Joanna McKittrick

Fluorescence properties of polycrystalline Tm3+-activated Y3Al5O12 and Tm3+-Li+ co-activated Y3Al5O12 in the visible and near IR ranges

Abstract The photoemission properties of polycrystalline powder Tm 3+ -activated Y 3 Al 5 O 12 and Tm 3+ - Li + co-activated Y 3 Al 5 O 12 were studied in the visible and near-IR ranges at 300 K. The polycrystalline materials were obtained through a novel combustion synthesis technique that yields chemically homogeneous and small particle size( 3+ -activated Y 3 Al 5 O 12 showed that the intensity of the blue emission is weak compared to the red emission. With the addition of Li + as a coactivator, the intensity of the blue emission increased by as much as 87%; however the red and IR emissions also increased. Efficiency measurements showed that the phosphor reached a maximum of 0.21 Im/W at a voltage of 11.7 kV and current of l μA/cm 2 . The optimal composition for maximum blue emission was found to be Y 2.93 Tm 0.07 Al 5 O 12 doped with 1.0 at% Li.

The influence of processing parameters on luminescent oxides produced by combustion synthesis

Rare-earth activated oxide phosphors have application in high energy photoluminescent (plasma panels) and cathodoluminescent (field emission devices) flat panel displays. These phosphors are composed of a highly insulating host lattice with fluorescence arising from the 3d! 3d, 5d! 4f or 4f! 4f transitions in transition metal or rare earth ions. Fabrication of complex host compositions Y 2SiO5 ,Y 3Al5O12, Y2O3, and BaMgAl10O27 along with controlled amounts of the activators (Cr 31 ,M n 21 ,C e 31 ,E u 21 ,E u 31 ,T b 31 ,T m 31 ) represent a challenge to the materials synthesis community. High purity, compositionally uniform, single phase, small and uniform particle size powders are required for high resolution and high luminous efficiency in the new flat panel display developments. This paper will review the synthesis techniques and present physical and luminescent data on the resulting materials. q 1999 Published by Elsevier Science B.V. All rights reserved.

Structure and mechanical properties of crab exoskeletons

The structure and mechanical properties of the exoskeleton (cuticle) of the sheep crab (Loxorhynchus grandis) were investigated. The crab exoskeleton is a natural composite consisting of highly mineralized chitin-protein fibers arranged in a twisted plywood or Bouligand pattern. There is a high density of pore canal tubules in the direction normal to the surface. These tubules have a dual function: to transport ions and nutrition and to stitch the structure together. Tensile tests in the longitudinal and normal to the surface directions were carried out on wet and dry specimens. Samples tested in the longitudinal direction showed a convex shape and no evidence of permanent deformation prior to failure, whereas samples tested in the normal orientation exhibited a concave shape. The results show that the composite is anisotropic in mechanical properties. Microindentation was performed to measure the hardness through the thickness. It was found that the exocuticle (outer layer) is two times harder than the endocuticle (inner layer). Fracture surfaces after testing were observed using scanning electron microscopy; the fracture mechanism is discussed.

Synthesis and optelectronic characterization of gallium doped zinc oxide transparent electrodes

Abstract In this work we report on pulsed laser deposition of gallium-doped zinc oxide (ZnO:Ga) transparent-conducting thin films grown on glass at different substrate temperatures. A widening in the optical bandgap and a good gallium-doping efficiency were observed in the films when the substrate temperature was raised from 150 °C to 300 °C, as determined from optical and electrical measurements. X-ray diffraction measurements revealed that the films grow preferentially oriented in the [002] crystallographic direction of the ZnO grains. The crystallinity of the films was also found to be strongly dependent on the substrate deposition temperature. The ZnO:Ga transparent films had excellent transmittance (85%) in the visible spectrum and a low electrical resistivity value (7 × 10 −4 Ω cm) in 200 nm thickness samples deposited on glass by laser ablation at 300 °C.

Rapid solidification processing

It has been over eight years since the subject of rapid solidification processing (RSP) has been given a general review. The early reviews of RSP were undertaken before the very rapid expansion of the late 1970s and early 1980s and the most recent general review was published in 1984, at approximately the height of RSP activity in the USA. More recent specialized reviews have been prepared for magnesium, titanium and aluminium alloys. The expansion of research and application testing resulted from the first major attempts to employ RSP on a large scale under both industrial and government sponsorship. The objective here is to develop the historical background of RSP, and then to update the present theoretical base for this technology. Examples of recent work on the rapid solidification of metals and ceramics will be given, along with examples of applications. Finally, an assessment of the future directions and prospects for RSP will be presented.

Physical properties of Y2O3:Eu luminescent films grown by MOCVD and laser ablation

Abstract Luminescent Y2O3:Eu3+ thin films were deposited on sapphire, polycrystalline Al2O3 and indium tin oxide coated glass or sapphire substrates by two different techniques: metallorganic chemical vapor deposition (MOCVD) and laser ablation. Microcrystalline Y2O3:Eu films were grown in a MOCVD chamber by decomposing and reacting yttrium and europium organometallic precursors in an oxygen atmosphere at low pressures (1–10 mTorr) and low substrate temperatures (500–700°C). The as-deposited films showed the characteristic red fluorescence spectrum of Y2O3:Eu with the main peak centered about 611 nm wavelength. The as-deposited films averaged 1.0 μm in particle size and 2.0 μm in thickness. Post-deposition annealing treatments in the temperature range 900–1200°C enhanced the luminescent intensity of the films. The as-deposited laser ablated oxide films were amorphous and required annealing at temperatures higher than 800°C to observe luminescence, which occurred in conjunction with crystallization. The as-deposited films averaged 500 nm in thickness and after post-annealing at 1000°C were composed of 15–200 nm grains.

Structural Design Elements in Biological Materials: Application to Bioinspiration

Eight structural elements in biological materials are identified as the most common amongst a variety of animal taxa. These are proposed as a new paradigm in the field of biological materials science as they can serve as a toolbox for rationalizing the complex mechanical behavior of structural biological materials and for systematizing the development of bioinspired designs for structural applications. They are employed to improve the mechanical properties, namely strength, wear resistance, stiffness, flexibility, fracture toughness, and energy absorption of different biological materials for a variety of functions (e.g., body support, joint movement, impact protection, weight reduction). The structural elements identified are: fibrous, helical, gradient, layered, tubular, cellular, suture, and overlapping. For each of the structural design elements, critical design parameters are presented along with constitutive equations with a focus on mechanical properties. Additionally, example organisms from varying biological classes are presented for each case to display the wide variety of environments where each of these elements is present. Examples of current bioinspired materials are also introduced for each element.

Comparison of the structure and mechanical properties of bovine femur bone and antler of the North American elk (Cervus elaphus canadensis)

Antler and limb bone have a similar microstructure and chemical composition. Both are primarily composed of type I collagen and a mineral phase (carbonated apatite), arranged in osteons in compact (cortical bone) sections and a lamellar structure in the cancellous (spongy or trabecular bone) sections. The mineral content is lower in antler bone and it has a core of cancellous bone surrounded by compact bone running through the main beam and tines. The mineral content is higher in the compact compared with the cancellous bone, although there is no difference in ratios of the mineral elements with calcium. Mechanical tests (bend and compression) on longitudinal and transverse orientations of dry and rehydrated compact bone of North American elk (Cervus elaphus canadensis) antlers are compared with known data on other antlers as well as bovine femora. Both dry and rehydrated bones are highly anisotropic, with the bending and compressive strength and elastic modulus higher in the longitudinal than in the transverse direction. There is no significant difference between the bend strength and elastic modulus between dry and rehydrated samples tested in the transverse direction. The elastic modulus measured from the bending tests is compared with composite models. The elastic modulus and bend strengths are lower in the rehydrated condition, but the strain to failure and fracture toughness is much higher compared with dry samples. All antler bone mechanical properties are lower than that of bovine femora. The antler has a much higher fracture toughness compared with bovine femora, which correlates with their main function in intraspecific combat as a high impact resistant, energy absorbent material. A model of compression deformation is proposed, which is based on osteon sliding during shear.

Minerals form a continuum phase in mature cancellous bone.

Bone is a hierarchically structured composite consisting of a protein phase (type I collagen) and a mineral phase (carbonated apatite). The objective of this study was to investigate the hierarchical structure of mineral in mature bone. A method to completely deproteinize bone without altering the original structure is developed, and the completion is confirmed by protein analysis techniques. Stereoscopy and field emission electron microscopy are used to examine the structural features from submillimeter- to micrometer- to nanometer-length scales of bovine femur cancellous bone. Stereoscopic images of fully deproteinized and demineralized bovine femur cancellous bone samples show that fine trabecular architecture is unaltered and the microstructural features are preserved, indicating the structural integrity of mineral and protein constituents. SEM revealed that bone minerals are fused together and form a sheet-like structure in a coherent manner with collagen fibrils. Well-organized pore systems are observed at varying hierarchical levels. Mineral sheets are peeled off and folded after compressive deformation, implying strong connection between individual crystallites. Results were compared with commercially available heat-deproteinized bone (Bio-Oss®), and evidence showed consistency in bone mineral structure. A two-phase interpenetrating composite model of mature bone is proposed and discussed.

Anisotropy in the compressive mechanical properties of bovine cortical bone and the mineral and protein constituents

The mechanical properties of fully demineralized, fully deproteinized and untreated cortical bovine femur bone were investigated by compression testing in three anatomical directions (longitudinal, radial and transverse). The weighted sum of the stress-strain curves of the treated bones was far lower than that of the untreated bone, indicating a strong molecular and/or mechanical interaction between the collagen matrix and the mineral phase. Demineralization and deproteinization of the bone demonstrated that contiguous, stand-alone structures result, showing that bone can be considered an interpenetrating composite material. Structural features of the samples from all groups were studied by optical and scanning electron microscopy. Anisotropic mechanical properties were observed: the radial direction was found to be the strongest for untreated bone, while the longitudinal one was found to be the strongest for deproteinized and demineralized bones. A possible explanation for this phenomenon is the difference in bone microstructure in the radial and longitudinal directions.