 So, recently a very jazzy term of nanomaterials has come into existence and because of that all the research and the funding has been directed to nanomaterials, science and nanomanufacturing. So, in this lecture we will learn about what is nanomaterials and how the science is dictated by various phenomena and how we can impart these knowledge, this knowledge into nanomanufacturing. So, there are couple of definitions which need which we need to understand before we progress into manufacturing those. So, first thing is nanoscience, nanoscience is the study, science study. So, nanoscience is the study of the fundamental principles of molecules and structures which lie between size range of 1 to 100 nanometer. Correspondingly we should not confuse it with nanostructure, nanostructure is some sort of a structure which is a size between 1 to 100 nanometer, it can possess variety of shapes, it can be tubular, it can be quantum dot, it can be a sphere, it can even be a conical or pyramidal shape, it can be a nano flower, any entity any structure which is a size there is between 1 to 100 nanometer and these go on to comprising what we call nanomaterials. Then there comes a term nanotechnology and nanomanufacturing. Once we have all these nanomaterials we need to have some sensitive instrument which can trap the nanoparticles and it can overlay them into forming certain bigger entity. So, nanotechnology is an art or an application of these nanostructures into some useful shapes which can make either components or devices to take advantage of these nanomaterials. So, we learn nanoscience is the study of these materials which range between 1 to 100 nanometer. Nanostructure is some sort of a structure which is size in between 1 to 100 nanometer and nanotechnology or and nanomanufacturing is nothing but the art of applying these nanomaterials into certain useful components or devices. Once we have those nanoparticles we want to manufacture them into some useful shapes. So, there can be two different techniques of nanomanufacturing or nanofabrication it means producing a device or component which is a size in between 1 to 100 nanometer. This one term called top-down nanofabrication this becomes a necessity when we have a bigger particle available with us and any bigger entity available with us. So, in this case we start with a much bigger material it can be couple of micrometers it can be millimeter or so on and then we start breaking it down into certain size range when it comes down to 1 to 100 nanometer. So, we try to make it smaller and smaller we take a bigger particle we can start milling it we can start machining it we should start impacting it. So, that it comes into very fine powder particles. So, now we are making it much smaller. So, we start with a bigger component and then we start going into making it much more finer. Second one is the bottom of nanofabrication in this case we start with individual atoms or molecules. So, we have some entity which is much smaller than nano because all the size range of atoms and molecules is an angstrom regime. So, we start building up those materials of the atoms individual atoms or molecules into a bigger shape. So, we can see top down we start with a bigger component we start fracturing it or breaking it down until we get a nano crystallinity in a material or nano material out of it. So, this is breaking it down into some very fine fine pieces which can again have some functionality. So, we had some bigger chunk and now we have some very finer chunk and that is nothing but top down approach. In bottom up approach we have very fine individual atoms and then we can put them into certain structure or certain shape to give a functionality to it. So, we get much finer, but certain functionality to it. So, in this case we have individual atoms they go on to arranging into some useful shape or size. That is what we can see in nano manufacturing we have two approaches top down approach we start with a bigger component and then we go on to forming a nano material and then bottom up approach in this case we utilize individual atoms or molecules to synthesize something which is a nanoscale. So, in this case certain examples of top down approach can be mechanical milling we start milling a bigger component and bottom up approach can be chemical vapor deposition or physical vapor deposition in this case we utilize individual atoms to start depositing on a substrate to form a nano coating. Again nanotechnology or playing or nanotechnology or synthesizing nano materials into some useful shape is nothing but playing with atoms. Many of us have played with Legos in that case we take each and every block and start constructing it. So, we get either buildings or dolls or some useful shape. So, the similar way this technology or art is based on manipulating individual atoms and molecules and then go on to making some very complex shapes or structures with defined atomic specification. So, we have certain blocks we can go on to making certain cards out of it or certain different structures out of it by placing each and every block into a specified location and that goes on to forming certain bigger complex complex structure. So, we can start depositing each and every individual atom into specified locations to make a bigger or complicated structure. It is a similar way we start playing with Legos we take individual blocks either to make cards robots dolls and in bridges or something like that. So, it is nothing but getting essentially every atom at the right place. So, we need to take and be able to manipulate each and every atom and that creates a difficulty because it is very hard to manipulate each and every individual atom or a molecule. So, we need to have some sensitive instrument which can first of all see those nanoparticles or these atoms individually be able to pick them and then place them into a right location. So, that again provides us a challenge in order to see those nanoparticles or those individual atoms manipulate it have a manipulator sort of a sub-nanomanipulator to pick up these individual atoms or molecules and place them at the right place and again because of instability of these atoms because as soon as we are putting an individual atom that may not be stable. So, again that stability of atom may come into picture in nanotechnology and again there are certain challenges in nano manufacturing like in most first law it has not predicted that the amount of the space which is required to install a transistor on a chip will string by roughly half every 18 months. It means that we need to double the electronic component such as resistors, transistors, capacitors everything on a smaller chip we need to double its density every 18 months. So, that is what it says the amount of space required to install a transistor on a chip has to string by roughly half every 18 months. So, the density has to increase very dramatically over time correspondingly there is a converse or a smooth second law which tells that the cost of building such a chip will also double every 36 months because if you are if we are making a particular chip and it has certain transistors and capacitors resistors everything and this density has to double we need to somehow enhance our capability of being able to install so many components in the certain limited dimension of the chip. So, if you have a dimension of x and y and we have the density of n in this case it has to increase by twice at least because the density has to increase. So, if we are increasing density we have to have at least 2 n and that that keeps going on to 4 n and so on. So, that is what I am again to synthesize a manufacture response the cost also will start enhancing dramatically because once we are increasing the overall density the overall technology has to change. So, that more number of transistors and such components can be installed in a similar dimension chip and thereby it also increases the cost of that particular chip and again the nano manufacturing it can again be subdivided into couple of broad categories. First thing is once you want to manufacture something if you want to manufacture a nano component the first thing is we need to have nano particles available to us which can be consolidated later on. So, first challenge first thing is for synthesizing a nano material or a nano device or a nano component first thing is we need to have a pure nano particle or a nano powder and then we need to integrate these nano particles or nano powder into some bulk nano structured component. So, from nano particles we go on to making certain component which can be such as aircraft brake pads or other some nano composite. So, they can be individual components which should be made from these nano particles and third stage is we consolidated all those nano components into certain device those can be electronic devices, magnetic devices, optics or even biological devices biomedical devices. So, we can see in nano manufacturing it is not a single step process, but it requires first the processing or synthesis of nano particles and again their purity, their size, their distribution is may be also of concern. The integration of these nano particles into bulk nano structure there are always some problems which are being imposed by these nano particles because of their size because of their high surface area and they are even there handling it can create certain problem in synthesizing them into some bulk components and again after they have been composed as a bulk component making them into certain device. So, that they can be sensitive to some stimuli or serve as a structural material with exposure to certain temperature or mechanical sense the overall properties of nano particles can themselves change. So, overall functionality of this nano materials or nano components can also change once they are composed as a device. So, again these may these are also certain concerns concerns in nano manufacturing, but as Richard Feynman has said basically for the electronic chips the principle of physics do not speak against the possibility of maneuvering things atom by atom. So, if we can manipulate atom by atom and then we can definitely achieve the class which we call nano material we can achieve any functionality by some of manipulating atoms on a individual scale and then making them into certain devices or components. So, let us now try to tackle each and every challenge which is being faced at individual level. So, that the first let us start with the challenges which are challenges associated with the synthesis of nano powder. So, first thing is synthesis of bulk amount of nano powder. If we are trying to pick atom by atom and then try to make a nano powder out of it. So, even to make couple of grams of nano powder we might require months because the process is very very slow. So, first thing is synthesizing nano powder in bulk scale itself is a challenge. So, we require certain technologies which can produce these powders in bulk. So, they can synthesize in large quantities and also it has to be economic. Otherwise if it becomes highly costly the cost may not be supporting the its commercial viability. So, that is what is required some technologies which can produce this nano powder nano particles in bulk. And right now these technologies are still in adolescent state they still need maturity. Even when we manufacture those nano powders they have tendency to agglomerate. So, because of their high surface area to volume ratio they have tendency to agglomerate because of their high surface area they tend to minimize their surface energy. So, they come in contact with the nearby nano particles and in term they tend to agglomerate. So, agglomeration is the inherent tendency of these powder particles. If we take a metallic nano powder particle they also have very high tendency to oxidize. So, they can form increased surface oxidation because of the high surface is now exposed to the environment or the atmosphere. So, they come in contact with oxygen and they tend to oxidize very easily. So, it is very hard to even produce a pure metallic powder because they tend to form oxides. Con concurrently if the nano particles which are of metallic nature they are tendency to oxidize so rapidly because of their high surface area it can be even very very explosion. So, they it can create even explosion hazard probably you might be aware that the aluminium is used as a fuel in the rocket engine because of its inherent oxidation tendency and nano aluminum powder particles they can create so much energy that it can impulse providing impulse to the rocket so that it can achieve very high speed. So, again so they can again have explosion hazard so because of the high surface area which is associated with the nano particles and that becomes a key issue for the metallic nano powders. If you want to make a component out of metallic nano particles first thing is they tend to oxidize and second thing is they can be highly explosive if they come in contact with the atmosphere. So, for handling all these we need an inert atmosphere where we can handle them as well as then we can start synthesizing them. Then comes the problem of handling and storage because again these nano particles we need some space for storing them if we have to create an inert atmosphere for so huge of a powder particle powder so much of a huge powder bulk then it requires much space and it also becomes costly to store and handle these materials. Even while handling if you are able to if you are exposing them to atmosphere it is creating surface oxide it also poses a threat of explosion. So, we need to take care of all these concerns. So, there are many much challenges which are associated with the synthesis of nano powder. First thing is being able to produce in a bulk quantity and then avoiding the agglomeration of these nano sized powders and somehow trying to limit the surface oxidation which is which is inherently there in the nano powder particles it can also create explosion hazard and also we need to minimize the cost and the handling and storage issues which are there with the nano powders. Again one more concern with the nano powders is this the bio safety because the nano powder particles if they go in our lungs it is very hard for them to come out. So, we need proper safety protocols which are required like the person has to be under proper safety protocols wearing safety gadgets all the mask which will not allow nano particles to see through the filters or respiratory filters and the same time this will not be get in touch with our skin or something like that to avoid any irritation or they are inducing toxicity effects to our body. So, there are many challenges which are associated with the nano powder particles and all of these are basically being listed here. Now, coming to the second stage in terms of processing them once you have the nano powder particles you want to consolidate them into certain device or components. So, there are certain challenges it is again challenges associated with the consolidation of nano powders. The first thing is as soon as we expose the nano powder particles to high temperatures it undergoes severe grain growth. Overall identity of the nano particle is lost if it starts growing it becomes enhanced the size of this nano particle enhances it has lost the characteristic of the nano particle. So, we need we want to avoid the severe grain growth which is now enhanced in the nano powder particles via conventional processing. Second thing is it also can undergo very non-uniform bulk transformation the surface is highly active it might be much more prone to surface reconstruction as well as transformation. So, it can render very non-uniform bulk transformation. Apparently the mechanical properties of the nano powder particles will very drastically up to a certain size range if you start reducing the grain size it will enhance the oral hardness and the strength of the material. But after a certain size range say below 10 to 15 nanometer if you go much below the grain size of 10 to 15 nanometer there is not enough grain area to support the pile up of dislocations. So, in turn it starts reducing the overall strength of the material that is called inverse hall-patch relationship. But still not the conventional knowledge was to start reducing the grain size it enhances the strength of the material. But in nano material the problem can be inverse hall-patch relationship it means the overall strength of the material starts degrading with the decrease in the grain size. So, again that may not be good for the longevity of lifetime of the nano device or nano component. Again there exists a lack of consolidation technique to fabricate large components till now the research has been directed to make very small pieces or discs of these nano components which can go as a maybe couple of millimeters or centimeters in that that scale the research has been directed and there is no commercial technique available to make bulk nano structures. Only a limited of such techniques might be available. But there is a overall limitation of synthesizing this nano particles into some bigger entities. And again so that again is a challenge which is associated with the processing and for that the manufacturing cost also should not basically exceed the cost of the required raw material and energy. So, in order to maintain a balance and so that it can be easily commercialized later on. So, manufacturing cost also has to be limited and it should be well below the cost of the raw material itself. That is again a challenge for the processing of nano material. Then challenges associated with the nano manufacturing of devices. If you want to create a device or a nano device first thing is we need an interdisciplinary approach because it requires knowledge of materials, knowledge of processing and also where it is going to be used if it is an electronic industry, it is a biomedical industry where is it or electrical industry. So, we need to have an interdisciplinary approach in terms of the material itself. What are the physics which is governing the overall functionality and materials processing and again the exact point where we are going to use this particular device. So, we need a combination of knowledge of these all areas to be able to make such a device. And also requires flexibility to operate smartly and also communicate and interface with other nano systems because once the device itself is nano it definitely needs to have some sensors or some connected which are again nano in size. Then only it makes sense to use this nano devices. So, it requires a very good connectivity with those systems so that it can communicate and interface with them and it can send the required information to the external world. So, in terms of packaging and interconnecting again it becomes a challenge. Then the reduced dimension also induce some different sort of responses with respect to the ordinary size components. Like in this case reduced dimension there is result enhanced friction in the nano devices because of the very low size once they come in contact with other devices the overall surface area is so high that the overall friction among the devices or the or the moving part becomes extremely large. And then it requires the need for improved tooling or computer edit design for their design. So, we utilize novel software for nano machining and focused ion beam and ultrafast laser for certain machining as well. And again the Moore's law they to to maintain with the Moore's law of the production there is always a thrust which is which is there to go into miniaturizing all these components. So, there is again certain pressure to be able to reach that particular level which is being predicted by the Moore's law. So, we can see there are certain challenges that we want to go for all for very lighter and lighter devices and components. So, we need to keep the pace with it in terms of manufacturing it. We require interdisciplinary ways. We require much more flexibility. We require that they can communicate or interface with the adjoining external world be able to reduce the dimension and come up with some improved tooling such as focused ion beam or even ultrafast laser to be able to keep pace with the technology. So, there are certain key issues which are existing in this nanomaterial. First thing is the preservation of the nanostructure for successful consolidation of nanomaterials into bulk size components. We need to preserve the nanostructure so the properties of nanomaterials can be retained. And secondly, the most challenging problem in the nano crystalline particle consolidation is the inability to fabricate sufficiently large parts. So, we need a technique to make these nanoparticles into bigger components bigger or bulk or large components which can be of usable size. So, right now we can make small small particles or devices which are functional but it is very hard to create a functional bulk material for a bigger functional component such as chair or such as elevators which can be made up entirely of these nanoparticles. So, the challenges in terms of enhancing the scale at which the nanomaterials are being synthesized into useful components. The conventional consolidation of particles has not been that alluring because in traditional consolidation techniques it is hard to retain the size of nano phase. The nano phase or the nanoparticles are inherently undergo green growth and again it can also lead to nanostructural instability. So, nanostructure may not remain stable even if there is a processing technique through which we can produce them but they still remain stable during the consolidation process. Even powder metallurgy it utilizes high temperatures or elevated temperatures for prolonged duration of time and that creates a diffusional mechanism. So, that the overall distribution of these nanoparticles it becomes much more homogeneous or it can also be it can basically go through or diffuse through the nearby phase and it can again lead to non homogeneity. The non homogenous part can be basically not be retained at the interfaces that is the problem with the powder metallurgy technique. Again the centering of nano versus micro. So, there are very conflicting theories on how these nano crystalline materials center. The conventional knowledge on the centering is very very different but when it comes to nano the overall centering kinetics itself is very very very different than what is being used for the bulk or the macro materials. Nanoparticles again they are very high tendency to agglomerate and there is one more problem with the nanoparticles is they are very high interparticle friction. So, it is very hard to obtain enhanced density. Again there is very high reactivity. So, because of that they the nanoparticles also undergo high contamination the surface oxidation or grabbing of some impurities from the atmosphere or even from the overall nearby container. It can also react and grab some impurity and it gets contaminated. But on the centering part because of their high surface area it also can it also undergoes rapid centering and it also occurs it also incurs rapid grain growth. So, these are certain problems which are associated with the nano and micro centering that there are various conflicting theories on the centering of nanoparticles. They are tendency to agglomerate they render very high interparticle friction. They are tendency to get contaminated because of their high reactivity. But they also undergo very rapid centering and they undergo rapid grain growth which may not be required to acquire their useful properties. The nanoparticles they generally show very depressed onset of centering temperature. It occurs at as lowest 0.2 to 0.3 Tm or the melting point 0.2 or 2.3 times that of a melting temperature. Whereas for conventional materials for conventional powders the centering starts at between 0.5 to 0.8 of the melting temperature. So, that is the key role of nanomaterials that they tend to have very low temperatures. So, they can start consolidating at very lower temperatures. The centering starts it onset at very in the initial stages itself. And such result is attributed to the structural instability of these nanoparticles because of their high surface area. Because of their high surface area they come in contact with the nearby particles and they incur various phenomena which we will be discussing in a few later on in a few slides. But because of that it can it can initiate or actuate these processes of solidification of densification as as as early as 0.2 to 0.3 times the melting temperature. And this occurs because they are fundamentally very energetic and they also render accelerated kinetics. So, because of their high surface area and their structural instability this one this is basically provides the accelerated kinetics or very high energetics to undergo centering and consolidation or even grain growth. Again when the nanoparticles are getting agglomerated this overall the overall four distribution of nanoparticles also is of very high concern. Generally the smaller the particles the larger the agglomerate. Because if the particle size is very very small they are very high surface area and they tend to render very high surface forces and in turn they form a very bigger chunk of the material or a very bigger agglomerate. And again they are very small pores within the agglomerate these are called inter agglomerate pores. And again they are some pores which are very large in size and they are intra agglomerate pores these are porosity between certain agglomerates and such a distribution will enter the overall packing density and there is now bimodal size of distribution of the porosity and again because of the high friction which is associated with the nanoparticles it can enter the densification mechanism. So we can see the smaller the particle larger the forces because of that it gives out very bigger agglomerate. Then there are smaller pores within the agglomerate the larger pores between the agglomerates and smaller pores within the agglomerate and this distribution will lead to change in the densification mechanism and it will enter the packing density of these nanoparticles. So we can see the bimodal pore distribution of nanoparticles it can be like this that we have an agglomerate we have an agglomerate. Then we can also have a second agglomerate which is out which comes in contact then we might have third agglomerate which basically comes in contact like this. Then we have fourth agglomerate which comes in contact so we can see that we have some porosity within the agglomerate which is called intra agglomerate porosity this entire thing is nothing but a agglomerate and then these are nothing individually they are called either crystallite or primary particle. Then we have this agglomerate there is some much space which is available so much space which is available out here this space this is nothing but interagglomerate porosity inter agglomerate so we can see there is some porosity between the the particles or the crystallite which is called intra agglomerate because this is within agglomerate and then we have between the agglomerates it is called interagglomerate porosity. So we can see the and there is a bimodal pore distribution because the size of this particular porosity the intra agglomerate porosity is much smaller is very very smaller than the interagglomerate porosity so we can see there is much difference in the size range of this porosity which is intra agglomerate porosity and then the interagglomerate porosity so that inter provides a distribution of pores which is in one is within the agglomerate so within the agglomerate because of porosity because of intra agglomerate porosity these particles will undergo certain shift and it will lead to friction and second friction is arises because of the link between the different agglomerates so there can be again friction along this side between agglomerate one and agglomerate two again we will see we will observe some friction between them and that arises because of the inter agglomerate porosity so there is a gap between the agglomerate that gives out the inter agglomerate porosity and that again it will reduce some friction and a friction because of this entity might be much higher the movement of individual powder particles it can render very high frictional forces and that will make the flow of these particles very very viscosity or very very it will basically stop the movement between the powder particles and that creates a problem in the flow of this powder particles if you want to fill a cavity for compression or hot pressuring or something like that it will it won't be that uniform so that is the reason the nanoparticles they need to have they need to be treated so that they can become an agglomerate spherical agglomerate and somehow they can reduce the overall friction between the particles and out here as well they are very high tendency to remain intact with the other particle so the movement between those particles will be very very limited now comes the consolidation of nanoparticles and as we said because of this porosity and the overall surface forces it becomes very hard to move the particles in relation to each other so that gives a very high frictional forces and since the density of the green component green compact depends on the frictional forces it becomes very hard to compress them to very high density and that basically originates from the electrostatic or Vandalov forces or the surface adsorption phenomena and these frictional forces are extremely high in the nanoparticles they form very hard agglomerate and enter inter agglomerate porosity which is basically very very large so it turns it's very hard to move those particles and then result a very dense compact out of it so we require very additional pressure to make them move and consolidate to a very high dense valet or a component so consolidation of nanoparticles it's of a bigger concern because the overall frictional forces are very very high and inter it requires very very high forces to consolidate into dense particles coming to the thermodynamics consolidation for the pore consolidation so based on the overall size of the pore so we can see that the thermodynamic treatment can provide the shrinkage of pore occurs and it requires very high pressure so the finest the pore size usually yields the highest densification rate so we can see that the pore size is very very fine it can easily get consumed by the surrounding and the larger pores require not only high temperature but also prolonged sintering times and the larger complete engulfment so consequently it becomes very difficult to retain the grain size in the nanometer regime so we can see first of all that the nanoparticles if you have a cluster of nanoparticles the finer grains are present within an agglomerate and it is very easy to eliminate these nanopores or make them densified porosities will go first so in turn the nano nature of this this is a nanoparticle and this is a nanoporosity so these will get consumed first so we can get a agglomerate a densified agglomerate and then we have second densified agglomerate and the third densified agglomerate so the overall grain size is now increased increased and in turn it has become difficult to retain the nanoparticles or nano grains so the consolidation of nanoparticles or retention of the grain size is very very difficult in the nanometer regime and this once it forms a bigger pore it becomes very hard to consume it and densify the material so in the process we are not only consuming the finer grains or finer particles we are also inducing certain porosity in the material which is very hard to eliminate so that is the requirement for the pore consolidation so we need to have a very uniform size of pore of pore throughout the material in order to retain the nanometer grain size while achieving a rapid centering so rapid centering becomes of key issue that we can achieve densification in very short duration of time while retaining the nano grains in the structure again why the vapor pressure of nanoparticles basically increases that that occurs because if you have smaller particle and a bigger particle first of all the flat surface will remain in equilibrium with the vapor as soon as the size is changing the free energy also keeps changing the smaller the wall the higher the free energy which is associated with the finer particle so the vapor pressure around the smaller ball has to increase and this pressure whether for gas or for particle the overall pressure basically is a function of the inverse of the radius so smaller the radius higher will be the vapor pressure so in turn this entity the smaller entity will render much more vapor pressure and then it will have very high energetic in terms of its growth or consolidation so that is the problem associated with the nanoparticles but then in turn it also yields very rapid kinetics for sintering we can see the smaller the smaller particle it has a direct dependence on the radius inverse relationship with the radius so higher the r lower in the vapor pressure so flat surfaces will be in equilibrium with the vapor whereas smaller entities smaller particles they have very small radius so in turn it will tend to increase the vapor pressure and sintering mechanism this sintering starts now occurring in the even when the earlier stage of sintering even when the temperature is around 0.2 to 0.3 times the melting temperature so overall lower activation energy is needed because of their high vapor pressure and overall even it is even lower than that is required for the conventional diffusion so for example of tungsten powder of size 40 nanometer the activation energy is approximately 134 kilojoules per mole and that is very very low even when you compare it either with the lattice diffusion or even when you consider with the surface diffusion so that is the kind of energy which is even lesser than the half of the surface diffusion so even surface diffusion cannot explain the overall densification which is occurring in the nano tungsten powder so even high diffusivity of this tungsten powder particle cannot be explained by the surface diffusion because of the high energiotics associated with the nano particles so even when we are able to achieve extremely fast sintering in the tungsten particle we cannot explain it using conventional wisdom of either lattice diffusion or even by surface diffusion so that requires that we explain these phenomena of sintering via some additional diffusion mechanisms so we can see the sintering mechanisms it starts occurring in the early stages as early point to point three times at melting temperature and requires activation energy which are much smaller than that required for either for surface diffusion or even for the lattice diffusion so these phenomena become unexplainable via conventional cognizance so we require to explain them with some other diffusion mechanisms so we can see that other diffusion mechanisms of this nano particles they have been explained via certain molecular dynamics which we through which we can really see what is happening in the material the first thing is the rapid dislocation motion so once the dislocation is generated in the material via the interfacing of these nano particles the dislocation created at the interface starts moving very rapidly and that leads to rapid shrinkage or densification second thing is grain itself can start rotating the first case rapid dislocation starts dominating the grain can also slip and again dislocation density is again driven by contact hergene stresses that exceed the shear strength so before the shear strength starts dominating the hergene stress itself is now available for the deformation of the material and then to yield to let the dislocation density change the structure of the material itself and lead to densification and after the neck forms the adjacent powder particle can also rotate to achieve a minimum grain boundary energy so let us see them in point by point the rapid dislocation movement so as soon as you have contact of the two powder particles the dislocation which is created at the interface because of the different orientation the dislocation itself can move very rapidly now the grains themselves can now rotate so once you have some deformation achieved at the necking this entire grain can rotate and result a much more denser interface grain boundary slip the grain boundary because of the temperature the grain boundary can also start slipping and the overall trace can be also obtained using this using the movement of the two different powder particles and rapid dislocation density is driven by the hergene stresses it is no more by shear it is no more by limited by the shear strength of the material it exceeds the shear strength of the material and because of contact hergene stresses which are developed at the interface they lead the movement of rapid dislocation density and again once the neck has formed the particles can also rotate to achieve a minimum grain boundary energy so we can see initially when we have two powder particles they might have different orientation of grains and first thing is there is overall movement of this dislocation density itself they come in contact they start forming neck and then this dislocation now start moving to some location so that you have some development of extra material and that extra material now can allow it slipping so the grain rotation can also occur to come and join the interface so now in intern you have prolonged neck area so the overall neck area enhances because of the movement of the grain rotation itself so in first case you have movement of dislocation and second case you can also have grain rotation and it can intern it can also lead to the twin boundaries or even dislocations in the final material but now it has come to a much more denser state and in this case again we can see much more the overall grain orientation or the grain size can also be very different than the starting material so we can see that in this case we can again have some sort of a orientation resemblance in the final stage so in this case the over initial grain orientation was very very different the overall planarity was very very different in this case we see the movement of dislocations and even the even the grain rotation has occurred in one case so that we can achieve a certain similarity near the neck region and also this neck area which was basically been promoted by the movement of dislocation and that comes in contact with the other grain and in turn we can achieve very rapid densification via sintering of nano particles and again in sintering of nano particles they are very very conflicting stories and there is no single theory which is available to universally explain what is happening in the nano materials so the variety of phenomena which might be dominant in the nano materials they can range from the movement of dislocations to what is happening at the grain either it is by surface diffusion or by the rotation of grain or it is by the herdsian contact which enhances the movement of these dislocations there are many many conflicting theories which are which describe the sintering of nano particles and any one of them might be much more dominant in one set of systems and comparison to the other set of systems there is no single theory which is able to explain the overall densification and grain growth and even the sintering of nano particles most of this techniques have some success as well as certain limitations one common feature of these techniques is that the application of sudden pressure temperature or other processing variables so in the consolidation we can apply a sudden pressure a sudden temperature or the cooling rates are very very sharp or very very high or any other processing variable so either the time is very very short the pressure is very very short or the temperature is attained in a very short duration of time sudden jerks can allow the retention of these nano particles in the sintered component but again the major conclusion which can come out of is this is that the synthesis of nano composites is much more much more feasible than that of a pure metal or a alloy because because of the high surface reactivity metal always undergoes certain reactions oxidation and also starts getting some impurity whereas in case of nano composites we are already inducing sudden inhomogeneity in the system so in that case it becomes much more feasible for the component to attach itself to a different species and then react with it and then again densify get densified so the overall synthesis of nano composites is much more feasible and also it renders enhanced mechanical properties surface properties encompassed into that of a pure metal or an alloy so in this case in the consolidation we are applying a rapid rate of either pressure or temperature or even the deformation or any time which is associated with this processing so in summary we can see that the nano manufacturing is a challenge because we need to incorporate first of all we need to make those nano particles then be able to process them and then make them into a useful shape which is much bigger in size so that it can be used effectively so it requires the retention of nano domains and again there are certain challenges with each and every stage of this nano manufacturing in terms of either handling the powders being able to manufacture them the oxidation, the surface reactivity then when it comes to devices again there might be certain challenges of interconnects of getting signals out of them making them into certain useful shape then again components it can again be a challenge because the overall pressure temperature of the working environment can also induce certain effects and it might become very hard to merge those nano components into a bulk or a useful shape then comes the rapid centering of nano particles this can also create certain challenge because retention of nano particles is very very difficult the nano pores which are present between the nano particles they are very easy to get consumed whereas the porosity between the agglomerates is very hard to eliminate so in turn the rapid centering or the rapid jerk which can be applied by a pressure, temperature or even by consolidation it requires a jerk or a short pulse so that they can be retained and the process can get completed in a very short duration of time nano particles additional diffusional mechanism also become predominant such as dislocation movement which becomes very very rapid in the hydrogen contact which can enhance the overall movement between the dislocation density and again there can be grain rotation to yield to a enhanced densification of these nano particles so we can see the overall nano manufacturing is totally dependent on the nano materials synthesizing them to certain devices becomes again a very big challenge and that requires understanding of the mechanisms or the centering mechanisms which are dominant in terms of creating these functional devices with this I end my lecture, thank you