The technology of water lubrication has been widely studied and applied in the area of mechanical transmission since 70′s, because water lubricated friction pair has many advantages over the conventional oil lubricated one. The key issue of water lubrication technology is the correct choice of friction materials under water lubrication. Ceramic friction pair is a focus of the study in water lubrication technology, because ceramic materals have excellent tribological performance in water. However, the brittleness of ceramic materials are difficult to overcome, so their application has been limited in many ways. Ceramics class composites are made of ceramic and metal composite, contain the some of the features both of ceramic and metal materials, has a high potential in water lubricaton. In this thesis, the tribological properties of two ceramics classes composites （the WC-Co cemented carbide and the Ti（C,N） Cermets） in water-lubrication were researched and discussed. The experiments were carried out by a pin-on-disk tribo-meter, Si3N4 ceramic balls as upper specimen, the lower specimens were disks made of Ti（C,N）, WC, Si3N4 and 316 stainless steel materials respectively.Deionized water lubrication was mian situation in this research, and both the 3% NaCl solution and anhydrous ethanol liquid were also selected as lubricant, to make a comparative analysis with deionized water. 3% NaCl solution used to simulate the lubrication of sea water. The purpose of ethanol use was to isolate water and friction pairs. Analysis the role of water through the performance of the friction pairs in the process of friction experiments in ethanol liquid without the hydration reaction.The the friction and wear conditions of friction pairs in de-ionized water, 3% NaCl solution, ethanol liquid were observed. The results of different loads and defferent sliding speeds were compared with. The changes in the different stages of worn surface were analyzed. Observed the three-dimensional micro-morphology of wear scars by optical microscope and non-contact profilers. The results show: the friction coefficient of Si3N4 ball/ Ti（C,N） disk and Si3N4 ball/ WC disk friction pairs can be reduced to less than 0.01 in de-ionized water. That means they can realize hydrodynamic lubrication. The Na+ ions in 3% NaCl solution may have effects on the friction process, which make the reduction in the friction coefficient smaller, and unable to realize hydrodynamic lubrication in some experimental conditions. The friction coefficient curve of all four kind of firicton pairs maintain as a horizontal line basically in the ethanol liquid, which demonstration that water lubrication is the main factor of friction reduction. In all conditions, the wear loss of both Ti （C, N） disk and WC disk is smaller than that of Si3N4 disk. It can make a conclusion that these two materials have excellent tribological performance in water.Under water lubrication the friction pair of Si3N4 ball and Ti（C,N） disk can realize hydrodynamic lubrication in both high speed low load and high speed high load conditions, while its friction coefficient only has a little decrease in low speed condition. This indicates that as the sliding speed increases the effects of water lubrication and load carrying capacity of the friction pair are improved. The high hardness and good corrosion resistance of Ti（C,N） material result in the least wear loss of the the friction pair in most conditions. And disk’s wear loss at high load is less than that at low load. It is deduced that the running-in process is as follows: metal and its oxide layers of disk were removed gradually at first, then the inner ceramic layer was polished, finally the lubrication film produced by tribo-chemical reaction was accumulated on the surface and hence reduced the friction coefficient. The Ti（C,N） cermet material contains seven kinds of chemical compositions, which may have negative effects on the formation of lubrication film, so its amount of lubrication film is less than that of Si3N4/WC pair.Under water lubrication the friction pair of Si3N4 ball and WC disk can realize hydrodynamic lubrication in all speed-load conditions. The wear can be mainly attributed to tribo-chemical wear. The friction products adhere to wear scar of the disk, which results in less wear loss of the disk. And the disk’s wear loss decreases as the load increases, showing good wear resistance. It can be deduced that the running-in process of Si3N4-WC pair is similar to that of Si3N4-Si3N4 pair: the polishing effects make surface more smooth; the lubrication film produced by tribo-chemical reaction between Si3N4 and water reduces the friction; and more lubrication film produced in high load condition result in better antifriction effect。The fact that friction pair of Si3N4 ball and WC disk can realize hydrodynamic lubrication at low speed shows they possess good adapatability.
Composites, especially composite laminate structures, have been applied more and more widely in aerospace industry. In order to increase the ratio of thrust to weight and improve the comprehensive performance of aircraft, optimization design must be carried out. Genetic algorithm (GA) is employed, which combines finite element method (FEM) and response surface method, to establish the reliability-based and dynamics comprehensive optimization design method of composite laminate structures in this thesis and the corresponding software code is written. The main research includes the followings:(1) Based on parametric and composite laminated structure FEM technique, the mechanical performance and modal of composite laminated structures are analyzed, the parametric program is designed.The failure function is defined and response surface–FEM is used to analyze the stability of composite laminate structures.(2) The dynamic multi-objective optimization model is obtained, and the weight, the maximum value of Tsai-Wu, the maximum displacement, the frequency of a structure are objective functions. Meanwhile, the last three variables are constraints in the optimization process. Based on the model the structure provides best performance with lighter weight. Considering the reliability, the integrated optimization model is built by regarding the reliability as constraint.(3) The dynamic optimization example is evaluated and validated by static tensile test and modal test. The results show that: the structure optimized provides better overall performance.(4) The results obtained by reliability and dynamic optimization example show that the structure comprehensively optimized provide better safety. Then the reliability and dynamics comprehensive optimization design is carried out for an aeroengine case.(5) Based on the characteristic of composite laminate structures, the encoding mode of GA is studied and a set of GA program is developed by VC++6.0. The operating system commands are called to achieve the overall optimization program design of genetic algorithm and parametric finite element.
In this work, ramie fiber was chosed as reinforcing material to prepare ramie reinforced polypropylene composites which had high cost-performance ratio and excellent mechanical properties. So that this paper can provide theoretical basis for practical production. Considering the high viscosity of PP resin, and the problems of dipping, molding, processing and costs, A new type yarn was prepared with the ramie yarn and polypropylene yarn by twisting frame. The formation of skin-core structure can make the matrix and fibers blend intensively.In this paper, 9 kinds of wrap yarns with different parameters were prepared, the cross-sectional and longitudinal section of the wrap yarns were analysized, and the mechanical properties of the wrap yarns were tested.Three different parameters of the wrap yarns(3Hz, 7Hz and 10Hz)were selected to prepare plain weave structure composite prefabricate, the mechanical properties of the wrap yarn fabrics were researched.The results showed that the structure of the yarn and subsequent weaving process were affected by the parameters of wrap yarns.In this paper,the influence factors for preparing the composite of ramie / PP wrap yarn fabric were discussed,such as temperature, pressure and dwell time, and the mechanical properties and the tensile fracture mechanisms of plastic composites were analyzed. The results showed that the mechanical properties of the composites reached the maximum when the molding temperature was 185℃, pressure was 10MPa, holding time was 10min. And the higher parameters can prepare soft covering yarn, which can improve the mechanical properties of composite materials.In this paper, the wrap yarn fabrics were processed by different treatments, such as alkali, maleic anhydride, acetylation, and A-151 treatment. The tensile properties of the composites under these different treatments were tested and the tensile fracture surface shape appearance were observed by SEM,the failure mechanism between fiber and the resin matrix was analysized. By comparison, alkali treatment and A-151 treatment can greatly improve the tensile properties of composites which could increased to 75.6 MPa and 72.8 MPa respectively.
Research on Dielectric Properties and Microstructure of Nanohybrid Polyimide Film Induced by External Field
The development of nanoscience and technology provide new ideas and ways to the exploitation of new materials and the modifying of present materials,and the appearance of nanodielectrics also open a new application field for the dielectrics. Polyimide/inorganic materials have been widely used in the electronic,electrical and other fields by its excellent heat-resistance,mechanical properties and corona-resistant performance. PI/Al2O3 material has gradually become the focus of the study at domestic and abroad.In this paper, the influence of inorganic nanoparticles on dielectric properties and breakdown characteristic of polyimide hybrid films are studied. The PI hybrid films are prepared by sol-gel and Al2O3 components are 0%, 5%, 10%, 20% and 0%, 4%, 8%, 12%, 16%, 24% respectively, （two groups samples）, which are used as the experimental films. The effect of Al2O3 components and external field on the dielectric properties and morphologies of the breakdown holes have been studied. Under the condition of high DC field, permittivity, loss tangent and breakdown voltages have been measured. The relationship of breakdown time and transmittance of film under the laser irradiation have been studied. The surface morphology and element distribution around the breakdown hole are investigated by scanning election microscope. By the above experiments, we explore the dielectric properties and the breakdown characteristics of PI hybrid films and the roles of nanoparticles in the films. The changes of electric field and the performance change of PI films by laser irradiation also have been analyzed.The experimental results show that the samples with 5%,10% and 20% components PI hybrid films have the same process on the absorption of laser. PI films absorb the laser energy and the temperature rise. When the injection power density of laser gets to the damage threshold of PI hybrid films, the film will be breakdown. By controlling of laser irradiation intensity, the nanoparticles can be induced to improve the performance of thin film under the precondition of the films without being breakdown. Under the condition of temperature 20°C, the humidity 20% and the frequency range 102-105Hz, the permittivity and loss tangent of the films with Al2O3 different components are inversely proportional to the relationship with the frequency. Under the fixed-frequency condition, for the 5% components sample, the dielectric constant has the biggest growth rate. The loss tangent has a drastic change for the 10% components sample. PI hybrid films produced a large number of Joule heat in the breakdown process, which is the process of high-temperature melting, breakdown and gradual erosion process. Accompanied the action of heat transmission, the films are damaged by thermal stress finally.
Since discovered by Iijima in 1991, CNTs had attracted great interests throughout the academic and industrial world due to its extraordinary electrical, mechanical and thermal properties, which makes them possible to possess percolation threshold at low loading in polymer matrix nanocomposites. But CNTs tend to form into aggregates for their high specific area and extremely high surface energy, which lead to heterogeneous dispersion in the polymer matrix and negative effects on the properties of the resulting composites. Therefore, great attentions were focused on the effective functionalize method on CNTs.In this paper, Multi-wall carbon nanotubes (MWNTs) were acid modified by refluxing with the mixture of H2SO4 and HNO3 under ultrasound, then were attended into the synthesis of PI/MWNTs hybrid films via in situ polymerization process. The dispersion situation of MWNTs in the nanofilms was characterized using scanning electronic microscopy (SEM); mechanical and thermal properties of nanoflims were tested, with the results indicating that the acid modified able to facilitate the homogeneous dispersion of MWNTs in the ethanol solvent, remain in fine monodispersity condition after one week; functionalized MWNTs can be dispersed homogeneously in the PI matrix with MWNTs content under 1wt%, the the addition of functionalized MWNTs increase the heat resistance of PI/MWNTs appreciably, slow down its thermal decomposition with the increased MWNTs content; as the MWNTs increased, the mechanical property has not an significantly discipline, tensile strength of nanofilms content reaches 110.05MPa at the MWNTs content of 0.36wt%, while its elongation is 31%, 15% lower than the pure PI films.
Preparation of Graphite Intercalation Compound with Acetic Acid and Triethyl Phosphate and Study on Their Structure and Inflation Properties
Intercalation reaction between graphite layers is an important way to get new graphite materials with unique properties, usually called graphite intercalation compounds (GICs). By intercalating phosphorous containing compound, a kind of GIC with high flame retardant efficiency could be prepared and used as environmental-friendly flame retardants. However, few papers have been delivered on this topic. This thesis aims at preparation of GICs intercalated with triethyl phosphate (TEP) and acetic acid (AA) by chemical oxidizing method and electrochemical method. AA-GIC, TEP-GIC and AA-TEP-GIC are prepared by direct intercalation and step-wise intercalation methods. The synthesis conditions are investigated by separately discussion of different factors and orthogonal design. The structure of GIC and its expanded product is characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Expansion volume is evaluated and several concerned factors are discussed.The following conclusions can be obtained in the thesis. Successful preparation of GICs is proved by XRD. Compared with graphite, GIC shows diffraction peaks at lower angle, meaning large interlayer distance in GICs. AA-TEP-GIC prepared by step-wise chemical oxidizing method has larger interlayer distance. For chemical oxidizing method, the dosages of oxidizing agent and intercalating agent exert profound influences on expansion volume while voltage and reaction time are of importance for electrochemical method. By controlling the voltage, time and concentration, GIC with larger expansion volume can be prepared. AA-TEP-GIC step-wisely prepared has larger expansion volume than AA-GIC and TEP-GIC directly prepared. As proved by SEM, expanded GIC by electrochemical method presents larger and more homogenous hole structure. Expansion volume of GIC linearly increases with weight gain during reaction and weight loss during expansion. GIC as prepared has good expansion properties at low temperature according to the results of expansion volume at different temperature.
In this paper, TiO2 nanotube was prepared by hydrothermal method with TiO2 powder and KOH solution. The effects of reaction temperature and time were studied on the structure and morphology of the samples by characterizing with powder X-ray diffraction （XRD） and transmission electron microscopy （TEM）. And the formation mechanism and conditions of TiO2 nanotubes were investigated. Nitride calcination was used to treat TiO2 nanotubes, finaly the N-doped TiO2 nanotube was obtained successfully. The N doping mechanism and optical absorption were investigated by X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS）, infrared spectrophotometry and UV-Vis absorption spectroscopy. At last, N-doped TiO2 nanotubes was applied for quick determination of Chemical Oxidation Demard （COD）.Comparing samples prepared at different temperature for same time, it was found that the anatase was obtained easily at low temperature and the rutile was obtained easily at high temperature; nanotubes could not be formed completely at too low temperature, while the structure of nanotubes would be broken up at too high temperature; with increasing temperature, the TiO2 nanotube grew wider. The best temperature rang was from 130℃to 160℃. When the reaction temperature was increased to 190℃, the sample composed was burnt out and was not tubular structure anymore. A very long time of hydrothermal reaction, the nanotube may change to nanorod.A general formation mechanism of TiO2 nanotubes was proposed in terms of the detailed observation of the products via characterizing the microstructure and morphology of samples. The overall formation can be summarized with three successive steps: 1. Layeded TiO2 formation; 2. Formation of nanotube via trititanate splitting and multilayer scrolling process of nano-planes around the c axis. 3. Lengthing process.N-doped TiO2 nanotube with different N doption were synthesized by varying the ratio of the amount TiO2 nanotube to that of ammonia. The results showed that the scrytal coefficient was different with standard data. So it was proposed that the variation was caused by N doption. Moreover, the replacement of N to O in TiO2 lattice resulted in narrowing the energy gap, and increasing shine optical absorption.As a type of high efficient photocatalyst, TiO2 nanotube was applied to determine COD of waste water. When COD range from 3 to 10, the COD was in a linear relation with absorbance change. The limit was 0.1mg/L.
Polypropylene (PP) has superior dielectric properties in high temperature and chemical corrosion-resistant properties, and it is extensively used to power capacitors insulation materials. But isotactic polypropylene (i-PP) easyily electrical aging, so that the application in the field of high-voltage insulation is limited. As the nano-particles have special properties,a small amount of nano-filler can change the property of polypropylene.In this paper, montmorillonite is used to modify polypropylene, and relationships between crystalline morphology and properties are studied. Isotactic polypropylene (i-PP) is used as the base material, and montmorillonite modified by octadesylamine as the nanofiller, and maleic anhydride grafted polypropylene (PP-g-MAH) as the compatibilizer, on this basis, PP/MMT nanocomposite is prepared by melting intercalation process. Effects of different contents of compatibilizer and montmorillonite and temperatures of melting intercalation on the crystalline morphology, space charge and electrical properties of polypropylene are researched. At the same time relations between crystalline morphology, space charge and dielectric properties are analyzed.In this article, crystalline morphologies of PP and PP/MMT nanocomposites are observed by PLM. The results indicate that MMT plays the role in heterogeneous nucleation. When melting intercalation temperature reaches 190℃and the content of compatibilizer is 3%, PP/MMT has the best crystalline morphology. Space charges of PP and PP/MMT nanocomposites are measured by PEA, and the results show that trap levels are changed and that deep traps are filled after adding the montmorillonite into the polypropylene. Studies of electrical properties show that the crystalline morphology has a great influence on it. It is concluded that there is the highest insulation resistivity and the lowest dielectric loss when melting intercalation’s temperature reaches 190℃and the content of compatibilizer is 3%, by comparing the different composites.
PTC polymer composites were widely used as self-limiting heating cables, current protected devices and sensors. In which, self-limiting heating cable played an important part in petroleum, chemical, medical, electronics for the temperature self-controlled, energy saving, low raw material cost and high security. However, some problems could not be ignored, such as the poor dispersion of CB, low stability of PTC effect and high start current which limited its further applications. Based on practical application, The effect of technology on the performance of the PTC polymer composites was studied beginning with dispersibility of CB in polyethylene.Based on statistical analysis of surface resistance, mechanical properties testing, analysis of micro-morphology and PTC effect testing. The dispersibility of CB, mechanical properties, micro-morphology and PTC effect of the composites filled with four kinds of CB that made in Yuanping, Beijng, Jiaozuo, Nanping were studied. The results showed that Jiaozuo CB had the best dispersibility and the Beijing CB had the highest PTC performance.The self-made pyrophsphate titanate, rare earth titanium, KH102, KH201, KH105, KH550 and stearic acid were used to treat CB, the surface resistance, mechanical properties, micro-morphology, rheological property and PTC effect of the samples were studied. The results showed that the comprehensive properties of the composites were improved by the coupling treatment, self-made pyrophsphate titanate had remarkable effect on improving mechanical properties and CB dispersibility, the rare earth titanate reduced the composite torque and enhanced the PTC intensity.The temperature, time, speed of the internal mixing technology were studied by uniform design and the optimizing technology was obtained by regression analysis, which was 30rpm,13.257min,175℃The starting current of PTC polymer composites was studied. The results showed that the starting current was decreased after coupling treatment, radiation crosslinking had an remarkable effect on decreasing the starting current of composites especially treated by rare earth titanate.
In recent years, with the rapid development of EHVDC transmission in domestic, the requirement of power system to EHVDC transmission cable has become increasingly high. The space charge which easily formed in the DC electric field can make the distribution of the electric field distort, make the polymer aging and even cause insulation breakdown. So, if the polymer insulation power cables shall meet the requirement of EHVDC transmission, it must restrain the concentration effect of space charge in dc field. According to reports, a small amount of nano-MgO added in PE can effectively reduce the effects of space charge and can satisfy the requirement of EHVDC transmission. But there is very few domestic study in this respect. In this paper, the synthesis technology and calcination technology of magnesium hydroxide were studied. Meanwhile, the surface treatment of nano-MgO by coupling agent and the preparation technology of MgO/PE composites were studied, and the test and analysis of mechanical properties and space charge distribution of MgO/PE composites is carried out.First, the Mg（OH）2 was synthesized using the homogeneous precipitation method with the Mg（NO3）2 as raw material and the CO（NH2）2 as precipitating agent. The optimum condition of synthesis of Mg（OH）2 was determined.The partical diameter and aggregation state of the nano-MgO were measured using the XRD and SEM. The effects of the calcining technology and the dosage of the dispersant PEG on the partical diameter and aggregation state of the nano-MgO were discussed. The optimal calcining technology of Mg（OH）2 and the dosage of the PEG were determined. The average size 50 nm and well dispersed nano-MgO was prepared under the optimal calcining technology and the dosage of the PEG.Secondly, the surface of nano-MgO was teated using the titanate coupling agent KH-105. The effects of the coupling agent dosage, treatment temperature and treatment time on the activation index of nano-MgO were discussed. This paper also tried to determine the optimal coupling agent dosage, treatment temperature and treatment time of the nano-MgO surface treatment. The results showed that the aggregation state was significantly improved after the treatment using the KH-105.Lastly, The MgO/LDPE composites were prepared using the direct method and masterbatch method. The mechanical properties and space charge distribution of MgO/LDPE composites were analyzed by the tensile test and PEA. The effect of the nano-MgO content on the mechanical properties and space charge distribution of MgO/LDPE composites were discussed. The results showed that the mechanical property of the MgO/PE composite was the best when the adding amount of the nano-MgO is 1wt%. It could reduce the space charge concentration of the MgO/PE composites effectively when the adding amount of nano-MgO was 0.5wt%2wt%. When the adding amount of nano-MgO was 2wt% it could achieve the best effect.