This work was carried out to study the effect of shot-peening on the fatigue behaviour of carbon steels. Differently heat treated medium and high carbon steel specimens were selected. Medium carbon steels, AISI 1141 and AISI 1151, were respectively air cooled and quenched-tempered. A high carbon steel, C70S6 (AISI 1070), was air cooled. The other material was a powder metal (0.5% C) steel. Each group of steels was divided into two. One was shot-peened. The other half remained in their original conditions. All were fatigue tested under fully reversed (R=-1) tension-compression loading conditions. Microhardness tests were carried out on both the grip and gage sections of selected non shot-peened and shot-peened specimens to determine the hardness profile and effect of cycling. Shot-peening was found to be deeper on one side of each specimen. Compressive residual stress profiles and surface roughness measurements were provided. Shot-peening increased the surface roughness from 0.26±0.03µm to 3.60±0.44µm. Compressive residual stresses induced by shot-peening reached a maximum of -463.9MPa at a depth of 0.1mm.The fatigue limit (N≈106 cycles) and microhardness profiles of the non shot-peened and shot-peened specimens were compared to determine the material behaviour changes after shot-peening and cycling. Also their fatigue properties were related to the manufacturing process including heat and surface treatments. Comparing the grip and gage microhardness profiles of each steel showed that neither cyclic softening nor hardening occurred in the non shot-peened condition. Cyclic softening was apparent in the shot-peened regions of all steels except powder metal (PM) steel. The amount of softening in the shot-peened region was 55.0% on the left side and 73.0% on the right in the AISI 1141 steel , 46.0% on the left side and 55.0% on the right in the C70S6AC steel and 31.0% on the right side in AISI 1151QT steel. Softening was accompanied by a decrease in the depth of surface hardness. It is suggested that although the beneficial effects of shot peening, compressive residual stresses and work hardening, were offset by surface roughness, crack initiation was more likely to occur below the surface. Surface roughness was not a significant factor in controlling the fatigue lives of AISI 1141AC and C70S6 steels, since they were essentially the same for the non shot-peened and shot-peened conditions. Shot-peening had very little effect on the push-pull fatigue limit of C70S6 steel (-2.1%), and its effect on AISI 1141AC steel was relatively small (6.0%). However, the influence of shot-peening on the AISI 1151QT and PM steels was more apparent. The fatigue limit of the PM steel increased 14.0% whereas the fatigue limit of the AISI 1151QT steel decreased 11.0% on shot peening.

Polymer light emitting diodes (PLED) were using a heterocyclic aromatic rigid-rod polymer poly-p-phenylene-benzobisoxazole (PBO) as an opto-electronically active layer; and poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) as a hole transporting layer. Aluminum (Al) and indium tin oxide (ITO) were served as device cathode and anode, respectively. [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) or derivatized multi-wall carbon nano-tube (MWCNT-C18), with great electron transporting ability, was doped into PBO to enhance the performance of PLED devices as well as the thin-film electrical conductivity. The optical length was changed by using different spin coating speeds and durations. From the research, the £fmax of electroluminescence (EL) was blue-shifted as PEDOT:PSS spin coating speed increased for a thinner layer. Once using a higher spin coating speed repeatedly to coat PEDOT:PSS, the £fmax of electroluminescence was red-shifted. If the PEDOT:PSS film thicknesses were similar, the EL spectra were almost the same, independent of device processing scheme. The injection current and EL intensity were enhanced by doping PC61BM or MWCNT- C18. The electric conductivity parallel to film surface (£m¡ü) was increased as the doping concentration increased. Because of the extremely different aspect ratio, the MWCNT-C18 had a lower percolation threshold concentration. Therefore, at a low MWCNT-C18 doping concentration, the injection current and the EL intensity were enhanced compared with those of PC61BM.

In this study, two series of polymers based on benzobisthiazole were synthesized. The poly(benzobisthiazoles) (PBTs) have been synthesized by the solution polycondensation of 2,5-diamino-1,4-benzenedithiol in poly(phosphoric acid)s (PPA). The diacids used were systematically varied to find the best for the solubilization of the aromatic heterocyclic rigid-rod polymers. The role of PPA is identified and the effects of phosphorous pentoxide and water on PBT during polycondensation are discussed. Polymer properties such as the inherent viscosity, decomposition temperature are correlated to systematically varied diacids. Finally, the effect of diacid architecture on the synthesis and microstructure of PBT is studied. The results are further discussed in terms of resonance, symmetry, and solubilization of the diacids. Next, we extend the rigidity and resonance of benzobisthiazole for the application as second-order nonlinear optics. Novel nonlinear optical (NLO) polyimides containing benzobisthiazole chromophores have been synthesized. The soluble polyimides containing different ratios of carboxylic acids (COOH) were first prepared and the precursors of NLO chromophores reacted with those carboxylic acids, followed by the benzobisthiazole derived chromophores synthesized at 300 oC under vaccum. The formation of benzobisthiazole was evidenced by FTIR and UV-vis spectra in combination with the analysis of model polyimides. The excellent thermal properties of those NLO polyimides were examined by TGA and TMA. PI-1 shows thermal decomposition temperature as high as 554 oC at 10 wt % loss and a Tg of 324 oC. The amorphous morphology of those polyimides was verified by XRD traces and some ordered alignments were found, due to the rigidity of the benzobisthiazole derivatize chromophores. The electrooptic coefficient of PI-1 (r33 = 5.3 pm/V) was obtained.

The focus of this study is mono-layer polymer light emitting diode (PLED). The emitting layer is poly-p-phenylenebenzobisoxazole (PBO). PBO is a fully conjugated heterocyclic aromatic rigid-rod polymer. Anode is indium-tin-oxide (ITO). Cathode is aluminum (Al). We used UV epoxy resin to package PLED devices, then measured current-voltage response, electroluminescence (EL) emission, and device lifetime. We demonstrate that the packaged mono-layer PBO LED reduced its demise from water and oxygen. Device lifetime increased from 1 hour to several hundred hours. At a larger bias voltage or current, emission intensity and device efficiency became higher. But decay rate increased leading to shortened device lifetime. Device temperature appeared linearly with current density. A red shift of the EL emission was observed. The £fmax. of emission spectra moved from 534 nm (initial) to 582 nm (after 100 hrs). After thermal annealing at 120¢J for ten hours, threshold voltage increased from 5 V to 12 V, current density decreased to several 10 mA/cm2, luminous intensity improved several ten times to 10-2 cd/m2, emission color changed from yellow-green to orange, luminous efficiency improved from 10-7 to 10-4 cd/A, but device lifetime declined to less than 20 hrs.

Polycondensation reaction was carried out for synthesizing rigid-rod polymer hPBI. Various molar ratios (50:1, 25:1, and 15:1) of 2-hydroterephthalic acid and 5-hydroisophthalic acid were also introduced in the synthesis for articulated rigid-rod polymer a-hPBI. The polymers were further derivatized with 1,3-propanesulton for pendants of lithium ionomer to become water soluble polyelectrolytes hPBI-PS(Li+) and a-hPBI-PS(Li+), respectively. Lithium salt doped cast film of the rigid-rod polyelectrolyte hPBI-PS(Li+) showed a room-temperature DC conductivity parallel to film surface as high as 4.02¡Ñ10-3 S/cm. Molecular weight of the rigid-rod polyelectrolyte was low indicating a small molecular aspect ratio. In cast film, the molecules were randomly distributed and highly isotropic facilitated Li cations mobility for a high film conductivity. The conductivity was also insensitive to the anion of lithium salt. No apparent layered structure was revealed by scanning electron microscope suggesting that the cast films had near three-dimensionally isotropic structure and conductivity.

Polymer photovoltaic cell (PV cell) utilizes a polymer to absorb photons for generating excitons. When excitons are separated into electrons and holes, the device has the photovoltaic effect. Polymer light emitting diode (PLED) injects electrons and holes respectively from cathode and anode into a polymer emission layer. Some of the electrons and the holes would recombine to induce light emission. This research used a heterocyclic aromatic rigid-rod polymer poly-p-phenylene- benzobisoxazole (PBO) as the opto-electronic layer, and a conducting material of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) as the hole transport layer. PV cells were fabricated using indium-tin-oxide (ITO) as anode and aluminium as cathode. Same layer arrangement was applied for PLEDs. These two kinds of devices were measured for electrical and optical response. It was evidenced that the addition of PEDOT:PSS layer facilitated the separation of excitons into electrons and holes at the PBO/PEDOT:PSS interface. Insertion of a LiF layer between PBO layer and Al cathode reduced their energy band gap and facilitated charge transport leading to an enhanced efficiency for PV cells and PLEDs. Thickness variations were found on spun PBO layer. According to emission intensity, we knew that the PBO layer quality was significant for electroluminescence. Introduction of a PEDOT:PSS layer improved the interface between ITO and PBO. The thickness of PEDOT:PSS layer depended on the ITO surface roughness. With a PEDOT:PSS layer, the opto-electronic efficiency of PV cell and PLED was improved.

Poly-p-phenylenebenzobisoxazole (PBO) and carbon nanotube (CNT) contain fully conjugated rod like backbone entailing excellent mechanical properties, thermo -oxidative stability and solvent resistance. Rigid-rod PBO is commonly processed by dissolving in methanesulfonic acid or Lewis acid. A CNT of multi-wall carbon nanotube (MWNT) was dissolved in a Lewis acid solution of PBO for dispersion, and then spun for thin film. MWNT concentration in the films was from zero up to 5 wt. %. Compared to that of pure PBO film, all PBO/MWNT composite films retained same but enhanced UV-Vis absorption peaks, according to MWNT concentration, showing that PBO and MWNT did not have overlapping electron orbitals affecting their energy gaps. The composite films were excited at 325 nm using a He-Cd laser for photoluminescence (PL) emission. All PL spectra had maximum intensity at 540 nm indicative of yellow-green light emission. The composite films were fabricated as light emitting diodes using indium-tin-oxide/glass as substrate and anode, as well as vacuum evaporated Al as cathode for respectively hole and electron injectors. In these light emitting devices, MWNT doped PBO would decrease threshold voltage for about 2 V. Up to 0.1 wt. % of MWNT, the device emission current was increased two orders of magnitude than those of the devices without MWNT. Further increase of MWNT caused a successive decrease in electroluminescence emission intensity attributed to a quench effect from aggregations of MWNTs. UV epoxy resin was applied to package the mono-layer and bilayer PBO light emitting devices. The UV epoxy resin had some gas release during encapsulation. The devices were packaged with vacuum and without vacuum encapsulation. It was demonstrated that the device encapsulation reduced its demise from water and oxygen. The vacuum encapsulation could remove gaseous volatile of the device to inhibit oxygen and moisture to prolong device lifetime. The main degradation of light emitting device was the oxidization of cathode. The interactions between nitrogen of PBO and H2O caused the formation of hydrogen bonding at room temperature. Oxygen and moisture diffused into PBO polymer and were suspected to form mid-gap state for the polymer. The mid energy band disappeared upon heat treatment before encapsulation. A device under a higher bias voltage was found to have a shorter lifetime, but a larger EL emission intensity. The EL emission intensity was not a constant under a constant current bias. The vacuum encapsulated device had two or twenty times lifetime than, respectively, the device encapsulation without vacuum evacuation or in ambient conditions. The sandwich structure of ITO/PBO/Al had no observable photovoltaic effect due to insufficient exciton separation into electrons and holes. Poly(2,3-dihydro thieno-1,4-dioxin):polystyrenesulfonate (PEDOT:PSS), a hole transferring medium, was spun into a thin-film between PBO and indium-tin-oxide to facilitate photovoltaic (PV) effect by forming a donor-acceptor interlayer to separate and to transport photoinduced charges. Optimum PBO thickness for the PV heterojunctions was about 71 nm at which the hole transferring PEDOT:PSS generated the maximum short circuit current (Isc) at a thickness of 115 nm. By using a layer of lithium fluoride (LiF) as an electron transferring layer adhering to Al cathode, the most open circuit voltage (Voc) and the maximum short circuit current (Isc) were achieved with a LiF thickness of 1-2 nm due to possible electric dipole effect leading to an increase of Voc from 0.7 V to 0.92 V and of Isc from about 0.1

Poly(p-phenylenebenzobisimidazole), PBI, is a rigid-rod polymer with a fully conjugated backbone having superior mechanical properties, thermo-oxi- dative and solvent stabilities. The stabilities cause processing difficulties and in terms limit its applications in critical technologies, such as conducting polymers, nonlinear optics, and solid polyelectrolytes. In this study, a chemical derivative of PBI, poly[1,7-dihydrobenzo[1,2- d:4,5-d¡¦]diimidazo-2,6-diyl[2-(2-sulfo)-p-phenylene]], sPBI, was synthesized by polycondensation reaction of 1,2,4,5-tetraaminobenzene tetrahydrochloride with 2-sulfoterephthalic acid in poly(phosphoric acid). Isolated sPBI was measured in 30oC methanesulfonic acid for an intrinsic viscosity as high as 10.5 dL/g. sPBI polymer was then reacted with 1,3-propanesultone in dimethylsulfoxide containing sodium hydride for water-soluble rigid-rod polyelectrolyte, poly[1,7- dipropylsulfobenzo-[1,2-d:4,5-d¡¦]diimidazo-2,6-diyl-[2,(2-sulfo)-p-phenylene]], sPBI-PS(Na+). sPBI-PS(Na+) was further converted to sPBI-PS(Li+) with hydrochloride and followed with lithium hydroxide. Various analyses were applied to ascertain chemical structure, purities and thermal properties of synthesized monomers and polymers. sPBI-PS(Li+) aqueous solutions were doped individually with lithium salts of LiI, LiBF4, and LiCF3SO3 at concentrations up to 1.7¡Ñ10-5 wt./wt., which were cast into freestanding films of 10-25 £gm in thickness. Direct-current conductivity measured at room- temperature parallel to the film surface was as large as 9.74¡Ñ10-5 S/cm. The ionic nature of the conductivity was revealed by constant-voltage depletion measurements. X-ray scattering results suggested that the cast film was in-plane isotropic but out-of-the plane anisotropic with the rigid-rod backbone lying in the plane of the film.

A heterocyclic aromatic rigid-rod polymer poly-p-phenylene-benzobisoxazole (PBO) was applied as the opto-electronic layer¡Fand a conducting material of poly(3,4-ethylenedioxythio-phene):poly(4-styrenesulfonic acid) (PEDOT: PSS) was used as the hole transport layer. Aluminum (Al) and indium tin oxide (ITO) were served as device cathode and anode¡Arespectively, fabricated into a bi-layer structure of ITO/PEDOT:PSS/PBO/Al for electrical and luminescence responses. This research demonstrated an increase of current density and a decrease of threshold voltage with a decrease of PBO layer thickness from 90 nm to 27 nm to facilitate electron tunneling and electron-hole recombination. With a lower spin coating speed, polymer chain would aggregate and inter-penetrate resulted in red-shift of electroluminescence (EL) emission spectrum. Furthermore, micro-cavity effect might influence EL spectrum by varying layer thickness. Modulation of PBO layer thickness led to tunable EL emission color. It was also demonstrated that an increase of current density and a slightly decrease of threshold voltage with a PEDOT:PSS film thickness changing from 96 nm to 17 nm at a constant PBO layer thickness of 90 nm. Micro-cavity effect thus influenced EL emission for a tunable emission color. Photolithography was applied to obtain ITO substrate of grating depth of periodic variation and then coated with a PEDOT:PSS leading to a grated PEDOT:PSS layer of periodic thickness. This led to ITO/PEDOT:PSS/PBO/Al device showing broadened EL emission spectra.

Thesis (Ph. D.)--University of Oklahoma. / Bibliography: leaves 120-130.