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Carbon nanotubes (CNTs) have captured a great deal of attention worldwide since their discovery in 1991. CNTs are considered to be the stiffest and strongest material due to their perfect atomic arrangement and intrinsic strong in-plane sp 2—sp 2 covalent bonds between carbon atoms. In addition to mechanical properties, CNTs have also shown exceptional chemical, electrical and thermal properties. All these aspects make CNTs promising candidates in the development of novel multi-functional nanocomposites. Utilizing CNTs as fillers to develop advanced nanocomposites still remains a challenge, due to the lack of fundamental understanding of both material processing at the nanometer scale and the resultant material properties. In this work, a new model was developed to investigate the amount of control specific parameters have on the mechanical properties of CNT composites. The new theory can be used to guide the development of advanced composites using carbon nanotubes, as well as other nano-fibers, with any matrices (ceramic, metal, or polymer). Our study has shown that the varying effect based on changes in CNT dimensions and concentration fit the model predictions very well. Metallic CNT composites using both single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT), have been developed through a novel electrochemical co-deposition process. Copper and nickel matrix composites were developed by using pulse-reverse electrochemical co-deposition. Uniaxial tensile test results showed that a more than 300% increase in strength compared to that of the pure metal had been achieved. For example, the ultimate tensile strength of Ni/CNTs composites reached as high as about 2GPa. These are best experimental results ever reported within this field. The mechanical results are mainly attributed iv to the good interfacial bonding between the CNTs and the metal matrices and good dispersion of carbon nanotubes within the matrices. Experimental results have also shown that the strength is inversely dependent on the diameter of carbon nanotubes. In addition to the mechanical strength, carbon nanotube reinforced metallic composites are excellent multifunctional materials in terms of electrical and thermal conduction. The electrical resistivity of carbon nanotube/copper composites produces electrical resistivity of about 1.0~1.2 x10-6 ohm-cm, which is about 40% less than the pure copper. The reduced electrical resistivity is also attributed to the good interfacial bonding between carbon nanotubes and metal matrices, realized by the electrochemical co-deposition.

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Huang Ching-wei. / Bibliography: leaves 111-112 / Thesis (M.Phil.)--Chinese University of Hong Kong, 1982

The development of nanostructured coatings which exhibit enhanced mechanical properties is currently of interest due to the importance of high performance coatings in a large range of applications. Single layer coatings have predominantly been used for these demanding applications, however the promising mechanical properties observed in multilayer coatings has shifted the focus of current research. In particular, there has been reports of the use of alternating materials with opposing mechanical properties, as seen in the abalone shell, which have exhibited hardness and toughness values significantly greater than either of their constituent materials. The main objective of this thesis was to fabricate Al/W nanostructured multilayers and determine if they exhibit enhanced mechanical properties. The Al/W nanostructured multilayers were fabricated using two different deposition techniques: pulsed magnetron sputtering and cathodic arc deposition. These two techniques differ in the energy of the depositing species and this results in significant differences in film properties. The indentation hardness of the coatings was measured using a Hysitron Nanoindenter. The relationship between the mechanical properties and microstructure was obtained using a range of characterisation techniques. Auger electron spectroscopy (AES), energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) were used to determine the chemical composition and stoichiometry, while cross-sectional transmission electron microscopy (XTEM) and energy filtered transmission electron microscopy (EFTEM) were used to explore the microstructure. The findings of this thesis showed very different results for the two deposition techniques. Although sputtering successfully produced well defined multilayers, no evidence of enhanced hardness was found for periods between 5 and 200 nm. On the other hand, arc deposited samples with intended periods between 1 and 200 nm showed a hardness enhancement above that of pure W, however the samples of highest hardness did not contain Al layers for much of their thickness. Arc deposited samples with the finest nominal periods (1 and 2 nm) contained W-Al intermetallics and were soft. The hardening mechanism was not attributed to a multilayer structure, rather to the introduction of defects in the W layers which acted as pinning sites for dislocations. A modified Hall-Petch equation for hardness enhancement fitted the data for W films prepared by pulsed cathodic arc in which the grain diameter was replaced by the nominal multilayer period. The difficulty producing Al layers on W surfaces in the cathodic arc was overcom e by changing the film growth mechanism by introducing Ar or O2 at the W/Al interface. In the latter case, Al2O3/W multilayers were formed but again showed no hardness enhancements. Complete microanalysis and characterisation of the multilayer structures is vital in determining the mechanisms which govern the hardness enhancements. The evidence in this thesis suggests that the defect density, and not the presence of interfaces are responsible for the hardness enhancement effect.

Rice hull ash (RHA) is a by-product of the rice industry. RHA is produced when rice hulls are incinerated, for example, when they are used to power steam engines in rice milling plants. Typically, this ash is disposed of in landfill sites, which may cause environmental problems. RHA has a naturally occurring silica content that is very high, ranging from 95 to 98%. This high silica content makes RHA a potential filler for polymer products. The aim of this project was to investigate the application of RHA as a filler in polypropylene. The study used a systematic approach to characterising the RHA physiochemical properties and comparing these to another commercially available grade of silica filler. The processing conditions for mixing RHA with Polypropylene (PP) were optimised to obtain the maximum tensile modulus value. Attempts were made to improve the interaction of RHA and PP by treating the RHA surface with silane coupling agents and adding functionalised polymers to the composite. Mechanical, rheological and morphological properties of the non-silanated, silanated and coupled composites were characterised and compared to determine their structure -property relationships. Rice hull ash (RHA) has a similar chemical structure to other silicas. Like any metals and metalloids, the surface of RHA contains -OH functional groups but these are very limited in quantity. RHA and PP composites have a wide processing window and the optimised processing conditions in a small batch mixer are 12 minutes, 60 rpm and 180°C. Addition of RHA into PP increases the modulus but decreases the tensile strength of the composites, attributed to poor compatibility between RHA and PP, as RHA is hydrophilic and PP is hydrophobic. The optimum loading of RHA is 20wt%. To improve the RHA and PP composites, 2 grades of silane and maleic anhydride grafted PP (MAPP) is used. Silane treated RHA composites have improved mechanical properties, especially tensile strength, attributed to enhanced interfacial interaction. The optimum is 1.5wt% for APS and 2.wt% for MPS in this system. The optimum MAPP concentration in this system is 3wt%. Properties of the RHA / PP composites show modest improvements compared to PP. The properties are not sufficient to make RHA a commercially attractive reinforcing filler for PP for high performance composite. It has potential for a cost reduction filler for low end application composites.

Polymers capability to withstand harmful interactions withdifferent environments can be determined by looking at thetransport and mechanical properties of the material exposed tothe medium. The diffusion of a penetrant in a polymer and itsswelling characteristics can be verified by a simplesorption-desorption experiment followed by methodical analysisof the data. Three different systems have been investigated andreported on using the sorption-desorption technique, tensiletesting, compression testing, stress-relaxation and curvefitting routines in Matlab. Fluoropolymers of different repeating unit structure andcrystallinities were exposed to tetrachloroethylene (TCE),water, hydrochloric acid (35%) and hydrobromic acid (47%) at70°C yielding solubility and diffusivity data. Thetransport properties were mostly controlled by the polarity ofthe polymer and to a less degree by the polymer crystallinity.Low solubilities were observed for the aqueous solutes andtheir diffusivities were best fitted using a dual sorption modeassuming no concentration dependent diffusivities. Thepolarisable non-polar TCE showed the highest solubility, andthe diffusivity was solute-concentration dependent. The rate atwhich the surface-concentration approached the saturation levelwas proportional to the product of the Young's modulus, thesquare of the dry polymer thickness and the logarithm of thesolute diffusivity. Data for water-hyperbranched polymer andlimonene-polyethylene confirmed the relationship. Low and high-density poly-ethylene (LDPE, HDPE), crosslinkedethylene vinyltrimetoxy silane (PEX), natural rubber (NR) andacrylonitrile-butadiene rubber were exposed to crude oilcomponents at 25 and 30°C. Solubility data indicated thatthe accessibility of the interfacial components decreased inthe order: cyclohexane, n-hexane/2,2-dimethylbutane andn-decane/n-tetradecane. The free-volume model describeddesorption data better than the semi-empirical exponentialmodel, but the numerical differences were for most practicalapplications negligibly small. The decrease in tensile modulus,yield stress and relaxation modulus for dry HDPE and PEXcompared to n-hexane, n-heptane, cyclohexane andtoluene-swollen samples was clearly caused by thepenetrant-induced plasticisation effect. LDPE and NR-sheets were exposed to limonene at 25°C.The limonene-NR displayed saddle-shape during sorption andcup-shape during desorption were most likely a consequence oflocal differences in limonene concentration that affected thestress state across the sheet thickness. The ratio in bulkmodulus between LDPE and NR was significantly smaller than thecorresponding ratio in tensile modulus and close to the ratioin the degree of anisotropy for the same polymers.Consequently, the bulk modulus is more accurate than thetensile modulus to use when predicting the degree of swellinganisotropy.

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