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Nanotechnology, shortened to nanotech, is the study of the controlling of matter on an atomic and molecular scale. Generaly nanotechnology deals with structures of the size 100 nanometers or smaller in at least one dimension, and involves developing materials or devices within that size. Nanotechnology is very diverse, ranging from extensions of conventional device physics to completely new approaches based upon molecular self-assembly, from developing new materials with dimensions on the nanoscale to investigating whether we can directly control matter on the atomic scale.
There has been much debate on the future implications of nanotechnology. Nanotechnology has the potential to create many new materials and devices with a vast range of applications, such as in medicine, eltronics and energy production.
A basic definition: Nanotechnology is the engineering of functional systems at the molecular scale. This covers both current work and concepts that are more advanced. In its original sense, 'nanotechnology' refers to the projected ability to construct items from the bottom up, using techniques and tools being developed today to make complete, high performance products. The word nanos is a greek word for Dwarf.
The fundamental difference between nanostructured materials and “ordinary” materials
originates from the ratio of surface or interface atoms to bulk atoms. This ratio is
negligible in ordinary materials, but can be very high in a nanostructured material (the
extreme case is that all atoms are surface or interface atoms). This means that in
nanostructured materials, surface and interface effects are important or even dominant.
Nanomaterials are the materials with dimensions in nanometers, which are characterized into several types, nanoparticles, nanotubes, nanowires, and nanorods, etc. Nanoparticles have a very high surface area to volume ratio, which provides a tremendous driving force for diffusion, especially at elevated temperatures.
One nanometer (nm) is one billionth, or 10−9, of a meter. By comparison, typical carbon-carbon bond lengths, or the spacing between these atoms in a molecule, are in the range 0.12-0.15 nm, and a DNA double-helix has a diameter around 2 nm. On the other hand, the smallest cellular life-forms, the bacteria of the genus Mycoplasma, are around 200 nm in length.
VARIOUS METHODS USED TO OBTAIN NANOPARTICLES:
There are several methods for creating nanoparticles, including both attrition and pyrolysis. In attrition, macro or micro scale particles are ground in a ball mill, a planetary ball mill, or other size reducing mechanism. The resulting particles are air classified to recover nanoparticles. In pyrolysis, a vaporous precursor (liquid or gas) is forced through an orifice at high pressure and burned. The resulting solid (a version of soot) is air classified to recover oxide particles from by-product gases. Pyrolysis often results in aggregates and agglomerates rather than singleton primary particles.
A thermal plasma can also deliver the energy necessary to cause evaporation of small micrometer size particles. The thermal plasma temperatures are in the order of 10,000 K, so that solid powder easily evaporates. Nanoparticles are formed upon cooling while exiting the plasma region. The main types of the thermal plasma torches used to produce nanoparticles are dc plasma jet, dc arc plasma and radio frequency (RF) induction plasmas. In the arc plasma reactors, the energy necessary for evaporation and reaction is provided by an electric arc which is formed between the anode and the cathode. For example, silica sand can be vaporized with an arc plasma at atmospheric pressure. The resulting mixture of plasma gas and silica vapour can be rapidly cooled by quenching with oxygen, thus ensuring the quality of the fumed silica produced. In RF induction plasma torches, energy coupling to the plasma is accomplished through the electromagnetic field generated by the induction coil. The plasma gas does not come in contact with electrodes, thus eliminating possible sources of contamination and allowing the operation of such plasma torches with a wide range of gases including inert, reducing, oxidizing and other corrosive atmospheres.
The working frequency is typically between 200 kHz and 40 MHz. Laboratory units run at power levels in the order of 30-50 kW while the large scale industrial units have been tested at power levels up to 1 MW. As the residence time of the injected feed droplets in the plasma is very short it is important that the droplet sizes are small enough in order to obtain complete evaporation. The RF plasma method has been used to synthesize different nanoparticle materials, for example synthesis of various ceramic nanoparticles such as oxides, carbours/carbides and nitrides of Ti and Si (see Induction plasma technology).
Inert-gas condensation is frequently used to make nanoparticles from metals with low melting points. The metal is vaporized in a vacuum chamber and then supercooled with an inert gas stream. The supercooled metal vapor condenses in to nanometer-sized particles, which can be entrained in the inert gas stream and deposited on a substrate.
The sol-gel process is a wet-chemical technique (also known as chemical solution deposition) widely used recently in the fields of materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials (typically a metal oxide) starting from a chemical solution (sol, short for solution) which acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers.
Nano-coatings are materials that are produced by shrinking the material at the molecular level to form a denser product. Nano-coating can be applied in many ways including chemical vapour deposition, physical vapour deposition, electrochemical deposition, sol-gel methods, electro-spark deposition, and laser beam surface treatment.
Main Advantage of Nano-coatings: Some of the main advantages of nanocoating are:
- Better surface appearance.
- Good chemical resistance, Good adherence on different type of materials.
- Decrease in permeability to corrosive environment and hence better corrosion properties.
- Optical clarity, Easy to clean surface.
- Increase in modulus and thermal stability.
- Anti-skid, anti-fogging, anti-fouling and anti-graffiti properties. Anti-reflective in nature.
- Better thermal and electrical conductivity.
- Better retention of gloss and other mechanical properties like scratch resistance.
- Chromate and lead free Main Issues in Nano-Coating:
- Main problem in using nano particles for coating purpose is dispersion and stability of nanoparticles. Agglomeration may take place because of high surface energy possessed by nanoparticles due to their large surface area.
- Pigments may lose their color on reducing their size to nano level and hence will loose their opacity.
- Stable binder is required to inhibit photocatalytic activities of nano TiO2.
- Hardening problems of ultrafine powder.
- Extensive use of nanoparticles may give birth to new type of environmental problems, such as newer type of toxic materials and other environmental hazards. Ultrafine particles can catalyze chemical reactions inside body which might be dangerous.
4.1 Bottom Up approach Techniques
4.1.1 Wet Chemical Synthesis of nanomaterials (Sol-gel process)
The sol-gel process, as the name implies, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The precursors for synthesizing these colloids consist usually of a metal or metalloid element surrounded by various reactive ligands. The starting material is processed to form a dispersible oxide and forms a sol in contact with water or dilute acid. Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape. Sol-gel method of synthesizing nanomaterials is very popular amongst chemists and is widely employed to prepare oxide materials. Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based precursors such as Si (OEt) 4 (tetraethyl orthosilicate, or TEOS). The reactions involved in the sol-gel chemistry based on the hydrolysis and condensation of metal alkoxides M(OR)z can be described as follows: MOR + H2O → MOH + ROH (hydrolysis) MOH+ROM→M-O-M+ROH (condensation) The sol-gel process can be characterized by a series of distinct steps.
Step 1: Formation of different stable solutions of the alkoxide or solvated metal precursor (the sol).
Step 2: Gelation resulting from the formation of an oxide- or alcohol- bridged network (the gel) by a polycondensation or polyesterification reaction that results in a dramatic increase in the viscocity of the solution.
Step 3: Aging of the gel (Syneresis), during which the polycondensation reactions continue until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from gel pores. Ostwald ripening (also referred to as coarsening, is the phenomenon by which smaller particles are consumed by larger particles during the growth process) and phase transformations may occur concurrently with syneresis. The aging process of gels can exceed 7 days and is critical to the prevention of cracks in gels that have been cast.
Step 4: Drying of the gel, when water and other volatile liquids are removed from the gel network. This process is complicated due to fundamental changes in the structure of the gel. The drying process has itself been broken into four distinct steps:
- the constant rate period,
- the critical point,
- the falling rate period,
- the second falling rate period. If isolated by thermal evaporation, the resulting monolith is termed a xerogel. If the solvent (such as water) is extracted under supercritical or near super critical conditions, the product is an aerogel.
Step 5: Dehydration, during which surface- bound M-OH groups are removed, there by stabilizing the gel against rehydration. This is normally achieved by calcining the monolith at temperatures up to 8000C.
Step 6: Densification and decomposition of the gels at high temperatures (T>8000C). The pores of the gel network are collapsed, and remaining organic species are volatilized. The interest in this synthesis method arises due to the possibility of synthesizing nonmetallic inorganic materials like glasses, glass ceramics or ceramic materials at very low temperatures compared to the high temperature process required by melting glass or firing ceramics.
The major technical difficulties to overcome in developing a successful bottom-up approach is controlling the growth of the particles and then stopping the newly formed particles from agglomerating. Other technical issues are ensuring the reactions are complete so that no unwanted reactant is left on the product and completely removing any growth aids that may have been used in the process. Also production rates of nano powders are very very low by this process. The main advantage is one can get monosized nano particles by any bottom up approach.
4.1.2 Chemical Vapour Deposition (CVD):
Chemical vapour deposition (CVD) was developed in Germany in 1994. It involves pyrolysis of vapours of metal organic precursors in a reduced pressure atmosphere. Particles of ZrO2, Y2O3 and nano whiskers have been produced by CVD method. The original idea of the novel CVC process which is schematically shown below where it was intended to adjust the parameter field during the synthesis in order to suppress film formation and enhance homogeneous nucleation of particles in the gas flow. It is readily found that the residence time of the precursor in the reactor determines if films or particles are formed. In a certain range of residence time both particle and film formation can be obtained. Adjusting the residence time of the precursor molecules by changing the gas flow rate, the pressure difference between the precursor delivery system and the main chamber and the temperature of the hot wall reactor results in the prolific production of nano sized particles of metals and ceramics instead of thin films as in CVD processing. In the simplest form a metal organic precursor is introduced into the hot zone of the reactor using mass flow controller. For instance, hexamethyldisilazane (CH3)3 Si NHSi (CH3)3 was used to produce SiCxNyOz powder by CVC technique. Besides the increased quantities in this Continuous process compared to GPC it has been demonstrated that a wider range of ceramics including nitrides and carbides can be synthesized. Additionally, more complex oxides such as BaTiO3 or composite structures can be formed as well. In addition to the formation of single phase nano particles by CVC of a single precursor the reactor allows the synthesis of 1. Mixtures of nanoparticles of two phases or doped nanoparticles by supplying two precursors at the front end of the reactor, and 2. Coated nanoparticles, i.e., n-ZrO2 coated with n-Al2 O3 or vice versa, by supplying a second precursor at a second stage of the reactor. In this case nanoparticles which have been formed by homogeneous nucleation are coated by heterogeneous nucleation in a second stage of the reactor. Because CVD processing is continuous, the production capabilities are much larger than in GPC (gas phase condensation) processing. Quantities in excess of 20g/hr have been readily produced with a small scale laboratory reactor. A further expansion can be envisaged by simply enlarging the diameter of the hot wall reactor and the mass flow through the reactor. The microstructure of nanoparticles as well as the properties of materials obtained by CVD has been identical to GPC prepared powders.
4.1.3 Physical Vapour Deposition (PVD)
PVD and CVD are largely complementary processes, used for tool coating applications. Combination CVD/PVD coatings are often utilized, with CVD comprising the first coating layer(s) and PVD comprising the smoother, finer top layer(s). Recently, PVD coating development has focused on new compositions, nanocomposite coatings and Al2O3. According to Quinto, AlTiN coatings applied via PVD have been called “the next best thing” to Al2O3, and until recently could only have been deposited on a commercial scale via CVD. This electrical insulating oxide coating had been a challenge for PVD, since depositing the correct coating structure had proven quite difficult. According to IonBond‟s Horsfall, PVD coatings that incorporate materials such as silicon and use new, nanotechnology-based materials perform better, enabling the newest machine tools to machine faster and, in many instances, machine under dry or neardry cutting conditions.
Nanocoating are nothing but applying of different nanoparticles onto the surface of other materials to obtain superior performance charecteristics.
While nanoparticles are often used merely as fillers, they can also serve as concrete components, as well as highly effective protective and functional coatings, catalysts and filtration systems. This new technology contributes to enhanced energy efficiency and solution of some global problems, for instance, supply of clean drinking water, and optimization of fossil fuel combustion.
By connecting and controlling structural entities on a molecular level, innovative property combinations have been achieved. ItN Nanovation uses the fact that nanoparticles show a very high sinter activity at relatively low temperatures. Thus microcrystalline and submicron powders are connected agglomerate-free by the so-called Nanobinder to ceramic coatings and, at the same time prevents agglomerates and improves the sinter activity of ceramics. Already at low curing temperature, these new coatings possess ceramic qualities as well as if necessary non-sticking, abrasion protection and self-cleaning properties. An application of these coatings on different substrates like metal, plastic and ceramic is easily accomplished using common spray or dip coating techniques. For instance the high-temperature-stable easy-toclean coating Nanocomp PP, which is used in the combustion boiler, on heat exchangers and on electrostatic filter funnels, prevents slagging, corrosion and abrasion and thus extends the run-time and the energy efficiency of the plant. The same principle can also be used in desalination plants, where the ceramic coating reduces crystallisation fouling, as well as in foundries, where the more efficient and semi-permanent nano-coating Nanocomp MC replaces graphite as a demoulding agent. While the above-mentioned systems have a dense and closed surface and structure, the nanoparticles can also be used to increase the porosity of ceramic coatings for catalytic properties. One square meter of such a NanoCat coating has a surface area of about 45,000 m2 and thus allows a wide range of catalytic applications from self-cleaning coatings in household ovens (decomposition of fat and oil already at 200°C) to the use as a su pport material for catalysts in chemical synthesis.
What means “Nano” in the fields of coatings?
- Use of new physical instruments, methods and processesDevelopment, pro-duction and analysis of nano-sized materials (structures, layers, particles, linkings)The differences between chemistry, physics and biology disappear. Below a certain order of magnitude particles get new, "atomic" properties, influencing optical, mechanical and chemical properties of surface.
Purposeful study at binders with increasing previous calculation instead of trial and error; new binders with tailor made properties and defined molecular struc-tures. One example: In particular the controlled radical polymerisation allows the construction of tay-lor made block and comb copolymers. The synthesis of strongly branched out polymers up to dendrimeres leads to binder agents with low viscosity in organic solvents as a basis for new High Solids systems and to new high-functional hardeners.
Hybrid-polymers as next example for nano structure are used already and have a great potential for future coatings systems: Combi-nations of organic polymers and organically modified, inorganic silicates. Using them it becomes possible to add the good qualities of different polymer classes. The combination of inorganic and organic ele-ments, in particular the mounting of silicon into the polymer structure, offers special ad-vantages. Hybrid polymers combine the sta-bility and scratch resistance of inorganic networks with the elasticity of the organic polymers. And - among other things - they show good adhesion on glass with which they are suitable for functional glass depositions as a basis at best. Synthesis ways as the sol gel technique.
SAM´s: Self assembling molecules can arrange themselves on a (metal) surface in a very regular and close manner and then polymerise in a second step. This very flexible, self healing up, strongly anchored layer may improve coatings adhesion, corrosion protection and mechanical and chemical resistance.
More perfect surfaces
Scratch resistance by imbedding of “ceramic” particles
Info-implementation (like DNA)
Optical or microbial effects (Ag),
UV-absorption (TiO2, ZnO, CeO2)
IR-absorption (ITO, others)
Antiadhesion-coatings for glass, ceramics and metals
Easy-to clean surfaces, reducing dirt
Filler materials as layer silicates or SiO2 are set in qualities' today mostly specific to the cost reduction and reaching certain properties in paint formulations. Specific filler materials will occupy in future more and more also the role of additives that are set in purposeful to quality improvement and strengthening of the surface.
Nanocorpuscles and nanolayers offer a very innovative chance of that. The only few atoms big particles show up to now hardly known quality profiles. Imbedded into the polymer matrix of the binder surface coatings which ultrathinly, permanently strongly, translucently and chemically constant can be produced with that.
In particular phyllosilicates, however, also Si02 that are unlocked nanofine in polymer solutions or polymer dispersion correct the mechanical qualities, for example the scratch resistance, as components of depositions. Or they control the rheological properties of the fluid paint without damaging qualities as gloss or transparency. Phyllosilicates dispersed in nanofine form increase the barrier effect of coatings considerably. They will find use as gas diffusion closures for example with drink bottles or hose materials. Here they seem also as an efficient closure against the CO2 -loss. Also the use for electroinsulation, for thermal insulation, for fire protection and to antistatic outfit of surfaces is known. Further examples: Anticustody depositions for glass, piece of pottery and metals as well as in polymers, that prevent dirt-, water- or oil-covering .
Micropatterns, made purposefully by coating on surfaces, favour the run off behaviour of water and the entrainment of dirt particles. This so-called "Lotos effect" (fig. 7 - 9) was copied after the nature by the Botanical Institute of Bonn University and is brought already into self cleaning roofing tiles and sanitary subjects. The use on bright surfaces as on cars is discussed again and again, not appearing however realistic at the moment. The hardest problems are the sensitivity against clean-ing, brushing and other mechanical treatments and the fact, that the artificial lotos surfaces are not self renewing like the natural leaf.
The controlled emission of substances from coatings is set in already today for the vegetation pollution prevention, antifouling e.g.. It can be used in future also for the operation of reactions at coated surfaces. Microporous coatings donated with catalysts can contribute to the decontamination of environmental damaging-gases, in the same way photocatalytically effective layers.
Just the border crossing conversion of scientific findings offers the paint indus-try a large innovation potential. During the itself cleaning surface always further advances into the paint sector and improves products considerably, develop-ments already suggest to themselves in the labs, that open an even further appli-cation field to ultra thin layers. Photochromic coatings for example, which are translucent according to light intensity incident or -dense. And coatings equipped with catalysts could add one day to the removal of pollutants in the air.
Finger prints in paints: Built-in theft protections, for example with codification as in genetic material, are supposed to guarantee in future individual assign-ments of coatings to the owner. They could determine the agency responsible also in case of car accidents. This is possible by addition of smallest amounts of components to the paint during an application.
Holografic structures also lead to interesting colour effects. At the development is worked by investigating such holografic effects influencing the colouring of coated surfaces.
Some further trends are in development, not yet in the market:
Photovoltaic Surfaces: High-flown project, up to now only 1000 W per car
Electronic selectability and color switching
Nano-coating of windshields and other high-tech glasses (already used for some applications)
Nano components will gain generally importance for the enamel development, after climbing the step from nano science to nano technology. New raw materi-als will result in using coatings more than up to now as functional layers.
Coatings with thicknesses of only a few micrometers or even only a few tens or hundreds of nanometers are gaining in importance due to their excellent properties. Hard material coatings of TiN, TiC or diamond-like carbon with thicknesses of 1 to 4 micrometers are already common for tools and engine components. Highly complex coating systems in the nanometer range have been developed over the past years to achieve scratch-resistant, soil-resistant, antistatic, reflecting or storage-capable surfaces.
STUDY OF FEW NANOCOATINGS:
TiO2 coated ceramic tiles are considered to be very effective against organic and inorganic materials, as well as against bacteria. With other words, the bacteria are killed faster than they can grow. The application of these tiles in hospitals and care facilities to reduce the spread of infections and the
threat to patients whose immune system have been weakened, in public and commercial facilities and schools to improve the hygienic conditions and in residential kitchens, baths and floors to promote family hygiene and to reduce housework is of general interest. Furthermore, these tiles show super-hydrophilic behaviour. Water forms a uniform sheet over the surface at a contact angle of 7 (exterior) and 25 (interior) degrees. Grease, dirt and other staining materials can easily be swept away with a stream of water. Superhydrophilicity, combined with the strong photocatalytic oxidizing properties makes this tile self-cleaning in exterior applications.
Although there are outdoor applications of TiO2 photocatalysis, in the literature no information on the interaction between titanium dioxide and traditional building materials like concrete, mortar, and plaster is available as yet. For example, most of the external building walls become spoiled from automobile exhaust gases, which contain oily components. By coating the original building materials with a super-hydrophilic photocatalyst, the dirt of the walls can easily be washed away by rain, keeping the building external wall clean for long times. Two effects should be considered: Firstly, a super-hydrophilic surface has a higher affinity to water than to oil. Secondly, ultraviolet illumination of TiO2 leads to the formation ordinary uncoated tile hydrophilic tile coated by TiO2 of a photogenerated hole-electron pair that reacts with oxygen and water in the environment to generate potential cleaning agents on the surface of the coated material. The agents (•OH, •OOH) decompose large organic molecules to smaller fragments. The combination of photocatalysis and super-hydrophilicity allows grease and dirt to be swept away with water. Here is the detailed investigation of the dependence of the photocatalytic activity of TiO2 on different building materials. The following items should be investigated:
- Possibilities of fixing TiO2 on building materials. Application of the spray-coating technology
The aqueous or methanolic TiO2 suspension is sprayed on the surface of the considered building material. This method has the advantage that the amount of TiO2 which shall cover a specific area of the sample can be regulated in a simple way. After spraying, the solvent can be removed by heating the sample to approx. 100° C.
- Application of the sedimentation technology
The sample is kept for a defined time in a TiO2 suspension. Then the suspension slowly is drained from the beaker. Again, the solvent can be removed by heating the sample to approx. 100° C.
- The photocatalytic decomposition of organic dyes (methylene blue, rhodamine B, and others) as model substances for organic contaminants
- The photocatalytic decomposition of grease and varnish
- Time-dependent measurements of the decomposition reactions