Heterogeneous azeotropic distillation completely reduces the occurrence of agglomerates and decreases the ZnO particle size. Lanje et al. The single step process with the large scale production without unwanted impurities is desirable for the cost-effective preparation of ZnO nanparticles. As a consequence, the low cost precursors such as zinc nitrate and sodium hydroxide to synthesize the ZnO nanoparticles ca.
In order to reduce the agglomeration among the smaller particles, the starch molecule which contains many O-H functional groups and could bind surface of nanoparticles in initial nucleation stage, was used. Another process of controlled precipitation of zinc oxide was carried out by Wang et al. This study was performed using a membrane reactor consisting of two plates of polytetrafluoroethylene PTFE , with stainless steel as a dispersion medium. The ZnO obtained had a narrow range of particle sizes, from 9 to 20 nm.
XRD analysis showed both the precursor and the ZnO itself to have a wurtzite structure exclusively. The particle size was affected by temperature, calcination time, flow rate and concentration of the supply phase. In a report of Jia et al.
From a fundamental point of view, these findings provide new insights into the growth of ZnO crystals and arm researchers with potential strategies for the controllable synthesis of ZnO in liquid media. In processes of synthesis of nanopowders based on precipitation, it is increasingly common for surfactants to be used to control the growth of particles. The presence of these compounds affects not only nucleation and particle growth, but also coagulation and flocculation of the particles.
The surfactant method involves chelation of the metal cations of the precursor by surfactants in an aqueous environment. Wang et al. The product was highly crystalline ZnO with a wurtzite structure and with small, well-dispersed spherical nanoparticles in size of 50 nm. It was found that CTAB affects the process of nucleation and growth of crystallites during synthesis, and also prevents the formation of agglomerates.
Li et al. The presence of the surfactant was found to affect both the shape and size of the resulting ZnO particles. Effect of sodium dodecyl sulfate SDS surfactant on the structure of a ZnO crystal created based on [ 45 ] with permission from Elsevier Publisher. The obtaining of ZnO nanopowders by the sol-gel method is the subject of much interest, in view of the simplicity, low cost, reliability, repeatability and relatively mild conditions of synthesis, which are such as to enable the surface modification of zinc oxide with selected organic compounds.
This changes in properties and extends its range of applications. The favourable optical properties of nanoparticles obtained by the sol-gel method have become a common topic of research, as reflected in numerous scientific publications [ 46 ]. Figure 3 shows two examples of synthesis by the sol-gel method: films from a colloidal sol Figure 3a , and powder from a colloidal sol transformed into a gel Figure 3b.
Overview showing two examples of synthesis by the sol-gel method: a films from a colloidal sol; b powder from a colloidal sol transformed into a gel created based on [ 72 ] with permission from Elsevier Publisher. Benhebal et al. The prepared zinc oxide has a hexagonal wurtzite structure with the particles of a spherically shaped. The resulting colloidal suspension was left for 30 min alternatively for 24 h , and was then washed with ethanol and water.
TMAH is a strong organic base, which comparably with an inorganic base e. This high pH means that metal oxides are not contaminated with the cation from the base, which may have an effect on the ohmic conductance of the oxide material. A determination was made of the effect of the quantity of ZEH used and the maturing time of the colloidal solution. TEM images showed that the ZnO particles obtained have sizes of the order of 20—50 nm. The quantity of ZEH has a negligible effect on the particle size.
Yue et al. High-filling, unifrom, ordered ZnO nanotubes have been successully prepared by sol-gel method into ultrathin AAO membrane. Integrating the ultrathin AAO membranes with the sol-gel technique may help to fabricate high-quality 1D nanomaterials and to extend its application as a template for nanostructures growth. The hydrothermal method does not require the use of organic solvents or additional processing of the product grinding and calcination , which makes it a simple and environmentally friendly technique.
As a result of heating followed by cooling, crystal nuclei are formed, which then grow. This process has many advantages, including the possibility of carrying out the synthesis at low temperatures, the diverse shapes and dimensions of the resulting crystals depending on the composition of the starting mixture and the process temperature and pressure, the high degree of crystallinity of the product, and the high purity of the material obtained [ 73 , 74 ].
An example of a hydrothermal reaction is the synthesis of zinc oxide as proposed by Chen et al. The process took place by way of reaction 5 :. The white Zn OH 2 precipitate underwent filtration and washing, and then the pH was corrected to a value of 5—8 using HCl. In the autoclave hydrothermal heating takes place at a programmed temperature for a set time, followed by cooling.
The end product of the process is zinc oxide the following reaction 6 :. The temperature and time of reaction were shown to have a significant effect on the structure and size of the ZnO particles. It was also found that as the pH of the solution increases, there is an increase in the crystallinity and size of the particles, which reduces the efficiency of the process.
A hydrothermal process was also used by Ismail et al. The resulting precipitate of Zn OH 2 was washed with water several times, and then underwent thermal treatment in a Teflon-lined autoclave. The shape of the particles is also affected by the time and temperature of the hydrothermal process. With an increase in time, temperature and surfactant concentration, the size of the particles increases.
Hydrothermal processing of the precursor, followed by drying, produced spherical particles of ZnO with sizes in the range 55— nm depending on the conditions of synthesis. Increasing the time of the hydrothermal process caused an increase in the diameter of the ZnO particles. It was found that the pH affected the size and shape of the ZnO particles.
The resulting nanoparticles cause distinct changes in the standard Raman spectrum of zinc oxide. A number of studies [ 54 , 55 , 75 , 76 ] have shown that the use of microwave reactors in hydrothermal synthesis processes brings significant benefits.
Microwaves make it possible to heat the solutions from which the synthesis products are obtained, while avoiding loss of energy on heating the entire vessel. Many chemical syntheses proceed with greater speed and yield when microwaves are used than in the case of traditional methods.
Similar fast and voluminal heating of the reaction substrates can be achieved using electrical current flowing through the substrates. Strachowski et al. The work was conducted in such a way that the reactions being compared took place at the same externally supplied power levels and with the same reaction vessel geometry.
Reactors were used with reaction energy supplied using microwaves, electrical current, Joule heating, high-voltage pulses, and heating of the whole autoclave. The powders produced in the other reactors showed the presence of other phases simonkolleite and hydroxyzincite besides zinc oxide. The use of a microwave reactor made it possible to shorten the reaction time several fold, and also produced the purest product.
Microwaves were also used by Schneider et al. Zinc oxide was obtained by heating, using microwaves, zinc acetylacetonate and a zinc oxime complex in various alkoxyethanols methoxy-, ethoxy- and butoxyethanol. Schneider et al. The size of the particles of the final product lay in the range 40— nm, depending on which precursor and alcohol were used. The smallest particles belonged to the zinc oxide obtained by heating a complex of zinc oxime in methoxyethanol.
With an increase in the concentration and chain length of the alcohol, the particle size increased. Thermal decomposition of both zinc acetylacetonate and zinc oxime enabled the obtaining of a product with the desired properties. Zhang et al. The authors suggested that the solvothermal process may involve the following reactions 9 and 10 :.
The system proposed by Zhang et al. It can be expected that a solvothermal hybrid and an ionothermal system may be successfully used to synthesize new materials with interesting properties and morphologies.
A solvothermal method was also used by Chen et al. The process involved the following reactions 11 and 14 :. The authors also determined the effect of the presence of water in the reaction system. The zinc oxide obtained had diameters in the range 24— nm, depending on the reaction conditions. The classic definition of an emulsion as a continuous liquid phase in which is dispersed a second, discontinuous, immiscible liquid phase is far from complete.
One very convenient way to classify emulsions is first to divide them into two large groups based on the nature of the external phase. Vorobyova et al. Zinc oxide was precipitated in an interphase reaction of zinc oleate dissolved in decane with sodium hydroxide dissolved in ethanol or water. The process as a whole involved the reaction 15 :. It was found that the reaction may take place in different phases, both in water and in the organic phase.
The conditions of the process temperature, substrates and ratio of two-phase components affect the size of the particles and the location of their phases. An emulsion method was also used in the work of Lu and Yeh [ 59 ]. The aqueous phase of the system was zinc acetate dissolved in de-ionized water, and the organic phase was heptane. To stabilize the water-in-oil emulsion, the surfactant Span 80 was added to the heptane.
NH 4 OH was added to the emulsion in order to obtain the zinc cation. Lu and Yeh concluded that ZnO precipitated in this emulsion system has a smaller range of particle sizes 0.
The product consisted of nearly spherical particles. Zinc oxide was also obtained by precipitation in an emulsion system with zinc acetate used as a precursor of ZnO, and potassium hydroxide or sodium hydroxide as precipitating agent [ 60 ].
Cyclohexane, as an organic phase, and a non-ionic surfactant mixture were also used for preparation of the emulsion. By applying modifications of the ZnO precipitation process, such as changing the precipitating agent, composition of substrates and the rate of substrate dosing, some interesting structures of ZnO particles were obtained.
The morphology of the modified samples was analyzed based on SEM scanning electron microscope and TEM transmission electron microscope images. Moreover the samples were analyzed by determination of their dispersive properties using the non-invasive back scattering method NIBS , parameters of porous structure BET and crystalline structure XRD.
For selected samples their electrical properties dielectric permittivity and electric conductivity were also measured. Zinc oxide structures: a solids; b ellipsoids; c rods; and d flakes [ 60 ]. Emulsions and microemulsions differ markedly from each other, which makes it relatively easy to identify the areas of their application. Microemulsions are stable, transparent, isotropic liquids consisting of an aqueous layer, and oil layer and a surfactant.
The drop size in a microemulsion is significantly smaller than in an emulsion, and lies in the range 0. In contrast to emulsions, microemulsions form spontaneously in appropriate conditions. In this case the microemulsion consists of a solution of heptane and hexanol together with a non-ionic surfactant such as Triton X The growth of nanoparticles involves the exchange of the substrates Zn NO 3 2 and NaOH between the microemulsion drops and the medium poly ethylene glycol —PEG , and aggregation of the formed nuclei.
Drops of microemulsion act as a microreactor in which the desired reaction takes place. Figure 5 illustrates the process of synthesis in microemulsion and the shape of ZnO nanoparticles as proposed by the aforementioned authors. Synthesis and morphology of crystalline ZnO synthesized in a microemulsion system: a without PEG ; and with the addition of: b Another process of precipitation of zinc oxide in the environment of a microemulsion was proposed by Singhal et al.
It is important that the solvents do not contain water, which might dissolve NaNO 3 during the reaction, causing contamination of the precursor. The resulting solution was filtered and dried. To ensure that the sample did not contain sodium, Zn NO 3 2 was also added to the solution. At the next stage an anhydrous microemulsion of alcohol in oil was prepared. The entire solution was mixed for 1 h.
The precipitate was separated from the solution by centrifugation, and the precipitate obtained was then washed twice. The first washing was performed using a methanol:chloroform mixture in a ratio of by volume, to remove surfactant and oil. The second used an acetone:methanol mixture to remove surfactant and excess oxalic acid.
Singhal et al. The technique of obtaining ZnO using microemulsion was also used by Yildirim and Durucan [ 63 ]. They attempted to modify the microemulsion method so as to obtain monodisperse zinc oxide. They did not obtain zinc oxide directly from the microemulsion process, but used thermal decomposition of the zinc complex precipitated in the microemulsion process, followed by its calcination. The process was modified in that glycerol was used as the internal phase of a reverse microemulsion Aeroloz OT:glycerol:heptane , similarly as Moleski et al.
The basic aim of the work of Yildirim and Durucan was to determine how the concentration of surfactant and the temperature of calcination affect the size and morphology of the resulting ZnO particles.
The zinc oxide obtained consisted of spherical and monodisperse particles measuring 15—24 nm. In turn, Xu et al. In all of these methods the starting solution used was zinc nitrate Zn NO 3 2. The emulsion consisted of Zn NO 3 2 and an appropriate surfactant cationic, anionic or non-ionic.
However the microemulsion was prepared from Zn NO 3 2 , cyclohexane, acrylonitrile-butadiene-styrene copolymer ABS , butanol, and hydrogen peroxide H 2 O 2. In summing up their work, Xu et al. There also exist many other methods of obtaining zinc oxide, including growing from a gas phase, a pyrolysis spray method, a sonochemical method, synthesis using microwaves, and many others.
Zinc oxide was obtained in the form of pure crystals by Grasza et al. Crystals of ZnO were grown from a gas phase air, nitrogen, atmospheric oxygen, gaseous zinc and arsenic. A wide range of values of heating time and temperature were used.
Particular emphasis was placed on analysing the surface during its interactions with the air, oxygen and gaseous zinc. It was found that thermal heating in the various gases led to similar changes in the crystal surface, although differences were observed in the rate of those changes. Grasza et al. Tests showed that the porosity of the crystal surface increases with increasing temperature and heating time.
A thin layer of zinc oxide was obtained by Wei et al. The results obtained by Wei et al. The PL spectra indicate that UV emission increases with increasing temperature. It was found that the quantity of UV emitted for a thin layer of ZnO made using powder was smaller than in the case of ceramic ZnO. Using an aerosol pyrolysis method, Zhao et al. A determination was made of the mechanism and kinetics of the thermal decomposition of zinc acetate dihydrate, as well as a correlation of the mechanism with the results of the aerosol pyrolysis process.
Zinc oxide synthesized by the aerosol pyrolysis method consists of particles in the range 20—30 nm. Hu et al. In summing up their work, Hu et al. The ZnO rods obtained can be successfully used in electronics and optoelectronics. The sonochemical method may be used in the future for the synthesis of single-dimensional structures of other metal oxides.
The search for new possible applications of zinc oxide, the need to reduce its content in rubber mixtures, and the major problem of its tendency to form significant agglomerations have encouraged researchers in recent years to carry out numerous studies to find an optimum method of modifying the surface of the compound without impairing its physicochemical properties.
Modification is also often carried out in order to improve its performance properties, such as high or low depending on application photocatalytic activity. In the following sections we will consider the methods of modification of zinc oxide proposed by various scientists.
Figure 6 presents a schematic that summarizes all the method of modifiction of ZnO mentioned in the text. Cao et al. The finest particles of ZnO were obtained by calcination of the precursor zinc carbonate hydroxide ZCH.
Modification of the ZnO particles made possible a solution to the problem of their agglomeration. Functionalization of the ZnO surface with an inorganic compound silica reduced the photocatalytic action of the oxide, while the organic compound HMDS increased the compatibility of the ZnO with an organic matrix.
The highly transparent modified zinc oxide surface was found to provide excellent protection against UV radiation, which represents a significant advantage of the use of these modifying agents.
A schematic representation of the synthesis of surface-modified ZnO ultrafine particles using an in situ modification method is shown in Figure 7. Schematic representation of the synthesis of surface-modified ZnO ultrafine particles using an in situ modification method created based on [ 41 ] with permission from Elsevier Publisher. Modification with the use of silica was also performed by Xia and Tang [ 84 ].
Molecules of TEOH are adsorbed by the silica, and the siloxane and silanol networks are broken as a result of the changes occurring in the SiO 2.
In accordance with the theory of maturing and aggregation, the resulting clusters are susceptible to rapid collision with other clusters of zinc oxide, leading to an appropriate concentration of the compound. An important role in the proposed modification technique is played by TEOH, which enables complex structures to be obtained. Hong et al. They also performed an additional modification using oleic acid. Zinc oxide was obtained as a result of the reaction of zinc acetate with ammonium carbonate, followed by calcination of the resulting zinc precursor.
To determine the compatibility between the inorganic nanoparticles and the organic matrix, the surface of the ZnO was covered with oleic acid. The FTIR spectra confirmed the presence on the surface of the modified ZnO of an organic layer and a chemical bond between the inorganic —OH groups and the organic chain macromolecules. The proposed mechanism for these processes was presented in terms of reaction 16 :.
Coverage of the zinc oxide surface with a thin film of amorphous silica improved the degree of dispersion, and thus reduced the agglomeration of nanoparticles. Moreover, based on photocatalytic degradation in aqueous solution using methyl orange, it was shown that silica-coated ZnO has lower catalytic activity than the original nanostructures. The work of Hong et al. Similar studies have been carried out and published by those authors in [ 85 ]. Hydrophobic ZnO nanoparticles were produced by Chen et al.
Results from the SEM observations and sedimentation test indicate that the new surface treatment would considerably reduce aggregates of particles and enhance long-term stability in an organic matrix. The coating obtained was highly uniform, and had a thickness of 5 nm. The pH at the isoelectric point for ZnO nanoparticles with an Al 2 O 3 layer moved from around 10 to a value of 6, which may improve the dispersion of ZnO particles. Wysokowski et al. The modification used zinc oxide synthesized by a hydrothermal method microwave dehydration.
The resulting system was then mixed using an ultrasound disintegrator in the case of silane the system was first mixed with a magnetic mixer, with simultaneous heating. The whole was mixed with a magnetic mixer, with heating, for 5 min. The addition to rubber mixtures of PEG-coated nano zinc oxide an increase in the degree of cross-linking of the vulcanizates, but there was also an increase in vulcanization reversion and a marked decrease in prevulcanization time.
The marked decrease in surface energy in the case of oxides modified with PEG and silane can be expected to facilitate their dispersion in nonpolar rubbers. Modification of the surface of ZnO particles using silane was also performed by Kotecha et al.
The modifier used was 3-methacryloxypropyltrimethoxysilane. Nanoparticles of zinc oxide were obtained using zinc acetate and potassium hydroxide as substrates. In this method the silane was introduced into the system during the precipitation. Concurrently with the formation of ZnO particles, a reaction takes place between silane and ZnO.
In the course of this reaction H 2 O is generated and a side reaction takes place, during which the pH increases to 9.
The silane-covered zinc oxide particles were introduced into an aqueous suspension and exposed to UV radiation. Based on interpretation of SEM images, the researchers concluded that unmodified zinc oxide contains particles around nm in diameter, forming agglomerates. The introduction of silane into the ZnO structure caused a decrease in the particle size 40— nm and an increase in the diameters of the aggregates, even to the order of micrometres.
UV radiation also changes the character of ZnO from hydrophobic to hydrophilic. Analysing the adsorption parameters, Kotecha et al.
The results of Kotecha et al. The resulting material has high porosity, large BET surface area, and hydrophilic properties. Figure 8 shows example mechanisms taking place during the process of modification of zinc oxide using a selected silanol binding compound. Chang et al. In their work, Chang et al. Change of state of the surface of zinc oxide nanowires through plasma treatment is one of the most promising methods of ZnO modification.
Experiments carried out by Ra et al. After treatment the concentration of the carrier and mobility of the ZnO decreased. There was also an improvement in properties relating to the detection of hydrogen by the modified nanowires, and the time of photocurrent amplification in UV radiation.
In summarizing their work, Ra et al. In turn, Kang and Park [ 93 ] modified zinc oxide using silver ions. The size of the ZnO particles obtained by the FEAG method depended on the conditions of the operation and the type of solvent.
Based on TEM images it was found that the ZnO obtained consisted of particles measuring approximately 12 nm. Next the ZnO was dispersed in a solution of silver nitrate in various ratios.
A commercial ZnO powder was activated mechanically by grinding in a vibrating mill with steel rings, under continuous air circulation. The process was continued for 30 and min. The product was subjected to comprehensive physicochemical analysis.
Wu et al. The devices were optimized by changing the number of layers of cross-aligned ZnO nanofibres and the growth time of CdS on the ZnO. In addition, the lifetime of carriers at the bulk heterojunction was investigated using an impedance analyser and was found to be dramatically increased after CdS modification. Over the past decade much work has been done on developing nanocomposites produced by the action of modified inorganic carriers with polymer matrices.
Such procedures make it possible to produce new classes of polymeric materials which combine properties of both inorganic particles and organic polymer matrices including process ability and elasticity.
Shim et al. The most important condition in the production of the composite is the interphase compatibility between the inorganic compound and the polymer. For this purpose the surface of the inorganic system should be treated with a hydrophobic organic substance. The obtained inorganic-polymer composites form persistent microspheres and combine easily into highly processed polymers. Similar studies have been carried out and published by other researchers [ , ]. Tang et al. The hydroxyl groups on the ZnO surface reacted with the carboxyl groups of the PMAA, producing a complex of poly zinc methacrylate on the surface of the zinc oxide.
Interpreting the particle size distributions, it was found that the ZnO modified with PMAA contains particles with smaller diameter ca.
Analysis of the dispersive stability of the ZnO showed that the modified particles of zinc oxide dispersed better in water than unmodified particles. Conventional inorganic nanoparticles have hydroxyl groups -OH on their surface, due to the effect of humidity and the environment and type of precipitation. These groups react with COO- groups to form small complexes of poly zinc methacrylate on the surface of the zinc oxide.
Poly methyl methacrylate was also used as a ZnO surface modifier by Hong et al. Nanoparticles of zinc oxide with a diameter of approximately 30 nm were synthesized by means of homogeneous precipitation followed by calcination. In order to introduce reactive groups onto the ZnO surface, a reaction was carried out between the hydroxyl groups and a silane coupling agent 3-methacryloxypropyltrimethoxysilane.
Graft polymerization was effected by means of a reaction between the ZnO, containing silanol groups, and the monomer. Tests showed that the polymerization does not alter the crystalline structure of the ZnO nanoparticles. Their dispersion in the organic solvent can greatly improve the graft polymerization of PMMA, and further improvement can be achieved by the addition of other surfactants. Modification of ZnO nanoparticles by grafted PMMA increases the degree of lyophilicity of the inorganic surface and reduces the formation of aggregates.
Polystyrene PS is also the subject of interest as a surface modifier of zinc oxide particles. Chae and Kim [ ] carried out a process of ZnO surface modification using that compound. For the structures obtained, the morphology, microstructure, thermal properties and mechanical properties were investigated. The tests confirmed that the solvent used is capable of breaking up the agglomerates that form, and prevents re-agglomeration during mixing of the solution. An object of interest in recent years has been the resistance connections of ZnO particles embedded in MIM metal-insulator-metal structures.
Work has focused on altering the layers of oxides, whose amorphous nature, porosity and lack of homogeneity constitute a problem.
Researchers under the direction of Verbakel [ ] investigated the resistance effects of the switching of diodes containing structures of nanometric ZnO covered with an active layer from a polystyrene matrix. Using an impedance spectroscope it was found that the electronic memory effect in nanostructured metal oxides can be affected by modification of the surface of the particles using coordinating ligands e. This process provides new prospects for ecological modification of the surface of ZnO powder using inorganic hybrid materials.
Modification of ZnO nanoparticles using polystyrene was also performed by researchers under the direction of Tang [ ]. Nanometric particles of zinc oxide particle size ca. The nano-ZnO surface had to have a hydrophobic character, in order to hermetically seal the ZnO nanoparticles perfectly in the monomer. This property was controlled by the creation of functional groups on the nano-ZnO surface with the use of a silane coupling agent MPTMS. MPTMS is an organic polymer chain which forms steric hindrances between inorganic particles, preventing their aggregation.
However it was not simple to obtain perfect dispersion of the hydrophobic nano-ZnO particles in an aqueous polymerization system. To ensure stability of dispersion, a surfactant OP was added to the system, in a quantity smaller than that which properly saturates the surface, so as to avoid the formation of micelles of emulsifier. Tests showed that the particles of the resulting polymer composite are monodisperse, with diameters in the range — nm. Studies with that compound were carried out by Chae and Kim [ ].
ZnO nanopowder particles of diameter 87 nm was dissolved in DMAc to break up agglomerates. The resulting precipitate underwent spectroscopic, thermal and mechanical analysis. The product exhibited better thermal stability than the starting material, due to the barrier role of ZnO.
Moreover the ZnO nanoparticles caused a reduction in the crystallization temperature of the modifier PAN and an increase in the width of the crystallization peaks. This is linked to heterogeneous nucleation and the reduced mobility of the polymer chains. The introduction of ZnO nanoparticles into the polymer chain caused an increase in the modulus of elasticity on stretching and a reduction in the dynamic load resistance.
Xiong et al. The compound was analyzed in terms of composition, structure, fluorescence and specific conductance. The tests showed that the polymer nanocomposite synthesized by means of a chemical reaction has better properties than its equivalent obtained through physical mixing. These properties mean that the obtained compound can be used in luminescent devices and in electronic apparatus.
Modification of zinc oxide using carboxylic acids such as stearic, tartaric, maleic, propanoic etc. An apparently promising method is modification in situ , which causes a significant increase in the surface area of the zinc oxide to as high as ca. Figure 10 shows an example mechanism taking place during modification of zinc oxide with maleic acid, and an FTIR spectrum confirming the effectiveness of the modification.
It has been experimentally demonstrated that alkanethiols may adsorb on ZnO surface [ — ]. For example, Singh et al. They also presented the first ultraviolet photoelectron spectroscopy UPS investigation of thiol adsorption on zinc oxide. It was found that the MT frontier orbitals are strongly perturbed by adsorption on ZnO , with the work function of the surface increasing by 0. X-ray photoelectron spectroscopy XPS and Raman spectroscopies confirmed adsorption, and in situ photoluminescence measurements showed the intensity of the visible emission peak is decreased by methanethiol adsorption.
The thickness and morphology of the encapsulating layer was controllable by the choice of thiol and preparation conditions. Singh et al. Because of its diverse properties, both chemical and physical, zinc oxide is widely used in many areas. It plays an important role in a very wide range of applications, ranging from tyres to ceramics, from pharmaceuticals to agriculture, and from paints to chemicals.
Figure 11 shows worldwide consumption of zinc oxide by region. In the Figure 12 summarized application paths of ZnO are presented. Global production of zinc oxide amounts to about 10 5 tons per year, and a major portion is consumed by the rubber industry to manufacture various different cross-linked rubber products [ ].
The thermal conductivity of typical pure silicone rubber is relatively low; however, it can be improved by adding certain thermal conductivity fillers, including metal powders, metal oxides and inorganic particles. The incorporation of nano-scale fillers can achieve high thermal conductivity even at a relatively low filling content. However, the ZnO nanoparticles tend to aggregate together to form particles of large size in the polymer matrix, due to the weak interaction between the surface of the nanoparticles and the polymer.
In order to solve this problem, surface modification techniques are applied to improve the interaction between the nanoparticles and the polymer.
In the work of Yuan et al. Yuan et al. Next the silicone coupling agent VTES was successfully incorporated onto the surface of the nanoparticles. Zinc oxide is a very effective and commonly used cross linking agent for carboxylated elastomers [ , ]. It can be used to produce vulcanizates with high tensile strength, tear resistance, hardness and hysteresis. The improved mechanical properties of ionic elastomers mainly result from their high capacity for stress relaxation, due to elastomer chain slippage on the ionic cluster surface and reformation of ionic bonds upon external deformation of the sample.
Moreover, ionic elastomers have thermoplastic properties and can be processed in a molten state as a thermoplastic polymer [ ]. However, there are some disadvantages to zinc-oxide-cross linked carboxylic elastomers. The most important are their scorchiness, poor flex properties and high compression set.
The vulcanization of XNBR with zinc peroxide mainly leads to the formation of ionic crosslinks; covalent links are also formed between elastomer chains due to the peroxide action. However, higher vulcanization times are required to achieve vulcanizates with a tensile strength and crosslink density comparable to that of vulcanizates cross linked with zinc oxide.
The last process, which decays with vulcanization time, most likely involves the formation of ionic species. The achieved vulcanization times are considerably higher than in the case of XNBR cross linking with zinc oxide.
Therefore, apart from the scorch problems, zinc oxide is still commonly used as a cross linking agent in carboxylated nitrile rubbers. In view of the fact that, during the cross linking process, zinc oxide reacts with the carboxylic groups of the elastomer, which leads to the formation of carboxylic salts ionic crosslinks , the most important parameters influencing the activity of zinc oxide are its surface area, particle size, and morphology.
These parameters determine the size of the interphase between the cross linking agent and elastomer chains [ ].
Przybyszewska et al. They concluded that the use of zinc oxide nanoparticles produced vulcanizates with considerably better mechanical properties and higher crosslink density, as compared with vulcanizates cross linked with micro-sized zinc oxide, which is used commercially as a cross linking agent. Vulcanizates containing the same quantity of zinc oxide nanoparticles exhibited a tensile strength about four times greater than that of vulcanizates with micro-sized particles.
This is a very important ecological goal, since zinc oxide is classified as toxic to aquatic species, and the European Union requires that the amount of zinc oxide in rubber compounds be reduced. Moreover, it should be noted that vulcanizates of carboxylated nitrile elastomer cross linked with zinc oxide demonstrate heat shrinkability.
It is the morphology of zinc oxide particles which mainly affects the activity in the cross linking process. Particle size and surface area do not seem to have a significant influence on the efficiency of zinc oxide as a cross linking agent. The specific shape and complex structure of ZnO aggregates, consisting of wires or plates growing from a single core, provide an increase in the size of the interphase between the elastomer carboxylic groups and the snowflake particle.
As a result, Przybyszewska et al. Moreover, the zinc oxide nanoparticles used by Przybyszewska et al. Environmental concerns are especially focussed on the effect of excess zinc on aquatic organisms [ ], which has led to various efforts to reduce zinc levels in rubber compounds [ ].
There are three basic methods of reducing the content of ZnO in rubber compounds:. To find an alternative to conventional ZnO, which in higher dosage is toxic to aquatic systems, Thomas et al. They studied the effect of these capped compounds on the curing and mechanical properties of natural rubber NR vulcanizates. The zinc oxide used in the research was prepared by a sol-gel method, and was then modified using accelerators such as BIAT and fatty acids such as stearic acid.
This capping technique reduces agglomeration of nanoparticles of ZnO and is an effective method to improve the curing and physicochemical properties of NR. By capping ZnO with BIAT and stearic acid, it becomes possible to save the extra time and energy required for these particles to diffuse onto the surface of ZnO via the viscoelastic rubber matrix.
This provides a further improvement in acceleration of vulcanization and improvement in the physicomechanical properties of the resulting vulcanizates. The mixture containing optimum concentration of BIAT-capped-stearic acid-coated zinc oxide ZOBS has superior curing and physicomechanical properties compared with other homologues and the reference mixture containing uncapped ZnO.
The increased crosslink density caused by the ZPS particles could increase the stiffness of vulcanizates containing ZPS. The capping technique could improve the scorch safety of rubber compounds by the delayed release of BIAT from the capped ZnO into the rubber matrix for interaction with CBS conventional accelerator.
Sabura et al. The researchers used this nano zinc oxide as a curing agent in neoprene rubber. The optimum dosage of ZnO was found to be low compared with commercial ZnO. The cure characteristic and mechanical properties of the rubber were compared with those containing conventional ZnO. It was found that a low dosage of zinc oxide was enough to give equivalent curing and mechanical properties compared to neoprene rubber containing a higher dosage of commercial zinc oxide. Due to its antibacterial, disinfecting and drying properties [ , ], zinc oxide is widely used in the production of various kinds of medicines.
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The influence of Nano materials. Currently this technology is received materials were characterized for their chemical oxide being used for the creation of new materials, devices and composition, crystalline and amorphous content, density, systems at molecular, Nano and micro-level. Nano materials fineness, specific surface area, and particle size distribution. The extremely fine size of Additionally, a limited number of investigations are dealing Nano-particles yields favourable characteristics.
Because of with the manufacture of nano sized cement particles and the their high surface area and excellent fire retardant development of nano binders. Addition of Nano-materials to cement and The study attempts to evaluate the impact of six different concrete can lead to significant improvements in the field of additives, including natural additives and nano-additives in M civil engineering.
The test design is adopted on for the mixes and the optimum amount of admixtures are determined for 1. Amorphous of the concrete. This effect depends on the amount of composition with decrease in weight on ZnO nano concrete. ZnO added to the hydrated paste. The transformation of zinc The use of chemical and mineral additives is the most effective hydroxide into calcium hydrozincate provokes the further way to improve the quality of the concrete, and to give them hydration.
Nanotechnology is a very active research field and has applications in a number of Fig Sample Zinc Oxide areas. Now a days Nano materials are used in construction along with the traditional building materials. Incorporation 2. Nivethitha et al. With the surface area and hence the properties of concrete. Nano-ZnO workability, strength and durability characteristics.
Anshul Pathak , investigated mechanical and durability 3.
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