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Sökning: WFRF:(Zhang Peng) > Doktorsavhandling

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1.
  • Deng, Dunyong, 1989- (författare)
  • On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718
  • 2019
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive manufacturing (AM), also known as 3D printing, offers great design flexibility for manufacturing components with complex geometries, and has attracted significant interest in the aero and energy industries in the past decades. Among the commercial AM processes, selective laser melting (SLM) and electron beam melting (EBM) are the two most widely used ones for metallic materials. Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties, weldability and low cost. Due to its excellent weldability, IN718 has been intensively applied in the AM filed, to gain more understanding of the AM processes and fully realize AM’s potentials.The study objects in the present thesis include both EBM and SLM IN718. The solidification conditions in EBM and SLM are very different and are different to that of conventional cast, leading to unique microstructures mechanical properties. Therefore, this thesis aims to gain better understanding of the microstructures and anisotropic mechanical behaviours of both EBM and SLM IN718, by detailed characterizations and by comparisons with the forged counterpart.The as-built microstructure of EBM IN718 is spatially dependent: the periphery (contour) region has a mixture of equiaxed and columnar grains, while the bulk (hatch) region has columnar grains elongated along the building direction; the last solidified region close to the top sample surface shows segregation and Laves phases, otherwise the rest of the whole sample is well homogenized. Differently, the as-built microstructure of SLM IN718 is spatially homogeneous: the grains is rather equiaxed and with subgrain cell structures. These microstructures also respond differently to the standard heat treatment routines for the conventional counterparts.Anisotropic mechanical properties are evident in the room temperature tensile tests and high temperature dwell-fatigue tests. The anisotropic tensile properties of EBM IN718 at room temperature are more likely due to the directional alignment of porosities along the building direction rather than the strong crysiii tallographic texture of ⟨100⟩ _ building direction. While for SLM IN718, the anisotropy is more likely attributed to the different extents of ‘work-hardening’ or dislocations accumulated between the horizontally and vertically built specimens. The anisotropy mechanisms in dwell-fatigue crack propagations at 550 ◦C for EBM and SLM IN718 are identical: higher effective stress intensity factor when intergranular cracking path is perpendicular to the loading direction, but lower effective stress intensity factor when intergranular cracking path is parallel to or slightly deviated from the loading direction.The 2160s dwell-fatigue cracking behaviours at 550 ◦C are of significant interest for AM IN718, of which test condition is similar to that of real service for IN718 disk in turbine engine. Generally, after conventional or short-term heat treatments, EBM IN718 shows better dwell-fatigue cracking resistance than SLM IN718. The damage mechanism is different for EBM and SLM IN718: the intergranular cracking in EBM IN718 is due to environmentally assisted grain boundary attack, while creep damage is active for SLM IN718. The considerably ‘deformed’ microstructure, specifically the subgrain cell structures in SLM IN718 resulted from the manufacturing process, is believed to activate creep damage even at a low temperature of 550 ◦C. And for SLM IN718, heat treatment routine must be carefully established to alter the ‘deformed’ microstructure for better time dependent cracking resistance at elevated temperature.
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2.
  • Zhang, Peng (författare)
  • Mesoporous magnesium carbonate as a drug delivery vehicle for stabilising amorphous drugs and regulating their release rate
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • In today’s drug discovery, the number of candidate drugs based on new molecular entities with poor aqueous solubility is increasing. Since poor aqueous solubility of an active pharmaceutical ingredients (APIs) is associated with low bioavailability and thus limite their therapeutic effect, this is often a great challenge in the development of new drugs when oral administration is the preferred route of administration. A number of different strategies have been developed to circumvent this problem where salt formulations of an API is the most widely employed method. However, new strategies are needed since there is no one solution that solves this issue for all substances. In recent time, the concept of stabilizing poorly soluble APIs in their amorphous form has gained a lot of attention since amorphous compounds exhibit a higher apparent solubility compared to their crystalline counterparts. Amorphous substances are prone to crystallize if left in a non-constricted environment and thus need to be stabilized if the amorphous state is to be conserved until administration. Inorganic mesoporous materials have been proposed as an interesting type of excipients that can conserve the amorphous state of APIs. In this work, the focus was to investigate the possibilities of using a mesoporous type of magnesium carbonate to stabilize the amorphous state of different APIs. Due to the nanometer sized pores in the material, complete conservation of amorphous APIs was obtained. This resulted in both an increase in in vitro release rate and a higher solubility of the substances which may translate to both a faster onset of action and an improved therapeutic effect of the APIs in a clinical situation. The long term stability of formulations was also investigated showing promising results.The results presented in this work show that mesoporous magnesium carbonate represents an interesting type of excipient for oral formulations of APIs with poor aqueous solubility.
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3.
  • Zhang, Peng (författare)
  • Multi-Agent Systems Supported Collaboration in Diabetic Healthcare
  • 2008
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis proposes a holistic and hierarchical architecture to Multi-agent System design, in order to resolve the collaboration problem in diabetic healthcare system. A diabetic healthcare system is a complex and social system in the case that it involves many actors and interrelations. Collaborations among various healthcare actors are vital to the quality of diabetic healthcare. The collaboration problem is manifested by the problems of accessibility and interoperability. To support the collaboration in diabetic healthcare as such a complex and social system, the MAS must have corresponding social entities and relationships. Therefore, it is assumed that theories explaining social activity can be applied to design of MAS. Activity Theory, specifically its holistic triangle model from Engström and hierarchy thinking, provides theoretical supports to the design of individual agent architecture and MAS coordination mechanism. It is argued that the holistic and hierarchical aspects should be designed in a MAS when applied to the healthcare setting. The diabetic healthcare system is analyzed on three levels based on the hierarchy thinking. The collaboration problem is analyzed and resolved via MAS coordination. Based on the holistic activity model in Activity Theory, Müller’s Vertical Layered Architecture is re-conceptualized in the Control Unit and Knowledge Base design. It is also argued that autonomy, adaptivity and persona should be especially focused when designing the interaction between an agent system and human users. This study has firstly identified some important social aspects and the technical feasibility of embedding those identified social aspects in agent architecture design. Secondly, a MAS was developed to illustrate how to apply the proposed architecture to design a MAS to resolve the collaboration problem in diabetic healthcare system. We have designed and implemented an agent system – IMAS (Integrated Multi-agent System) to validate the research questions and contributions. IMAS system provides real time monitoring, diabetic healthcare management and decision supports to the diabetic healthcare actors. A user assessment has been conducted to validate that the quality of the current diabetic healthcare system can be improved with the introduction of IMAS.
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4.
  • Zhang, Pimin, 1990- (författare)
  • Performance of MCrAlX coatings : Oxidation, Hot corrosion and Interdiffusion
  • 2019
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • MCrAlY coatings (M=Ni and/or Co) are widely used for the protection of superalloy components against oxidation and hot corrosion in the hot sections of gas turbines. The drive for coating systems to bestow adequate oxidation and corrosion resistance upon the components becomes urgent as an inevitable result of the necessary improvement in engine combustion efficiency and service lifetime. Through the careful design of the composition, MCrAlY coating performance can be optimized to meet the needs under different service conditions and component materials, therefore, “MCrAlX”, with “X” stands for the minor alloying elements, is used to highlight the effect. In the present thesis, the performance of new MCrAlX coatings is investigated with respect to oxidation, hot corrosion and interactions between coating-superalloy substrates.Oxidation of MCrAlX coatings can be generally categorized into initial, steady and close-to-end stages. Coating performance can be affected by various factors at different stages, therefore, experiments were designed by targeting the oxidation stages. Investigation on the initial stage oxidation behavior of MCrAlY coatings with post-deposition surface treatments reveals the different growth mechanisms of alumina scales. Surface treatments significantly reduce the alumina growth rate by suppressing transient alumina development and aiding the early formation of α-Al2O3, which improves the long-term oxidation performance of the coating. Similarly, the modification of minor alloy elements in MCrAlX coatings also serves the purpose. The oxidation behavior of new MCrAlX coatings was investigated at the steady oxidation stage, followed by the microstructure observation, thermodynamic and kinetic simulations. As an alternative reactive element addition of Y, Ce shows a negative effect on the formation of columnar alumina scales of high strain tolerance. In comparison, Fe or Ru addition shows no influence on alumina growth, rather than strengthening the phase stability in the coating and reducing the interdiffusion between coating-substrate through different mechanisms. As the oxidation proceeds to the close-to-end stage, a reliable criterion to estimate the capability of coating to form α-Al2O3 is of great importance to accurately evaluate coating lifetime. A temperature-dependent critical Al-activity criterion was proposed to better predict the formation of a continuous α-Al2O3 scale based on correction of elemental activity using thermodynamic database to replace the empirical Al-concentration based criterion.Severe interdiffusion occurs between coating-substrate during high temperature oxidation, accelerating the degradation of the system. Interdiffusion behavior of diffusion couples of superalloys-MCrAlX coatings were examined. It is highlighted that the recrystallization of superficial layer of the substrate contributes to the secondary reaction zone formation and element interdiffusion controls subsequent zone thickening.Study on Type I hot corrosion behavior of new MCrAlX coatings shows that the addition of Fe has no influence on basic fluxing reactions before severe Al depletion from the coating occurs. Instead, it boosts the “effective” Al supply of coating by shifting the equilibrium concentration of Al in the γ phase to a low Al level. Besides, the pre-mature coating degradation at the coating-substrate interface was due to the fast growth of corrosion products from substrate induced large local volume expansions, resulting in early coating spallation.
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5.
  • Zhang, Peng (författare)
  • Summer Climate Variability during the Past 1200 Years in Central Scandinavia – A Tree-Ring Perspective
  • 2015
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • To set the current 20th century warming in a long-term context, significant efforts have been made to reconstruct hemispheric-to-global temperatures beyond the instrumental period. Tree-rings, which have annual resolution and can be precisely dated, have been widely used to infer past climate variability. In Fennoscandia, tree-ring maximum latewood density (MXD) provides so far the best high-resolution natural archive of summer temperature, and has been used to infer regional temperature variability for the last millennium. However, most of the temperature reconstructions have been based on data from northern Fennoscandia. In central Scandinavia, MXD based temperature reconstructions have not been able to reach the Medieval Climate Anomaly (MCA) when the climate conditions in some regions are analogue to the current warming, but without strong influences from human activities. This is a key period to evaluate if current warming can be reached without anthropogenic influences. To improve our understanding of past summer climate variability in central Scandinavia, in this thesis work, efforts were made to 1) find Scots pine (Pinus sylvestris L.) tree-ring samples from the central Scandinavian Mountains to increase the sample replication before 1750 CE and to extend it over the MCA, 2) examine if tree-ring data can represent annual conditions by comparing annual and summer temperature variability at different timescales in central Scandinavia. The results show that the local tree line in central Scandinavia during the MCA and early Little Ice Age (LIA) was about 140 m higher than at present. The temperature sensitivity of pine growth might be dampened by more humid growth condition. The result implies that temperature reconstructions predominantly based on the tree-ring widths from lake-shores may need to be re-evaluated. Focusing on tree-ring density, it was shown that mean absolute MXD values varied notably with elevation, with higher elevation having lower MXD values due to occurrence of the temperature gradients along altitudes. Heterogeneous temporal distribution of tree-ring samples at different elevations could seriously bias the long-term trend of the temperature reconstruction based on these samples. A mean-adjustment method was developed to overcome this bias. The reconstruction based on unadjusted data yielded 0.4°C lower average warm-season temperature during the period 850-1200 compared to the mean-adjusted reconstruction. The new warm-season (April-September) temperature reconstruction in central Scandinavia covering 850-2011, suggests a MCA during ca. 1000-1100, followed by a transition period before the onset of the LIA proper in the mid-16th century. During the past 1200 years, the late 17th century to early 19th century was the coldest period in central Scandinavia, and the warmest 100 years occurred during the 20th century. The new reconstruction suggests lower temperature during the late MCA (ca. 1100-1220) and higher temperature during the LIA (1610-1850) than the previous reconstruction, and shows regional differences in temperature evolution between northern and central Scandinavia before 1300 CE. Overall colder climate conditions are recorded in central Scandinavia before 1200 CE and warmer conditions during 1200-1300 leading to a mismatch in phase at multidecadal to century timescales before 1300 CE. During 1100-1250, central Scandinavia is dominated by warm, cloudy and wet summer conditions, while during the LIA the region was dominated by cold and sunny summers and partly wet conditions. The transition period between the MCA and LIA (around 1350-1550) was dominated by relatively dry conditions. During this period, temperatures were positively correlated with sunshine hours at multidecadal to century timescales, which was different from MCA and LIA. For central Scandinavia, the summer temperature overall is not a good ‘proxy’ for the annual temperature especially at the 2-16 year timescales.
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