Corrosion control of magnesium for stent applications

Biomaterials used for implants may be metallic, ceramic, polymeric or composite. Currently, metals that are gradually broken down in the body have been attracting much attention, as a new generation of biodegradable implants. Magnesium (Mg) and related alloys are promising candidates for degradable biomaterials, comprising temporary mechanical properties with biological acceptance to the human body. However, the target periods set clinically, with respect to the practical uses of Mg for biodegradable stents, have yet to be achieved. Hence, improved understanding of the corrosion behaviour of Mg in the biological environment is needed. Novel Mg narrow walled minitubes, for degradable stent applications, have been produced using radio frequency magnetron sputtering (RF-MS) physical vapour deposition (PVD). The microstructural development of the as-deposited minitubes have been investigated, as a function of annealing temperature, using the combined complementary analytical techniques of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffractometry (XRD) and microhardness indentation. The as-deposited minitubes exhibited columnar grain structures with high levels of porosity, but were very brittle. Slight alteration to the crystal structure, from columnar to more isotropic grain growth, was demonstrated at elevated temperature, along with increasing material densification, hardness and corrosion resistance. It is suggested that stabilisation of the columnar grains and the formation of oxide layers during the sequential Mg-layer deposition process, acted as a barrier, preventing the development of a fully dense, equiaxed structures. The onset and development of Mg corrosion may be addressed by the use of coatings or near surface modification processes. Accordingly, the corrosion resistance of ~ 1-2 µm thick Al coatings, deposited by RF-MS on polished Mg surfaces, within Ar and Ar/H2 environments, were appraised. The coatings were heat-treated at 300°C and 450°C, with the aim of inducing the formation of bioinert Al2O3, and samples were corroded within phosphate buffered saline (PBS) solution at 37°C to mimic the biological environment. Both as-deposited and heat-treated coatings were found to delay the onset of corrosion, but showed higher initial corrosion rates, once established, as compared to the polished Mg surfaces. Slight improvement in coating performance was achieved through the addition of H2 to the system, which acted to inhibit Al-Mg alloying and enhance Al2O3 formation. However, localized accelerated corrosion associated with substrate polishing damage emphasised the need for improved process control and coating uniformity. Si-H coatings deposited on Mg surfaces within Ar/H2 ambient using a PVD technique was also investigated. The as-deposited coatings comprised dense, crack-free amorphous a-Si-H layers with thickness of ~ 1 µm. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses provided evidence for the presence of SiH2 as well as SiOx. The corrosion resistance of a-Si-H coated Mg increased significantly in contact with PBS, in both electrical and immersion tests, due to improved coverage of the substrate. The effect of rapid thermal processing techniques on the corrosion resistance of Mg surfaces was also investigated. Mg surfaces treated by large area electron beam (LAEB) irradiation showed refinement of the surface grain structure, with increased grain boundary delineation, although localised ablation, roughness and crater formation increased with increasing cathode voltage and number of pulses. The corrosion potential and corrosion rate of LAEB modified surfaces generally increased with increase the energy imparted to the surface. The extended corrosion performance of low energy EB processed surfaces, under immersion testing was consistent with the trend of improved corrosion resistance during the early stages of immersion in PBS. However, surfaces over-processed at high energies experienced higher corrosion rates in both potentiodynamic and immersion testing, due to the development of inclusions, craters and cracks on the modified surface. Further, Mg surfaces, modified by laser surface melting (LSM) under conditions of low energy laser irradiation, experienced rapid melting, causing surface smoothening and grain refinement centred along the laser beam tracks, whilst coarser grains decorated the overlapping regions, due to the Gaussian shape of the laser beam profile. More uniform surface processing was achieved by increasing the laser beam spot size, which acted to improve the corrosion resistance of Mg. Under high energy LSM processing conditions, Mg surfaces showed conventional laser melting rippled patterns, along with craters and cracks, and the redeposition of MgO particles, causing an increase in surface roughness and corrosion rate. The corrosion performance under immersion testing showed the corrosion rate similar to that of the original polished Mg samples, due to non-uniform surface modification and the mixed development of fine and coarser grains. However, observation revealed that refined grain regions along the centre of the laser tracks were able to resist corrosion for longer times. Generally, annealed Mg-minitubes produced by PVD, and the near surface modification of Mg by EB and LSM, showed that fine grained Mg can affect the electrochemical response of Mg within the physiological environment, due to the rapid, enhanced development of the passivation layer, promoted by improvements in surface homogeneity and an increase in grain boundary density

Corrosion control of magnesium for stent applications

Biomaterials used for implants may be metallic, ceramic, polymeric or composite. Currently, metals that are gradually broken down in the body have been attracting much attention, as a new generation of biodegradable implants. Magnesium (Mg) and related alloys are promising candidates for degradable biomaterials, comprising temporary mechanical properties with biological acceptance to the human body. However, the target periods set clinically, with respect to the practical uses of Mg for biodegradable stents, have yet to be achieved. Hence, improved understanding of the corrosion behaviour of Mg in the biological environment is needed. Novel Mg narrow walled minitubes, for degradable stent applications, have been produced using radio frequency magnetron sputtering (RF-MS) physical vapour deposition (PVD). The microstructural development of the as-deposited minitubes have been investigated, as a function of annealing temperature, using the combined complementary analytical techniques of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffractometry (XRD) and microhardness indentation. The as-deposited minitubes exhibited columnar grain structures with high levels of porosity, but were very brittle. Slight alteration to the crystal structure, from columnar to more isotropic grain growth, was demonstrated at elevated temperature, along with increasing material densification, hardness and corrosion resistance. It is suggested that stabilisation of the columnar grains and the formation of oxide layers during the sequential Mg-layer deposition process, acted as a barrier, preventing the development of a fully dense, equiaxed structures. The onset and development of Mg corrosion may be addressed by the use of coatings or near surface modification processes. Accordingly, the corrosion resistance of ~ 1-2 µm thick Al coatings, deposited by RF-MS on polished Mg surfaces, within Ar and Ar/H2 environments, were appraised. The coatings were heat-treated at 300°C and 450°C, with the aim of inducing the formation of bioinert Al2O3, and samples were corroded within phosphate buffered saline (PBS) solution at 37°C to mimic the biological environment. Both as-deposited and heat-treated coatings were found to delay the onset of corrosion, but showed higher initial corrosion rates, once established, as compared to the polished Mg surfaces. Slight improvement in coating performance was achieved through the addition of H2 to the system, which acted to inhibit Al-Mg alloying and enhance Al2O3 formation. However, localized accelerated corrosion associated with substrate polishing damage emphasised the need for improved process control and coating uniformity. Si-H coatings deposited on Mg surfaces within Ar/H2 ambient using a PVD technique was also investigated. The as-deposited coatings comprised dense, crack-free amorphous a-Si-H layers with thickness of ~ 1 µm. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses provided evidence for the presence of SiH2 as well as SiOx. The corrosion resistance of a-Si-H coated Mg increased significantly in contact with PBS, in both electrical and immersion tests, due to improved coverage of the substrate. The effect of rapid thermal processing techniques on the corrosion resistance of Mg surfaces was also investigated. Mg surfaces treated by large area electron beam (LAEB) irradiation showed refinement of the surface grain structure, with increased grain boundary delineation, although localised ablation, roughness and crater formation increased with increasing cathode voltage and number of pulses. The corrosion potential and corrosion rate of LAEB modified surfaces generally increased with increase the energy imparted to the surface. The extended corrosion performance of low energy EB processed surfaces, under immersion testing was consistent with the trend of improved corrosion resistance during the early stages of immersion in PBS. However, surfaces over-processed at high energies experienced higher corrosion rates in both potentiodynamic and immersion testing, due to the development of inclusions, craters and cracks on the modified surface. Further, Mg surfaces, modified by laser surface melting (LSM) under conditions of low energy laser irradiation, experienced rapid melting, causing surface smoothening and grain refinement centred along the laser beam tracks, whilst coarser grains decorated the overlapping regions, due to the Gaussian shape of the laser beam profile. More uniform surface processing was achieved by increasing the laser beam spot size, which acted to improve the corrosion resistance of Mg. Under high energy LSM processing conditions, Mg surfaces showed conventional laser melting rippled patterns, along with craters and cracks, and the redeposition of MgO particles, causing an increase in surface roughness and corrosion rate. The corrosion performance under immersion testing showed the corrosion rate similar to that of the original polished Mg samples, due to non-uniform surface modification and the mixed development of fine and coarser grains. However, observation revealed that refined grain regions along the centre of the laser tracks were able to resist corrosion for longer times. Generally, annealed Mg-minitubes produced by PVD, and the near surface modification of Mg by EB and LSM, showed that fine grained Mg can affect the electrochemical response of Mg within the physiological environment, due to the rapid, enhanced development of the passivation layer, promoted by improvements in surface homogeneity and an increase in grain boundary density

Causality deficit-inflation: Wavelet Transform

Few researchers have addressed the problem of the causality inflation-deficits, and even Previous worktreated this causality they have only focused on the Granger causality technique in which the results of thisapproach raises many doubts. This investigation aim to study the relationship between inflation and the pri-mary deficit using the wavelet transform for different euro area member countries from 1980 until 2014 withquarterly data. We have characterized the inflation-primary deficit relationship in a time-frequency scale.First we found that the deficit causes inflation in log term. We detected also, that inflation-primary deficitrelationship is highly consistent during the post-financial crisis in the euro area in the long run. Finally wecan judge through the wavelet transform that this relationship is non linear

Causality deficit-inflation: Wavelet Transform

Few researchers have addressed the problem of the causality inflation-deficits, and even Previous worktreated this causality they have only focused on the Granger causality technique in which the results of thisapproach raises many doubts. This investigation aim to study the relationship between inflation and the pri-mary deficit using the wavelet transform for different euro area member countries from 1980 until 2014 withquarterly data. We have characterized the inflation-primary deficit relationship in a time-frequency scale.First we found that the deficit causes inflation in log term. We detected also, that inflation-primary deficitrelationship is highly consistent during the post-financial crisis in the euro area in the long run. Finally wecan judge through the wavelet transform that this relationship is non linear

Stability analysis of embankment dam and forecast of failure scope based on point safety factor

Abstract In the context of verification of civil engineering structures stability and determination of sliding surface and safety factor, a careful analysis of several parameters was carried out to guarantee their safety against failure. To quantitatively forecast failure scope, the embankment dam located on Oued Rhiss in the province of Hocemia is chosen as the model of this study. A static stability analysis is performed by using the Slope/W software. A parametric study performed to evaluate the influence of dam's height, the height of water in the reservoir and the length of drains on the safety coefficient and pore pressures. Reliability analysis elaborated by using the Comrel application, and it allows to statistically quantifying the probability of failure by employing the Monte Carlo distribution. Results show that the dam structure has some weak zones and not strong enough as the safety factor is less than one, it is related to structure's parameters and the drainage system

Development of a New Slit-Slotted Shaped Microstrip Antenna Array for Rectenna Application

Abstract-These paper presents a new 3X3 array design using a microstrip patch array antenna to operate at 2.45GHz. The aim of this antenna array construction is to obtain high directivity. The element of the array is microstrip square patch antenna using V-shaped symmetric-slit along with rectangular slot in diagonal direction at the centre of square patch radiator to achieve the circularly polarized radiation and each element is fed by inset feed. The size and feed position of the single microstrip square patch is determined through the theoretical design and CST microwave studio software simulation. Based on which an array of six elements with equal sizes and equal spacing is designed on a planar substrate. The simulation results in this paper can be used as design reference for the practical design of the rectenna

Rozvoj lidských zdrojů a jejich vztah ke kvalitě hotelových služeb

Import 23/07/2015Thesis deals with hotel management issues in Libya focusing on human resources aspect and its influence on hotel service quality. It proposes several tools as a help for decision making in the field of particular hotel activities, demonstrates their utilization and com-ments their results. It also comes from a survey in selected hotels in the Libyan capital, and based on this survey it shows current quality issues faced by these hotels. It con-cludes, that there is a desperate need for promotion of Libya as a future attractive tourist destination, with a lot to offer to tourists, including a sufficient hotel infrastructure, which is still developing.Disertační práce se zabývá hotelovým managementem v podmínkách Libye se zaměřením na řízení lidských zdrojů a jeho vliv na kvalitu hotelových služeb. Navrhuje několik nástrojů jako pomůcek pro rozhodování v oblasti dílčích hotelových aktivit, ukazuje možnosti jejich využití a komentuje jejich výsledky. Vychází rovněž z průzkumu ve vybraných hotelích libyjského hlavního města a na základě tohoto průzkumu ukazuje současné problémy v oblasti kvality, s nimiž se uvedené hotely potýkají. Závěrem kon-statuje, že Libye se musí v budoucnu zaměřit na propagaci země jako budoucí atraktivní turistické destinace s bohatou nabídkou pro turisty, včetně dostatečné hotelové infra-struktury, která se neustále vyvíjí.Prezenční115 - Katedra managementuvyhově

Dimensional considerations on the mechanical properties of 3D printed polymer parts

Additive manufacturing offers a useful and accessible tool for prototyping and manufacturing small volume functional parts. Polylactic acid (PLA) and thermoplastic polyurethane (TPU) are amongst the most commonly used materials. Characterising 3D printed PLA and TPU is potentially important for both designing and finite element modelling of functional parts. This work explores the mechanical properties of additively manufactured PLA/TPU specimens with consideration to design parameters including size, and infill percentage. PLA/TPU specimens are 3D-printed in selected ISO standard geometries with 20%, 60%, 100% infill percentage. Tensile and compression test results suggest that traditional ISO testing standards might be insufficient in characterising 3D printed materials for finite element modelling or application purposes. Infill percentage in combination to design size, may significantly affect the mechanical performance of 3D printed parts. Dimensional variation may cause inhomo-geneity in mechanical properties between large and small cross section areas of the same part. The effect was reduced in small cross section parts where reducing the nominal infill had less effect on the resulting specimens. The results suggest that for 3D printed functional parts with significant dimensional differences between sections, the material properties are not necessarily homogeneous. This consideration may be significant for designers using 3D printing for applications, which include mechanical loading