Step-by-step guide to sample mounting

Step-by-step guide to sample mounting and making electrical contacts

Mounting the sample

Introduction

Every experiment will be different. On the techniques page, various different approaches, and the pros and cons of the different techniques are discussed. It is very likely you will need to mix and match different approaches and even develop new methods to find something that works for your unique experiment and sample.

This step-by-step guide illustrates one approach to sample mounting, relevant to a fairly common experiment; mounting a ~2 mm long matchstick-shaped sample to the strain cell and then attaching four electrical contacts for an electrical transport measurement.

 

 

 

Diagrams of a CS1X0 cells with parts labelled.

Preparation

Your sample will need to be clean, polished and have the correct shape – typical sample dimensions, that are large enough for handling but also, small enough (for most materials) not to excessively load the stress cell, are a few mm in length, and ~100 x 300 µm in cross section.

If you plan to use the top sample plate (see the techniques page for a discussion), then you should prepare your spacers now. Firstly, decide on a suitable approximate epoxy thickness: usually thinner epoxy lines are desirable. If the epoxy is thinner, strain is applied to the sample more efficiently, and higher strains are possible. On the other hand, some groups report that thicker glue lines reduces the likelihood that sample breaks at low strains at the point the sample enters the glue.

An epoxy thickness of 30–50 µm is a good place to start.

Sand the spacers down to match the thickness of the sample, plus twice the desired epoxy thickness. E.g. if the sample is 100 µm thick and ~30 µm-thick epoxy layers are desired, sand the spacers down to 160 µm thick.

Sand the spacers in a way that their upper and lower surfaces remain parallel. Wash the spacers thoroughly afterwards – conductive metal dust can damage the device and cause problems with your experiment.

Note that when using an FC1X0 it is usually not required to sand the spacers as there are fewer disadvantages to having somewhat thicker glue lines.

Titanium spacers, prior to sanding.

Holding the stress/strain cell

Before any fine work is carried out, it is necessary to hold the stress or strain cell so it is held securely with the sample mounting holes accessible. Most Razorbill Instruments devices will lie flat on a table (UC200, CS2X0T) or are provided with a work stand (FC1X0). The CS1X0 cells must be bolted to a holder to be worked on. This can be as simple as bolting them gently to a metal block with a hole drilled through it. Alternatively work stands for the CS1X0 cells can be purchased separately from Razorbill Instruments. Note that the ‘bridge’ of the cell should not be clamped or held or the piezoelectric stacks may be damaged!
 

 

A CS120 mounted in a CS1X0 stand. The stand allows the strain cell to be held in position under the microscope.

Positioning the sample mounting plates

To ensure the sample plates are straight and well positioned, sample plate guides are used. Please note that in the FC100, sample plate guides are not necessary because the teeth on the bottom of the sample plates prevent the sample plates from rotating.

Attach the brass sample plate guides as shown below. Do not tighten the screws yet – the sample plate guide should be able to slide beneath the screws.

 

 

The sample plate guide mounted to the cell.

The next stage is add the bottom sample plates. To make sure they are correctly positioned, it is helpful to also screw in the M2 screws that will eventually hold sample plates in place. Keep these screws very loose – they are only there to make sure the sample plates correctly line up with the sample mounting screw holes

 

The bottom sample plates, guides and mounting screws. Nothing is tightened and everything is still free to move.

Position the sample plates to an appropriate spacing. Once you have the bottom sample plates positioned where you want them, you can gently push the two halves of the sample plate guide together, gently pressing the sample plate into position. The objective is to not be so tight that you can’t slip in the top sample plate when you need to but tight enough that the plates don’t move or rotate when you later tighten the screws. Carefully tighten the screws in the auxiliary screw holes (holding the sample plate guide). For now, remove the screws from the sample mounting screw holes, placing the screws to one side.

At this stage, some users report that they deposit a tiny droplet of GE varnish on the edge of the bottom plates that forms a meniscus around the edge affixing the plate in position. Only a very small drop should be used because the plates will need to be removed after you experiment. Do not use so much GE varnish that it flows into the space around the flexures and/or into the capacitor; this will damage the cell.

To check that your plates are positioned correctly you can rest your sample so that it spans the gap. There should be a long enough unsupported section in the middle of the sample but also a long enough section overlapping with the sample plate to provide enough area for a strong glue join. For a 2 mm-long sample, it is common to have a 1 mm exposed length, and ~500 µm embedded in the glue at each end; not that if you will want to make electrical contact to the ends of the sample, you may want these ends to protrude over the central holes in the sample plates. When you are satisfied remove the sample again.

 

 

Left) The sample plates are correctly positioned, the screws in the sample plate guide have been have fully tightened and the screws in the sample mounting screw holes have been removed. Right) A sample is placed spanning the gap to check that the spacing has been set correctly.

 

Gluing the sample in position

The next stage is to affix the sample in position using epoxy. The end goal is not to clamp the sample, but to have to surrounded by cured epoxy, sandwiched between the sample plates but not in contact with them. The sample should be electrically isolated from the sample plates and minimal stray epoxy should have run over the top, bottom or sides of the part of the sample that is spanning the gap. A small fillet of epoxy extending beyond the sample plates is sometimes desireable to help strengthen the joint, but it shouldn't reach far enough to prevent you attaching contacts or to upset the strain homogeneity in the section of the sample being measured

In this guide we use Stycast 2850 FT Black Epoxy - with catalyst 23LV, but there are other options. Please see the discussion on the techniques page.

To stop the epoxy spreading out and causing a mess, you can use a higher viscosity epoxy, or allow your Stycast to partially cure (becoming more viscous) before you use it.

Some samples, like Strontium Ruthenate do not make electrical contact easily, so will rarely cause issues with shorting to the sample plates. Other samples can have electrical shorts to the sample plates if they happen to touch the metal through the epoxy. To prevent this solid spacers can be added to the epoxy. These can be specially made spacer spheres such as those from Cospheric. A low-tech cheap and easy solution can be to pull a few cotton fibres from clothing and place these in the epoxy where you sample is going to rest to prevent the sample sinking through and touch the bottom sample plate. Some groups paint a very thin layer of epoxy on the sample plate in advance allowing it to cure so there is a physical barrier between the sample and the sample plate.

Deposit two small droplets of epoxy on the bottom sample plates. To apply the glue you will need a tool with a very tiny tip. Options include a single fibre from a paint brush, an eyelash or strand of small diameter wire (such as magnet wire) glued to a handle such as a cocktail stick.

Once the epoxy droplets are added, place the sample on top. Add another droplet of epoxy to completely cover each end of the sample.

The danger at the point is that the sample sticks to the tool you are using to apply the epoxy and is moved from the correct position, possibly getting covered in epoxy. To prevent this it is helpfully to have a second clean tool to gently hold the sample down. Beware of breaking the sample by crushing it in the tips of the tweezers.

Now your sample is spanning the gap and each end is coated in epoxy you could choose to neglect the top plates and simply tighten the sample mounting screws and wait for the epoxy to dry. The top sample plates are designed to improve strain homogeneity and the glue bond strength but are an added complication that is not necessary in all cases.

If you are planning on adding the top sample plates, then add the spacer now. Carefully rest it so that it is directly above the screw hole.

 

Left) Epoxy droplets are carefully deposited on the edge of the sample plates. Right) The sample is lain on top of the epoxy droplets.

Carefully place the top sample plates so that they sit on top of the spacers and rest directly over the position of the bottom sample spacers. The plates should gently slide in. If the sample plate guide has been pressed together too tightly then you may have to briefly loosen them to let the top plate in (or lightly sand the top plates, until they fit). Make sure the top sample plates are resting flat in the correct position and the sample plate guides have been retightened.

Add the M2 screws. Keep inspecting the sample plates and sample to make sure nothing is moving. Gently tighten, alternating between the screws, stopping immediately if you see any displacement or rotation of the plates.

 

 

Left) The top sample plates are lain flat on top of the spacers. Right) The brass sample mounting screws have been carefully tightened.

Once the sample mounting screws are fully tightened, the sample plate guides can be removed and the epoxy can be left to cure for 24 hours. To check that the epoxy has completely cured, it is useful to keep the container the two parts were mixed in to test the hardness, using the tip of a tweezer. If you need to heat the epoxy to cure it, keep the temperature below 90℃ and short the drive wires, so that large thermal voltages are not generated in the piezoelectric stacks, which could damage them and break the sample.

Once the glue is cured the sample is ready to be strained.

 

 

Detail of a sample mounted in a CS120 strain cell.


 

Attaching electrical contacts

Introduction

Many experiments require measuring electrical transport. This requires attaching four wires to the sample. It can be particularly challenging in a strain measurement as the samples are small and are suspended across a gap. Some groups choose to attach electrical contacts with their sample before they mount their sample in the strain cell (see link). In this example we illustrate the process if it is done afterwards.

Contact pads

You will need to position your electrical contact pads near your sample. Your research group may have a preferred way of doing this. Some groups use a small piece of stripboard or homebuilt PCB stuck to the device. Razorbill Instruments can supply a WP100 wiring platform for this purpose. Wiring platforms are provided free with the FC100 and UC200 strain cells, the PPMS kit/probe or can be purchased separately. This tutorial used the WP100 Razorbill Instruments wiring platform.

 

The WP100 wiring platform can be screwed onto one of the ancillary tapped holes and places contact pads very close to the sample.

 

Adding electrical contacts requires particularly precise manipulation. To make things easier, we have mounted the sample stand into the Razorbill Instruments sample mounting table and positioned the table under a stereoscopic microscope. The mounting process can be further aided by mounting a micromanipulator to the mounting table for very fine control.

Adding the gold wires

We use 50 um gold wire to make the electrical connections. Using a very fine soldering iron tip, solder short sections of gold wire to the contact pads on the wiring platform. Make sure you only let the gold wire briefly make contact with the molten solder, or the gold will dissolve into the solder. For cryogenic use it is best to use lead solder (instead of lead-free).

 

 

The wiring platform allows the contact pads to be placed very close to the sample. The 50 um gold wire can be soldered so that the wires stick up and over the sample.

Once you have all 4 wires soldered so that they are sticking up vertically, you can use end-cutting tweezers to trim them to a more suitable length. Using tweezers, bend them so the end of the gold wires are making contact with the top of the sample. You should aim for all of the contact points to be in a line and clustered in two groups of two. Be very careful to very gently manipulate the gold wires as the soft metal is very easily crushed or nicked by the tip of the tweezers. If they do not make contact with the sample along a straight line, and you are measuring with an applied magnetic field, you will get some hall effect mixed in with your resistance measurement which you will have to correct for in your data later.

Using silver epoxy

Add a single drop of conductive epoxy to the tips of the wires. Use a single eyelash or fine magnet wire deliver the drops of silver epoxy.

In this tutorial we use Epotek H20E, but please see the discussion on the techniques page for a discussion of the different options.

Make sure the drops do not make contact with each other. Mistakes are very hard to undo, so it is worth practising many times on a similar sized but worthless sample before you attempt it for real.

Try and ensure the drop of epoxy covers the tip of the gold wire and wets the surface of the sample.

 

 

(left) The Gold wires are bent so that they rest on the sample at the correct position. (right) four tiny droplets of conductive epoxy are deposited to ensure the wires make a good electrical contact to the sample.

The device can be carefully transferred to an oven and cured at the lowest possible temperature (preferably below 90℃) to cure the epoxy. Note that whenever the stress or strain cell has its temperature changed it is very important to ensure the ends of the drive wires are shorted together. This is very important to prevent large thermal voltage building up in the piezoelectric stacks which could damage the device!

After a room temperature test to make sure all the contacts are well formed, the stress or strain cell can now be loaded into the cryostat for an electrical transport measurement.