 Hello everyone, welcome to this material characterization course. In this class, I would like to discuss about the sample preparation techniques for TEM analysis. You have to understand one thing, the sample preparation is the biggest task as far as the TEM analysis goes. It takes lot of pains to prepare a suitable sample and getting a suitable sample is almost becoming you know it is a very long process because you need to produce a sample with lot of care because you want to produce a very thin section without I mean incorporating an external strain or I mean contamination in order to characterize the material in its original form. So it becomes very difficult to prepare a sample and also it requires lot of patience and time. So I will just go through some of the basic principles which is been I mean which is underlying the specimen preparation and also I will just show some of the laboratory demonstration in taking you through all the tools which we have in our electron microscopy laboratory. So I will just briefly give a presentation on various techniques adopted for a sample preparation whether it is a metal, a ceramic or a polymer or a powder and so on. So then we will take up corresponding individual case studies. So most of the information is taken from this textbook by David Williams and Barry Carter. If you look at the overview of the specimen preparation, specimen preparation could take time from 10 minutes to 1 or 2 days depending upon what we want to learn from the material. There are 2 types of specimens, self-supporting specimen, specimen resting on a supporting grid. We will look at what is these 2 types and mechanical thinning. When I say mechanical thinning, I am keeping a material like metals and ceramics in mind. So we will also show the procedures of what is mechanical thinning and the ideal way to do this job is to using the spark erosion or electric discharge machining instead of doing a manually. This is also we will demonstrate. A rule of thumb is any abrasive will produce a damage to 3 times their grid size above and below the surface. Hence the final disc must be thicker than the 2x the damage depth or else the mechanical damage depth will always be visible in the final specimen. So what we are trying to say here is we are simply talking about a mechanical thinning. You will see that we will be demonstrating how to prepare a thin metallic sample or a ceramic sample where we will be rubbing this specimen against an emery paper or an abrasive paper. So you have to be very careful about this because this abrasive paper will have in itself you know it will produce a damage up to 3 times its grid size. So the grid size of the emery paper is important. So you have to take a very fine emery paper if you have a very thin sample otherwise it will produce its own damage which is not a characteristic of a material itself. So you have chemicals which we use in specimen preparation. You have HCN, HF, HNO3, HCLO4. You have to be very careful because most of them are poisonous and explosives and then you have this perchloric acid that is HCLO4 plus ethanol or methanol. Mixer is called universal polish. That means most of the metals if you use this as an electrolyte that will polish. So what we are now specifically talking about is a mechanical thinning followed by an electrolytic polishing. We will get into the detail. But since in earlier days the analysis was primarily done on a metallic sample and these are the techniques were developed in the beginning. So now you have sample preparation techniques for other materials such as ceramics, polymers and biological and powder and so on. So we will just also follow that order. We will just look at first metallic samples and still more relevant people are using it and then this technique which is specific for ceramics and polymers I will just keep mentioning it then and there. So the electrolytic mixture perchloric acidic and water phase diagram is very important because as I mentioned in the previous slide this is mostly used as universal polishing electrolyte. So if you see that phase diagram this is perchloric acid water and then this is acidic acid. It shows the hazardous regions and the recommended density line for a safe use of all the perchloric solutions. See that you have a boundary with which if you mix these three acids it will be a flammable or it could be an explosive also. So you have a safe line under which you have to operate whenever you produce a electrolyte for the metal thinning or material thinning. So just to give that point this slide is brought and some of the sample preparation accessories are shown in this slide. This is a punching machine, a sheet punching machine and then these are all grits of various kinds and this is the you would say that you know this is foldable grits where you can keep your sample inside and then it is locked so that it is safe and intact. So we will go through all this when we actually take up some practical examples. A mechanical punch for stamping a disc from this thin sheets of ductile materials. So this punching machine will produce a metallic specimen of 3 mm disc in order to suitably placed in a sample holder in a TEM. So we will also show you the actual sample holder how it is being used. A sheet sample is placed in the punch as indicated and the handle on the right is pushed down ejecting 3 mm diameter disc suitable for thinning. So it is I will show the live demonstration so do not worry about it and why 3 mm because it has to suit the sample holder. A variety of specimen support grits of different mesh size and shape at the top right is the oyster grade useful for sandwiching small silvers of thin materials. So this is for sandwiching this sample and then you also have a dimpling apparatus this is all coming under the pre-thinning apparatus category we will we have this machine in our laboratory we will demonstrate but the principle is simple surface dimpling using a chemical solution example to remove the silicon from one side of the disc the light pipe permits the visual detection of the perforation using the mirror so this is the setup. So you have the etchant and then this is the jet and this is a sample is kept on this and you have the mirror to view it and the light will pass through this perforation once the etching is complete which produces a hole in this tube. A dimpling apparatus showing a grinding tool and a specimen supporting block so you have the sample will be kept here typically a ceramic sample will be kept here and then this will will grind this specimen and make a dimple that is why it is a name dimpling apparatus we will actually show you the demo how it is done and then mostly the electro polishing is called final thinning so you have the typical IV characteristics that is current voltage characteristic of an electrolytic reaction you have etching region you have a polishing region and then pitting region. So before you start any of this electrolytic polishing you have to characterize the electrolytic action so you have to choose an appropriate current and voltage in order to stay in this polishing region otherwise it is again a very tricky situation and you can see that you have this kind of a layer which forms on the specimen you have the specimen here viscous fluid film and then electrolyte and you have a thin solid oxide film also forms on the specimen we will see how to handle this and one of the criterion for using this electro polishing technique is it is only electrically conducting materials no mechanical damage but surface chemistry can change and hazardous to the health you have to be careful better to wear a precautionary tools and safety devices before we engage in this exercise and so what this curve shows electro polishing curve showing the increase in the current between the cathode as the applied voltage is increased polishing occurs on the platoon etching at the lower voltage and pitting at the higher voltage so the ideal conditions for obtaining a polished surface require a formation of a viscous film between the electrolyte and the specimen surface. So this is an ideal condition for the specimen preparation so how do we make sure that so we have an automatic machine which characterize the ideal conditions for polishing I will show you when I demonstrate this action in the laboratory and the another type of jet polishing is shown here basically you have the two methods what we have in our laboratory is this method you have a twin jet you have the sample in between and this is a research circulated electrolyte and then you have the pump which passes the electrolyte through these two nozzles and then producing the dimple on both sides and eventually make a perforation inside the sample and that forms your final sample for the TM analysis. So first one is a retro polishing by allowing a single jet of electrolyte and this is a twin jet electrolyte so this is a gravity led electrolyte to thin a disc supported on a positively charged gaze so this is one method and what we are going to demonstrate in the laboratory is this and as I mentioned this is a twin it is called a twin jet polisher so the disc must be rapidly extracted from the electrolyte and washed in the solvent to remove any residual electrolyte which may etch the surface this is very important operation in jet polishing once the sample is taken out from the jet polishing unit you see you have to remember that the electrolyte is still sticking to both the surfaces so you have to rapidly clean this specimen which is taken out of this electrolyte and thoroughly washed with the water not in one beaker or you have to keep three beakers with an alcohol and then you have to rapidly agitate the specimen to remove in order to remove the electrolyte from both sides so that the etching action is stopped and then you are which will eat away your material slowly which you may not see it with your naked eye but it will only once you put it under microscope you will see that that is more important I will also show this aspect I am telling this right now itself because this is one place where people fail and then they do not get the good sample of after electrolytic polishing so this is very important step. Undoubtedly you get a better at electro polishing with the practice obviously this is a kind of you know it is a recipe everybody will have their own way of doing things because electrolytic reaction is depending upon so many factors which one has to look at the current to the voltage the specimen condition the sample surface condition everything will vary a person to person or the specimen to specimen with which they are handling so even your pre-thinning operations will have a significant role to play in electrolytic polishing for example if you are not very careful with the abrasive thinning or mechanical thinning where and then you may be using a glue to stick this foil onto a bulk sample and if you do not remove the a glue properly then also your electrolytic reaction will not be proper maybe one side it is stuck with the glue the other side is exposed to the electrolyte so it will be a non-uniform thinning will takes place and one very important aspect everybody forgets in this exercise is suppose if you do the electrolytic polishing I mean before you come to the electrolytic polishing you better to look at the sample under the optical microscope both sides whether you have any contamination sticking onto your sample it could be a debris from the emery paper or abrasive paper or it could be a glue which is still sticking at one of the edges or it should be if it is stored in an open space or dust particles or any other foreign material which will be sitting on the sample which you may not observed with a naked eye and it is essential that you observe this sample under an optical microscope before you come to electrolytic polishing then you will if you follow that procedures and then follow the jet polishing procedures I am 100% sure you will be able to produce a nice hole in the sample with a very large thinning area so the importance I will just show you when we put the sample in the microscope and then you will appreciate what I am now talking about. So it comes with the practice but reproducing the correct condition of temperature electrolytic solution chemistry stirring rate applied voltage polishing current etc can only be achieved through trial and error so this is another important point you have to remember you have to do few trials before you come out with the proper combination of parameters keeping the records of each experiment is very important of this to stabilize this jet polishing the another important technique is iron milling this particular technique is especially meant for a non-metallic samples especially a ceramic samples so we will briefly go through this schematic where it demonstrate the how the the basic functions of the iron milling machine you have the the specimen which is kept there and then you have the circuit iron gun which comes and impinges on the specimen surface and it will started to thin we will see what it is so the schematic diagram of an iron beam thinning device will look like this argon gas bleeds into an ionization chamber where a potential up to 6 kilo electron volt creates a beam of argon ions that impinge on the rotating specimen so this specimen is rotating that is what shown in this arrow so the beam of argons will fall on this specimen surface we will see we will also show the actual mission how this operation is carried out the whole apparatus is under vacuum the specimen may be cooled to a liquid nitrogen temperatures and perforation is detected by a penetration of ions through the specimens so in fact you will see that the specimen is being thinned and by both the directions so and also in some of the machines you have the the cooling system which can go up to liquid nitrogen temperature and some of these systems will not have the liquid nitrogen setup so the the iron impingement on the both sides will be there and you can see that and then how nicely the thinning takes place in this particular region where both the beam impinges on the sample on a come on the straight line please remember the the pre-thinned samples are only put on the iron million missions not the bulk sample which is very important so this is again very very delicate to delicate to handle it because if you are not performing the pre-thinning exercise much more carefully again this iron milling is not going to help you which is going to have lot of problems which we will demonstrate in a laboratory so another important aspect of this argon thinned specimen surface you can see this micrograph these are all bright field images of cadmium tellurium showing the defects in the argon thinned specimen and this is an undamaged crystal thinned by reactive iodine iron milling so you have to be extra careful about this iron milling is going to create lot of surface damage and in order to use this you have to you have to keep that fact in mind it is going to create a lot of defects so if you do not choose an appropriate parameters for this thinning here again you are going to spoil your sample or you may be without noticing this if you start doing the microscopy you will be wrongly characterizing the damage which is generated be sample preparation which you will interpret as the material characteristic which is quite dangerous so you have to keep that in mind so if you want to produce a cross section specimen then the here are some of the procedures which is shown here the sample is cut into thin slices normal to the interfaces which are glued together between the spacers which could be silicon glass etc wider than the slot in the grid so this is the spacer so you put all the slices and then put it inside the club sandwich is then itself glued into the grid and iron milled to the perforation so the whole thing is put into this spacers and then and then it is tightly glued into the grid like this and then finally iron milled so this surface get iron milled and then you will be able to see the cross section specimens so it is very involved procedure here again and one of the most conventional way of preparing the sample was through electrolytic polishing is the window method of for metals and alloys where you have the an electrolytic setup for polishing electrolytic polishing and you have the sample window which is completely covered with some kind of a lacquer or so and then it will go through the sequence of electrolytic etching like this so we will go through the remarks a sheet of metal 1 centimeter is lacquered around the edges and made the anode of an electrolytic cell and if you look at the progress during the thinning the initial perforation usually occurs at the top of the sheet like this lacquer is used to cover the initial perforation and the sheet is rotated 180 like this the thinning continues to ensure that final thinning occurs near the center of the sheet if the final edges smooth rather than a jagged it is probably too thick so if you have a very thin region you will your final specimen will appear like this if it is too thick it is it will be like this it is true for even a twin jet polishing also if you have a thin specimen your final surface will look like otherwise it will be like this see one important advantage of doing this window technique is you will have a large thinning region as compared to a twin jet polished 3 mm disc and here it is not 3 mm disc it is a big window you are getting so you will get a very large area of thin region to examine which is a great advantage for a TEM analysis. The another important technique is ultra microtomy this is primarily used for a polymeric material soft material and we will just go through the principle and then here again we will show the actual demonstration in the laboratory how to do this you see the basic operation is shown in the schematic you have the base with the knife and then you have a small trough we call it as a boat sometime and the specimen is kept in a or stuck within a I mean you can this is a mobile arm I would say that thermally advanced arm and this is the feed direction like your lathe and that is how the specimen moves ups and down and this is a knife the dark triangle and this is a trough so the specimen will impinge on the knife edge and then it will be coming out like this so this is the the trough in the other view it is enlarged view and this is the knife and this is your arm this is how it look like so it will cut through like this the specimen is going to just float on the trough because the trough will be filled with water and then you have this is a cutting surface and you have the sample here it is a sample block and the direction of motion is this up and down so it will be taken as a simple thin section of a desired thickness which can be taken with the TM copper grid you can just fish it out and then directly look at under the microscope so the sample is first embedded in the epoxy or some other medium or the whole sample is clamped or moved across a knife edge see if your sample this all depending upon the sample dimensions if you have the very small samples you have to stick it with an epoxy or something else or your sample itself it is a bulk you can directly clamp it on to the arm and then perform this operation you produce a thin flakes of that floats off onto the water or an appropriate inert medium from where they are taken on to the copper grid like this see another important technique is a replication or an extraction technique which is very important for some of the analysis of a second phase particle in the metallic specimen for example if you have very you know suppose if you have a magnetic sample for example iron or a steel and so on and then you have the second phase particle which is there in the material in order to obtain the information from the second phase particle alone and if you put the metal foil the metal foil also will influence the electron beam and because of that you will not be able to focus or obtained a specific information from the second phase particles to in order to take the you know the information from the second phase particle alone and this extraction replica technique is being used see the one of the bay to produce a replica is shown in the schematic so you see the bulk sample which has a second part phase particle like this so after etching you are going to clean this and then it will all your second phase particle will come almost to the top surface and then you do a coating with typically with a carbon or anything any any any thin film you coated and then you that is just extract this the the carbon film from the bulk so that will have this kind of a situation where all your second phase particle will stick to this carbon or any thin film which you coat and if you are able to extract this I mean since the film is carbon film is extracting this particle from the bulk material that is why it is called a carbon extraction replica and this replica can be put on the grid and then you can view it under the microscope so here it is shown with some kind of a typical plastic and which will take the impression and it will within a chemical etching you produce this kind of a self supporting replica then you can view it this is this kind of a technique is for some fracture surface you can analyze you take the impression of the fracture surface and then look at the features carefully and once the replica is produced then you can just slide it on the water then it will float like this so that is also clearly demonstrated so this is very important technique if you want to take the information T or if you want to perform a TM analysis on a second phase particles without the interference of the matrix then you will be able to use this technique and finally you see that after doing all this jet polishing or final polishing then if you want to clean the surface of the specimen because after all these things your specimen is highly corrosive or amenable to all the oxidation and so on then it is better to clean that surface before you put the specimen in the TM column so in order to clean that you have something called plasma cleaners the plasma consists of energetic electrons and ions that bombarded the surface and break the carbon hydrogen bonds with the short duration of exposure the surface if the specimen itself is unaffected the hydrocarbon gradually reduced in the molecular weight and pumped oxygen nitrogen and argon are the most commonly used gases the hydrocarbon gradually reduced in molecular weight and pumped away in the vacuum of the cleaner oxygen nitrogen and argon are the most commonly used gases for this kind of operation so this is the typical plasma cleaner this is how it look like so the the instrument and cross section details are shown in this schematic you have the this is a main housing and this is the specimen holder which after it inserts into this chamber how it look like so you have the quartz tube and this is a gas inlet and which comes and the all the vacuum the specimen this is the o-ring with which you know you the specimen will be inside the vacuum of the microscope similar chamber is here and then the cleaning operation is done so now what I will do is I will just quickly go to the laboratory demonstration and I will just start with a simple low speed saw and this is a typical diamond blade which is being used to produce any thin section so we will start with from this how we proceed with the cutting action so you have the specimen you can put it in the diamond saw slow speed diamond saw the advantage of this machine is you can produce a sections of very thin sections you can produce typically 0.1 mm and these are the the details of the equipment you have the coolant for the slow speed axon some of the coolants are commercially available so we can directly use it and the the material which is being taken for this demonstration is some aluminum alloy and you see that it is now almost producing a thin section and you can and the specimen thin section is I will the thin section is further polished by mechanical thinning for that it has been pasted on the bulk sample like this and then being rubbed on the very fine emery paper and then you just measure the foil thickness with the micrometer the thumb rule is it should be below 100 micron so you have about 67 micron thick foil and then you take that foil into the the punching machine so this is how the 3M discs are produced in this using the punch machine you can clearly see the punching action with the punch and the die and your 3M disc will be collected in this a container which is kept below this so after punching this you can produce as many number of 3M disc from the a lot thin disc which you have mechanically polished and then you can produce as many specimens as possible from this you can punch out and then you will do this jet polishing okay I will stop here and then I would like to tell you something before we proceed with this jet polishing demonstration in the if you do not produce this thin section with the slow speed hacksaw properly then you will have the problem throughout the subsequent operations for example if you want to produce a parallel thin section it is very important that you remove one section by the diamond blade you remove that do not take the first because you may not know whether whatever the the sample you are putting for the first cut is it parallel the surface is may not be parallel so in order to produce a parallel surface you remove first section do not use it from the second section you start using it the you can go up to very thin section depending upon your sample availability so take that and also make produce the section with a proper cutting parameters for example I did not talk about the kind of load you can use for the cutting the speed and the coolant flow everything is important and if you are not following this procedure then you may spoil the the specimen there itself or if you produce a taper section from the slow speed saw itself then that will carry on you will you will have a problem until the final jet polishing so a slow speed hacksaw if you if you if you know how to use it you better learn it and then produce a parallel uniform thin section and then that parallel uniform thin section should be maintained during the your mechanical polishing you should not apply a lot of mechanical force because you will be generating a damage by your own force so you prepare a very thin section and then get into this 3 mm disc so now you continue this we will continue this jet polishing unit so this is the specimen holder in a twin jet polisher the 3 mm disc is kept in the specimen holder and then now this will be put it inside the twin jet polisher so so this is a liquid nitrogen bath we are going to or now this this is an electrolyte and then this will be kept in a mixed with an liquid nitrogen bath and then the setup will be immersed in that so the electrolyte is being cooled so the whole the electrolytic thin jet unit is being is kept on the bath and then you insert the sample in the slot and now you can look at the polishing recipe on the digital display and here you can have a standard recipe which is provided by the machine supplier or you can have your own recipe as I mentioned in the presentation the IV characteristics of the sample should be first established and you have to choose an appropriate electrolyte temperature polishing time and so on so the to in depending upon the specimen and the thickness the machine gives a kind of a rough range of current and voltage optimized and then you can choose that guideline and then further proceed with the polishing machine so what is being now done is just first do a scan that will give you an idea depending upon the thickness of your sample it will give you the appropriate thinning region so that is called a scan or I can say that a simple scan it will show some rough idea of the polishing IV characteristics so if you follow that method you will avoid the the spitting or not pitting region or as well as or if it is not good enough for the polishing so in this case this point has been identified as the polishing region then you go ahead with the actual electrolytic polishing I repeat the recipe what we are following in this electrolytic method it is not done in one go it is done with trial and error you have to do two three times to see whether you are getting the kind of specimen thinning which you wanted so what I will do is I will just go to the thinning after thinning this is the important step once the thinning is complete it gives the beep sound because it will sense the light through the specimen and then you have to remove this very quickly and then you have to wash it in an alcohol much more rigorously you have to wash 3 4 petri dish you can keep it and wash it thoroughly and then only you will be able to look at you can see that a small hole and this is going to be put on the TEM holder this is the 3 mm disc which is being loaded on to the TEM specimen holder yeah this is the a single tilt holder and now before we put it into the microscope this this is being cleaned by a plasma cleaner so this is a plasma cleaning unit again it has got its own recipe you have to follow a procedures and since it is done in a vacuum again you have to check with the vacuum and like a jet polishing mission this also has the readymade recipes available for type of specimen or type of operation so now the vacuum is on so the pumping is being done and so you will see the so this getting ready now so this cleaning is done typically for few minutes so there is a recommendation depending upon the sample so typically it is 4 minutes in this case and then you see that the indication with the glass glow in this schematic and you can also view through this window how this plasma is getting I mean specimen is getting cleaned by the plasma and then it is all very simple automatic process then now your sample is ready for the viewing in the TEM so now so that metallic part is over now what we are now going to see is suppose if you have the non-metallic sample in case of a ceramic sample typically a ceramic sample now I will just demonstrate how a ceramic sample is being pre-thinned for example it has to go through a dimple grinder which I just showed in the presentation and for this dimple grinding this the simple video which we are now going to show this is a stub and this is a glue typical a polymer based glue which is commercially available so you put this glue you melt it on the hot plate and put a glue on this stub in this case for just for a demonstration purpose we have not taken a ceramic sample but it is again a metallic sample just for demonstration but typically a ceramic sample would be it should be used for this kind of thing so the specimen is being stuck on the sample stage and this is a magnetic stage and then you have the a simple microscope to look at the specimen surface and alignment you can see that so it will be done very fast so you see that now the microscope will be mounted on the specimen stage so that you can just look at the thinning action it is a pre-thinning action I would say so now you can look at this and then align align the specimen stage in order to produce a proper dimple in the center of the 3 mm disc so that is very important you have to produce a dimple in the center of the sample that is why this microscope is used then you choose again here in appropriate parameters so you have the grinding wheel which will touch on the sample and that this is the dial gauge which controls the thickness so you can control the thickness in microns and then speed of the grinding wheel is controlled and you here again you have a specified recipe or you have to arrive at your own recipe for thinning it and this is an a ceramic abrasive glue which is placed on the specimen and then now the grinding action will takes place that is dimpling action you can see that the specimen stage is also rotating in order to produce a uniform a dimple and the center so this is how the dimpling is done so it is very important that the dimpling is done in the center of the specimen and after this dimpling you can you can take this sample for the further thinning in the iron miller so this is a pre-thinning typical pre-thinning operation for as I mentioned it is a ceramic sample so after this this sample is taken to the iron miller or if you are dimpling in a in a bulk sample you have to produce a 3 mm disc a 3 mm disc cannot be produced with the disc punch which we have shown because only metallic metallic specimens are prepared by that disc punch so for a ceramic material you have something called an ultrasonic disc cutter with that you prepare a 3 mm disc of specimen from the ceramic sample then you can perform this dimpling action then go to iron miller so there are 3 stages the the in the next class I will demonstrate the how the ultrasonic disc cutter functions and then followed by the iron milling equipment and then we will move on to the next technique thank you