In recent years, multiferroics was one of the most popular materials and were widely studied by many scientists. Among all we interested the most is BiFeO3, which exist a room temperature ferromagnetic and ferroelectric properties with high ferromagnetic transition temperature Tc that can provide various kind of applications. However, the drawback of the BiFeO3 is difficult to synthesize the pure phase and to eliminate the leakage of the current. According to others¡¦ reports, with a proper doping can reduce the evaporation of Bi atoms and stabilize the crystal structure. Therefore, we choose Pb as the mixed elements and wish to reduce the unstable oxygen vacancies around the Bi atoms and to increase the dielectric property of Bi1-xPbxFeO3. In our experiment, SrRuO3 is chosen as the buffer layer material, which can grow as a strain relaxed film on the substrate. It is found that the strain relaxation transforms the SrRuO3 crystal structure to a nearly cubic one which has a lattice matching with Bi1-xPbxFeO3. As a result of this, we might improve the leakage problems of Bi1-xPbxFeO3.

In recent years multiferroic materials have great application potential in the ferroelectric random access memory and emerging spintronics development setting off the boom of the multiferroic materials research. It was assume that the ferroelectric and magnetic properties cannot coexist at the same temperature range or their coupling is weak making the searching for multiferroics in dream. Multiferroic materials have been discovered in recent years. Multiferroics may even be induced due to the strong coupling between artificial layers by growing multilayer technique. [1] [2] Among of which the BiFeO3 compound contains of antiferromagnetic and ferroelectric properties at room temperature. The BiFeO3 thin films manifests a larger spontaneous porlarization than that of the bulk and is highly capable of industrial application. In this study, Pb doped BFO (BPFO) this films were grown on top the conductive SrRuO3 layers. By varying BPFO¡¦s thickness, the strain effect the relative physical properties were studied. It is found that the lattice constants of SRO is totally different to that of STO substrate indicating strain relaxation. Similar phenomena are also discovered for BPFO thin film which is believed due to the tilting effect on the SRO/STO interface.

The strain effect on La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 thin films on SrTiO3 (001) substrate with different thicknesses has been studied by X-ray absorption near edge spectroscopy (XANES), which can reveal the details of the coupling between cations and anions. The strain may suppress the TC of LCMO films while enhancing that of LBMO films. The theoretical calculation results suggest the unoccupied states of the third structure of XANES are formed by much more complex hybridization of O 2p to Mn 4sp, La 6s and (Ca 4sp or Ba 6sp) orbitals. The change of the absorption intensity of the second and third structures is compatible to the TC change of both films due to the strain effect. This strongly suggests that the strain effect on LCMO and LBMO thin films is mainly associated with the bonding situation between O and La(Ca or Ba) ions.

This thesis research synthesizes a new class of composite materials and investigates their properties, performance, and potential applications. The new materials that are multi-phase and multifunctional are considered for use in car cooling systems, internal combustion engine waste-heat-power generators, and engine crack healing which are major problems plaguing the auto industry. This research uses primarily experimental approaches to study the magnetocaloric compound, Ge5Si2Ge2 (GSG), that has large strain effects. Such a material was formed into a composite using Al as a substrate. The newly developed composite, GSG-Al, is the first material of its kind that possesses self-healing effects in cracks. X-ray diffraction was used to determine the crystal structures that existed within the material. It is found that the sintering process used to create the composite caused the formation of GdAlGe that is a magnetic compound with a high Curie temperature. The GSG-Al has a wide variety of crystal structures, ranging from face centered cubic for aluminum phases to monoclinic and orthorhombic phases for GSG. The discovery of GdAlGe confirmed that alpha-ThSi-type tetragonal and YAlGe-type orthorhombic crystal structures existed. Transmission electron microscopy (TEM) was used to analyze the wear debris collected during tribo-testing. The debris were also analyzed using energy-dispersive X-ray spectroscopy (EDS) for chemical analysis. The GSG-Al was put through tribological studies at several different temperatures to determine the thermal effects on the composite. The GSG-Al, although found to be ductile, showed high resistance to wear when compared to a common aluminum alloy, Al 6061-T651. The wear rate decreased with increasing temperature when the temperature was increased from the room temperature to 150 degrees C. Results showed that with GSG, the composite did not show cracking common in Al alloys. This was due to the unique thermal expansion properties of the GSG-Al. The phase transformation induced a significant volume expansion in the material and thus a giant strain effect. This research opens new approaches in energy conversion and improving efficiency of automobile engines. The composite developed here is important for future scientific investigation in the area of multifunctional materials as well as materials that exhibit self-healing tendencies.

Yes / This paper studied the effect of different types of multi-walled carbon nanotubes (MWCNTs) on the dynamic mechanical properties of cementitious composites. Impact compression test was conducted on various specimens to obtain the dynamic stress-strain curves and dynamic compressive strength as well as deformation of cementitious composites. The dynamic impact toughness and impact dissipation energy were, then, estimated. Furthermore, the microscopic morphology of cementitious composites was identified by using the scanning electron microscope to show the reinforcing mechanisms of MWCNTs on cementitious composites. Experimental results show that all types of MWCNTs can increase the dynamic compressive strength and ultimate strain of the composite, but the dynamic peak strain of the composite presents deviations with the MWCNT incorporation. The composite with thick-short MWCNTs has a 100.8% increase in the impact toughness, and the composite with thin-long MWCNTs presents an increased dissipation energy up to 93.8%. MWCNTs with special structure or coating treatment have higher reinforcing effect to strength of the composite against untreated MWCNTs. The modifying mechanisms of MWCNTs on cementitious composite are mainly attributed to their nucleation and bridging effects, which prevent the micro-crack generation and delay the macro-crack propagation through increasing the energy consumption.

The effect of strain (stress) on the below threshold output of InGaAsP diode lasers has been investigated theoretically and experimentally. The degree of polarization (DOP) and the polarization- resolved spectral output (PRSO) were obtained as a function of the external stress applied to the device. A correlation between the DOP and the peak of the PRSO as a function of the stress was found. This correlation suggests that below threshold, DOP can be used to measure the strain in the active region of lasers. A model based on a strain modified Shockley matrix for the band calculation and a strain modified dipole moment for the optical emission has been constructed to bridge the correlation between the DOP and PRSO. / Thesis / Master of Engineering (ME)

In this thesis, we have performed systematical study of anomalous electric and magnetic behaviors of the 3d transition metal oxides; colossal magnetoresistance (La1−xRxMnO3 where R is a divalent alkaline earth ion) and Multiferroic (Ho1-xLaxMn2O5) systems by X-ray and Neutron scattering techniques. In our study, the enhancement of the transfer temperature for La0.8Ba0.2MnO3 under strain effect from the SrTiO3 substrate could be possible due to two reasons which one is Sr diffusion from SrTiO3 substructure, and other one is the octahedral MnO6 high symmetry are increasing. We focus the intrinsic strain effect on La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 films, and findings show that due to the different ionic sizes of doped Ca or Ba ions, the strain effect acts differently in the way it deforms. The interfacial strain effect produces opposite influences on the lattice symmetry, the average Mn¡VO bond lengths, the average oxygen disorders, the coupling symmetries inside and in the vicinity of the MnO6 octahedrons, as well as producing an opposing trend in metal-insulator and magnetic transition temperatures of the strained La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 films. The strain effects on the electronic structures of La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 thin films have been studied by O K-edge x-ray absorption near edge structure (XANES) spectroscopy. For La0.67Ca0.33MnO3, the first-principles calculations reveal that the features in the XANES spectra are associated with hybridized states between O 2p and Mn minority-spin 3d t2g and eg, La 5d/Ca 3d, and Mn 4s/Ca 4p states. An analysis of these features shows that the tensile strain decreases substantially La¡VO and Ca¡VO hybridization and TC for La0.67Ca0.33MnO3. For La0.8Ba0.2MnO3, the small compressive strain enhances slightly La¡VO and Ba¡VO hybridization and TC. In this thesis, the influence of the local structure distortion on the magnetic transition in La doped HoMn2O5 Multiferroics has been investigated systematically. The orthorhombic crystal structure of Ho1−xLaxMn2O5 is maintained up to x¡Ø0.2 but decomposed into multiphase for x¡Ù0.25. By doping La ions to a concentration of 0.1¡Øx¡Ø0.2, the formation of the RMnO3 1(13) phase can be suppressed and single-phase Ho1−xLaxMn2O5 (0.1¡Øx¡Ø0.2) compounds can be formed under 1 atm flowing oxygen. For x=0.2, a ferromagnetic FM transition at 150 K is superimposed on the paramagnetic background, which implies that the compound undergoes a ferromagnetic to antiferromagnetic (AFM) transition. This unique FM to AFM transition is observed for the first time. The FM transition is attributed to the formation of magnetic clusters in a host paramagnetic matrix. The anomalous magnetic clusters phenomena observed in Ho0.8La0.2Mn2O5 can be directly attributed to the different properties between Ho and La ions, and the differences of Ho and La ions are not only in the ionic radius but also in the electron negativity. During 90~150K, X-ray scattering diffraction presented the new addition peaks indicates the new electric density distribution, and the Neutron powder scattering diffraction (NPD) refining results show that the local structure of R-O (R: La, Ho) is un-symmetry which is conflict to the La Extended X-ray absorption fine structure (EXAFS) (which shows that the local structure of La-O becomes more symmetry than H-O. Since the refining values of the NPD are an average of entire crystal, such that it cannot tell the local changes. X-ray absorption spectrum (XAS) and EXAFS, in contrarily, can provide the local information. They implies that the temperature evolutions of the coupling strength with O 2p or unoccupied density state are opposite for the Ho and La ions in our Ho0.8La0.2Mn2O5 sample. Therefore, local change of ions position and charge redistribution happens in this specific temperature range.