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Totally found 76 items.

  • [会议] Environmental effects in biaxially orientated Polymethyl Methacrylate
    Environmental stress cracking (ESC) is commonly found in polymers when submerged in an environment under applied stress. In such conditions, a crack may initiate and propagate from a material defect until it reaches a critical size and causes catastrophic failure. It is known that materials with a denser molecular structure are less prone to ESC, hence amorphous polymers often suffer greatly from this effect. A fracture mechanics approach was employed to investigate the fracture mechanism and crack growth in both air and environment. Two different grades of PMMA (amorphous and biaxially stretched PMMA) were tested in solvents with similar solubility parameters as this is known to hasten crazing. Time for crack initiation and crack speed were obtained and plotted against their corresponding fracture toughness. Thus, the ESC resistance of each material can be compared and component life expectance can also be predicted. To validate the experimental results, scanning electron microscopy (SEM) was used to examine the fracture mechanism of ESC.
    关键词: Environmental stress cracking;Biaxially orientated PMMA;Fracture mechanics;Crazing
  • [会议] Environmental effects in biaxially orientated Polymethyl Methacrylate
    Environmental stress cracking (ESC) is commonly found in polymers when submerged in an environment under applied stress. In such conditions, a crack may initiate and propagate from a material defect until it reaches a critical size and causes catastrophic failure. It is known that materials with a denser molecular structure are less prone to ESC, hence amorphous polymers often suffer greatly from this effect. A fracture mechanics approach was employed to investigate the fracture mechanism and crack growth in both air and environment. Two different grades of PMMA (amorphous and biaxially stretched PMMA) were tested in solvents with similar solubility parameters as this is known to hasten crazing. Time for crack initiation and crack speed were obtained and plotted against their corresponding fracture toughness. Thus, the ESC resistance of each material can be compared and component life expectance can also be predicted. To validate the experimental results, scanning electron microscopy (SEM) was used to examine the fracture mechanism of ESC.
    关键词: Environmental stress cracking;Biaxially orientated PMMA;Fracture mechanics;Crazing
  • [会议] Properties of Porous Magnesium Using Polymethyl Methacrylate (PMMA) as a Space Holde
    Porous magnesium has been recognized as a promising biodegradation metal for bone substitute application because of its excellent biocompatibility, low density, ability to biodegrade in vivo and excellent mechanical properties. In the present work, porous magnesium was fabricated by powder metallurgy using spherical polymethyl methacrylate (PMMA) as a space holder. To determine the optimum sintering temperature for porous magnesium fabrication, the porous magnesium was fabricated using double step sintering process at various sintering temperature (550℃, 585℃ and 620℃) in first stage of research. The porous magnesium fabricated was then characterized for morphology, porosity, density and compressive strength. Density of porous magnesium increases and the porosity decreases with increasing sintering temperature. The mechanical characterization indicated that porous magnesium sintered at 620℃ exhibited the highest compressive strength and density with optimum range of porosity of 39.37%. For second stage of research, porous magnesium with porosities of 39.38 - 40.82% was produced with different sizes of PMMA particles (38-63 μm, 63-90 μm and 90-125 μm). The compressive yield strength ranges between 19.95 MPa to 23.28 MPa and increases with decreasing PMMA particles size and porosity. These results proven that the PMMA has a potential to be used as space holder in porous magnesium for biomedical application.
    关键词: -
  • [会议] Properties of Porous Magnesium Using Polymethyl Methacrylate (PMMA) as a Space Holde
    Porous magnesium has been recognized as a promising biodegradation metal for bone substitute application because of its excellent biocompatibility, low density, ability to biodegrade in vivo and excellent mechanical properties. In the present work, porous magnesium was fabricated by powder metallurgy using spherical polymethyl methacrylate (PMMA) as a space holder. To determine the optimum sintering temperature for porous magnesium fabrication, the porous magnesium was fabricated using double step sintering process at various sintering temperature (550℃, 585℃ and 620℃) in first stage of research. The porous magnesium fabricated was then characterized for morphology, porosity, density and compressive strength. Density of porous magnesium increases and the porosity decreases with increasing sintering temperature. The mechanical characterization indicated that porous magnesium sintered at 620℃ exhibited the highest compressive strength and density with optimum range of porosity of 39.37%. For second stage of research, porous magnesium with porosities of 39.38 - 40.82% was produced with different sizes of PMMA particles (38-63 μm, 63-90 μm and 90-125 μm). The compressive yield strength ranges between 19.95 MPa to 23.28 MPa and increases with decreasing PMMA particles size and porosity. These results proven that the PMMA has a potential to be used as space holder in porous magnesium for biomedical application.
    关键词: -
  • [会议] Thermal Shock Behavior and Bonding Strength of MoSi_2-BaO-Al_2O_3-SiO_2 Gradient Porous Coating with Polymethyl Methacrylate Addition for Porous Fibrous Insulations
    In order to improve the thermal shock behavior of high temperature resistant coating on porous fibrous referactory insulations, the MoSi_2-BaO-Al_2O_3-SiO_2 (MoSi_2-BAS) gradient porous coatings were designed by preparing a dense surface layer and a porous bonding layer with the method of brushing and subsequent sintering at 1773 K. The porous bonding layer was obtained by adding polymethyl methacrylate (PMMA) as pore former. As the content of PMMA increases, the MoSi2-BAS coatings changed from a dense structure into a gradient porous structure. The interface bonding strength and thermal shock resistance of the MoSi_2-BAS coatings were investigated. The result shows that the as-prepared coating with gradient porous structure exhibited excellent thermal shock resistance, which remained gradient structure without cracking after thermal cycling 100 times between 1773 K and room temperature. And the interface bonding strength of the gradient porous coating reached 1.5±0.08 Mpa, which was much better than that of the dense coating.
    关键词: polymethyl methacrylate;porous insulation;gradient porous coating;BAS glass
  • [会议] Structural Changes in Polymethyl Methacrylate during Laser Machining
    The focus of the article is an issue of laser machining of polymethyl methacrylate materials. Specifically, it deals with evaluation of structural changes in polymers as due to the effect of concentrated energy there appears vaporization of the material in the place of cut as well as heat transfer into the material around the place of cut and thus to the heating. As a result of the raised temperature, different structural changes may occur in the material; some high-molecular substances begin to break down into low-molecular substances, and depoly-merization or degradation of the material may occur. All these changes may result in a change of the final properties of the product. For experimental machining and subsequent study using the "Fourier Transform Infrared" vibration spectroscopy, PMMA was chosen as it is one of the most feasible polymeric materials in this technology.
    关键词: Laser Technology; Structural Changes; Polymeric Material; Vibration Spectroscopy
  • [会议] Deep hole drilling of polymethyl methacrylate with small diameters by actively compensating the hole straightness deviation
    Deep hole drilling is very demanding in terms of process control and precision of the used machine tools. In particular for drilling of polymethyl methacrylate (PMMA) with small drill diameters below 1 mm specific aspects have to be considered compared to regular drilling. Instances are the low glass temperature and the poor heat transmission from the material's viewpoint and the hole straightness deviation and the inability of proper process monitoring from the process viewpoint. Furthermore it hast to be evaluated individually if other manufacturing technology are possibly more suitable for the desired application. In this paper the problems are discussed which occur when drilling polymethyl methacrylate with small diameters. On this basis the results of initial drilling tests are discussed and presented whereby the used concept for live condition monitoring of the process is introduced.
    关键词: Micro Manufacturing;Deep Hole Drilling;Polymethyl Methacrylate
  • [会议] Effect of ZnSO_4 and SnSO_4 additions on the morphological of α-Al_2O_3 flakes for pearlescent pigment
    The flaky α-Al_2O_3 is one of desirable substrate materials for pearlescent pigments. In the present work, the flaky α-Al_2O_3 with aspect ratio larger than 100 were prepared via a novel routine combining the sol-gel process with molten-salt growth method. Furthermore, ZnSO_4 or/and SnSO_4 were selected as the morphology modifiers and the effects of the ratio and amount of the modifiers on the morphologies of α-Al_2O_3 platelets were investigated by Scanning Electron Microscopy (SEM). The results showed that the flaky α-Al_2O_3 with an average size of ~30 μm and ~0.2 μm in thickness were prepared using 2 wt% ZnSO_4 and 0.2 wt% SnSO_4 as the modifiers by fired at 1100℃ for 5 h.
    关键词: -
  • [会议] The influence of the fabrication developing time on plasmonic bowtie nanoantenna metastructures
    Plasmonic bowtie nanoantennas, which are metastructures that manipulate light and generate an intense localized electric field, demonstrate potential for surface enhanced Raman spectroscopy (SERS) signaling. Nanoantenna-based SERS can be employed to amplify molecular fingerprints, which is important for biomolecular and chemical reaction sensing. Nanoantenna electric field (or hot spot) optimization occurs when the plasmonic resonant wavelength of the structure closely matches the wavelength of the incident light. Depending on the excitation wavelength, the fabrication procedures can have tighter constraints. In this work, we fabricated several device arrays following nanoantenna design optimization for operation at 532nm wavelength. Fabrication steps utilized electron beam lithography (EBL) and nanopatterning tools, development, physical vapor deposition (PVD) by electron beam evaporation deposition (EBED), and resist removal by lift off. The polymer resist employed consist of a bilayer of polymethyl methacrylate (PMMA) in anisole (495k M.W. and 950k M.W.) sourced from MicroChem, Corp. The developer used (also sourced from MicroChem, Corp) was methyl isobutyl ketone: isopropyl alcohol (MIBK:IPA) in a 1:3 ratio. The influence of the developer step on the shape and quality of nanoantenna arrays was studied as the bowtie nanoantennas merged together or lost their defined shape. This study explored a specific bowtie nanoantenna design of 90-nm side lengths and 50-nm gap size, and 532-nm by 1.5 urn separation from its nearest neighbor. These nanoantennas were patterned in 10×5 and 10×25 grids with varying developer exposure times (10-100 seconds). Results reveal that the final device footprint has clear and defined shapes at as well as merged, undefined shapes across at 10-second window (40s-50s). At the upper end of the window, a greater than 40% increase in nanoantenna surface area is consistently observed.
    关键词: -
  • [会议] Noise measurements from reconstructed digital breast tomosynthesis
    In this work, we investigated and measured the noise in Digital Breast Tomosynthesis (DBT) slices considering the back-projection (BP) algorithm for image reconstruction. First, we presented our open-source DBT reconstruction toolbox and validated with a freely available virtual clinical trials (VCT) software, comparing our results with the reconstruction toolbox available at the Food and Drug Administration's (FDA) repository. A virtual anthropomorphic breast phantom was generated in the VCT environment and noise-free DBT projections were simulated. Slices were reconstructed by both toolboxes and objective metrics were measured to evaluate the performance of our in-house reconstruction software. For the noise analysis, commercial DBT systems from two vendors were used to obtain x-ray projections of a uniform polymethyl methacrylate (PMMA) physical phantom. One system featured an indirect thallium activated cesium iodide (CsI(TI)) scintillator detector and the other a direct amorphous selenium (a-Se) detector. Our in-house software was used to reconstruct raw projections into tomographic slices, and the mean pixel value, noise variance, signal-to-noise ratio (SNR) and the normalized noise power spectrum (NNPS) were measured. In addition, we investigated the adequacy of a heteroskedastic Gaussian model, with an affine variance function, to describe the noise in the reconstruction domain. The measurements show that the variance and SNR from reconstructed slices report similar spatial and signal dependency from previously reported in the projection domain. NNPS showed that the reconstruction process correlates the noise of the DBT slices in the case of projections degraded with almost uncorrelated noise.
    关键词: Digital breast tomosynthesis;Signal to noise ratio;Sensors;Reconstruction algorithms;Image processing;Breast;X-rays
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