant, but the spring constant will still determine the maximum strain that can be achieved with the available force.

Sample Finish

Many of the materials studied using these devices are highly brittle. Consequently, the samples can break under strain well below their theoretical ‘ultimate strain’ if the samples have any surface defects. The samples that reach the highest possible strain before breaking will be the ones free from defects and this can only be achieved by careful polishing or some other kind of smoothing process, such as an isotropic etch, prior to mounting.

It is worth noting that most users find that the samples will withstand much higher strains under compression than tension and there is often as much interesting physics in that direction.

Unconventional sample geometries

There will be numerous experiments in which the standard sample plates will not be suitable. Sometimes it will be necessary for the end user to design a custom sample plate for a unique experiment.

Razorbill instruments have developed several alternative sample plates that can be purchased for particular experiments. Two are described below.

 

Flexure sample plates

There will be some samples that can only be fabricated in small crystals. Some samples will have photolithographic structures or patterning which will mean they cannot be cut to a matchstick shape. Some materials are inherently two dimensional will not stretch unsupported between a ~1 mm gap between two separate sample plates. In all of these cases it may be more convenient to have a pre-set two-hundred micron gap set between a set of flexure guided sample plates.

Raised sample plates

For some experimental techniques it is necessary to have very good access to the top surface of the cell. This could be because, in the case of a scanning probe technique a physical probe must be able to touch sample without being obstructed by the screwheads or top sample plates. Similarly, some optical techniques require the top surface of the sample to be as close as possible to the optical window. In these situations, raised sample plates with counterbored holes for the mounting screws. This enables the top surface of the sample to the highest point in the cell.

Choice of Epoxy

Each sample is different, so finding a suitable epoxy may involve some trial and error to what works for you. The following are a few suggestions to get started with.

Masterbond EP21TCHT-1

A high-quality epoxy that is certified as cryo-compatible, well thermally matched to titanium, NASA-rated low-outgassing and mechanically hard is Masterbond EP21TCHT-1 epoxy (https://www.masterbond.com/tds/ep21tcht-1).

The disadvantage is that it is expensive and not widely available. It is also slow to use: for the epoxy to reach full strength it must be left overnight and then cured at 90℃ for two hours. Note: please ensure the drive wires are ALWAYS shorted during any temperature change or the piezoelectric stacks may become damaged, recently purchased cells are provided with shorting caps for this purpose.

Stycast 2850 FT

In the published literature on the subject, almost all researchers use Stycast 2850 FT with catalyst 23LV or 24LV to mount their samples in the stress/strain cells. It tends to be readily available and cures at room temperature (so is a bit kinder on the device).

The disadvantage is that it is not specifically designed as a cryogenic epoxy (though it is reported to work well), and that when it is freshly mixed, it is very inviscid (runny) which can lead to it flowing to areas its not desired. 

Generic Epoxies

Although there are few epoxy specifically designed for low temperature use, it is our expectation that most widely available household epoxies (such as araldite) would probably give good results at low temperature. A rapid curing epoxy could even speed up the sample mounting process.

Optional Accessories

WP100 Wiring platform

If you are planning on performing an electrical transport measurement it can be very convenient to have a set of electrical contact pads held close to the sample.

Device Stand

When the sample is mounted it can be helpful to hold the cell upright using a stand. Our stands can then slot into an optical table (shown below, available separately) which a micromanipulator can be mounted on to make very fine work easier.

Please note that the FC100 is provided with a stand for free, but for the other models they are sold separately.

Making electrical contacts

Bonding methods

Using silver paint/paste

Silver paint is commonly used to quickly make contacts and fix broken connections. It consists of conductive silver particles suspended in a solvent. This is then painted on a surface, the solvent evapourates and the solute precipitates forming a conductive join. The advantage of the technique is that the join forms quickly, usually without requiring annealing and it sticks to most surfaces. It is also cheap and widely available.The disadvantages are that for very small connections it is extremely difficult to ensure a tiny droplet remains wet before it is deposited on the surfaces and the joins created tend to be mechanically weak.

The key to using silver paint effectively is getting the consistancy exactly right so that it does not dry on the brush before it is brought to the electrocal connection and but it is not too dillute to form a good bond.

Due to the flaws listed above, we recommend avoiding the use of silver paint unless there is a lot of existing skill in your research group or there is no other option due to the types of materials you are investigating.

Using silver epoxy

Silver epoxy is an adhesive that consists of one or more cross-linking polymers with conductive silver particle suspended in it. A given trigger, for example the combination of the two polymers, the addition of a catalyst, the application of heat, will cause a crosslinking chemical reaction and the polymer matrix will harden. The conductive silver particles are loaded into the matrix at such high concentration that there is always a conductive pathway through them.

This technique can lead to strong, reliable electrical contacts. The disadvantage is that it can be challenging to produce neat, distinct joins and removing epoxy that has been accidently deposited is tricky. Until the epoxy is cured the connections will be very fragile so may spontaneously break. Despite these disadvantages, silver epoxy is, in most cases, the best approach to making electrical connections.

Using wire bonding

Some sample materials can form good electrical contacts using a wire bonding machine. This is a quick and very neat way to make electrical contacts to the sample. The distadvantages of the technique are that wire bonding only makes good contacts to fairly few materials, and the wire bonding machine often had to press down quite firmly to make a connection (which has a chance of breaking an already-mounted sample) . Adhesion of the wire bond can be improved by preparing the surface well (polishing it smooth and in some case depositing a layer of gold).

 

The images above, show a Nickel-Titanium sample that has had electrical connections added using wirebonding. Note that in this case a customer-designed wiring platform has been used. Many thanks to Alexander Kunzmann of IFW Dresden for sharing this image

We are always interested in hearing more about how our cells are used. If you know a technique – either in your lab or one you’ve seen published – which would fit on this page please let us know!