Synthesis and characterisation of oxychalcogenides as promising thermoelectric materials for waste heat recovery

This dissertation describes the preparation and investigation of crystal structure and thermoelectric properties of solid-solutions within three families of layered bismuth oxychalcogenides: BiOCuCh (Ch = S, Se, Te), Bi2YO4Cu2Se2, and Bi2O2Ch (Ch = Se, Te). The crystal structures of all materials were investigated using powder X-ray and neutron diffraction (for BiOCuCh). BiOCuCh (Ch = S, Se, Te) compounds crystallise in the ZrCuSiAs structure type (P4/nmm space group), and are composed of fluorite-type [Bi2O2]2+ and anti-fluorite-type [Cu2Ch2]2- slabs, stacked alternatively along the c – axis. Results show that BiOCuCh (Ch = S, Se, Te) are p-type semiconductors. The electrical conductivity increases while thermal conductivity decreases systematically with changing from S to Te in these compounds. Analysis of neutron diffraction data shows that the rattling behaviour of copper in a rigid framework (BiOCh) is at the origin of their low thermal conductivity. The figure of merit increases with increasing atomic weight of the chalcogenide. BiOCuSe shows the larger potential for thermoelectric applications in terms of its combination of economic cost and properties. Therefore, the effect of doping with divalent cations (Pb2+, Cd2+, Zn2+) on BiOCuSe was studied. Results show that substitution of trivalent Bi3+ with a 4-5 at.% of divalent Pb2+ leads to an enhancement of the power factor and a high figure of merit (ZT ~ 0.62 at 673 K), whilst the substitution of monovalent Cu+ with divalent Cd2+ or Zn2+ leads to an increase in the magnitude of the electrical resistivity and the Seebeck coefficient. In addition, a reduction of the thermal conductivity (κ ~ 0.77 W m−1 K−1) is achieved in ball-milled Bi0.95Pb0.05OCuSe. Bi2YO4Cu2Se2 crystallises in the Sr2Mn3Sb2O2 structure type (I4/mmm space group), and consists of fluorite-type [Bi2MO4]+ and anti-fluorite-type [Cu2Ch2]- layers stacked alternatively along the c – axis. It possesses metallic behaviour, with hole charge carriers and a fairly low figure of merit (ZT ~ 3x10-2 at 673 K). This behaviour is related to the oxidation state of the copper (+1.5) in which more hole charge carriers have been produced. Bi2O2Ch (Ch = Se, Te) crystallises in the anti-ThCr2Si2 structure (I4/mmm space group) and comprises fluorite-type [Bi2O2]2+ and square net Ch2- stacked alternatively along the c – axis. Results show that Bi2O2Te1-xSex (0 ≤ x ≤ 0.25) are n-type semiconductors, and that Bi2O2Te shows the highest figure of merit (ZT ~ 1.3x10-1 at 573 K) while Bi2O2Te1- xSex (0.5 ≤ x ≤ 1) and Bi2O2Se1±δ(0.05 ≤ δ ≤ 0.15) solid solutions show insulating behaviour

Entrepreneurial orientation and social ties in transitional economies

Recent research suggests that entrepreneurial orientation (EO) has a more complex effect on performance (i.e. non-linear instead of linear) than previously considered. We extend this view by examining the non-linear effect of each individual dimension of EO (i.e. innovativeness, proactiveness, and risk-taking) on firm performance in the context of a transitional, collectivist economy. Drawing upon social capital theory, we also examine under which social capital conditions (i.e., business and political ties) each dimension of EO is most effective. Using survey data from 137 firms in Vietnam Top 500 Companies, this study shows that innovativeness and proactiveness have inverted U-shaped relationships with firm performance, while the effect of risk-taking on firm performance is also non-linear but in the form of increasing returns. Findings also show that social capital from business ties differentially moderates the effects of EO dimensions on performance. Similarly, social capital from political ties has different moderating effects on the innovativeness-performance and proactivenessperformance linkages. The findings urge managers of firms operating in transitional economies to take the levels of social capital from business ties and political ties into consideration when making their decision on which entrepreneurial strategy to pursue

The dynamic relationship between greenfield investments, cross-border M&As, domestic investment and economic growth in Vietnam

Funding: NAFOSTED (grant number 502.02-2020.09).This paper investigates the dynamic linkages between different types of foreign direct investment (FDI), domestic investment and economic growth in Vietnam. We decompose the aggregated FDI level into its two major components: greenfield investments, and cross-border mergers and acquisitions (M&As). The empirical results reveal that greenfield investments and cross-border M&As exhibit different impacts on economic growth. While greenfield investments appear to complement domestic investment, which subsequently promotes long-run economic growth, cross-border M&As exert a significant crowd-out effect and subsequently impede growth in both the short- and the long-run. These results provide important implications for policies to attract FDI in order to stimulate sustainable growth.Publisher PDFPeer reviewe

The impact of recentralisation on FDI : evidence from a quasi-natural experiment

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant No. 502.02-2020.09.Although decentralised governance has been one of the most salient political regimes worldwide over the past few decades, many countries have started to realise various shortcomings associated with their decentralisation process. As a consequence, a number of central governments have attempted to pursue recentralisation reforms in order to reclaim authority from the localities. This government reform can lead to significant changes in institutional arrangements, and subsequently, may influence various aspects of socio-economic activities. However, the real impact of recentralisation reform still remains ambiguous. In this paper, we examine how recentralisation may affect foreign direct investment (FDI) inflows. We exploit the pilot recentralisation reform that temporarily abolished the intermediate legislative branches in some provinces in Vietnam as a quasi-natural experiment. The result shows that recentralisation leads to a significant reduction in FDI inflows. Our results are robust to a number of sensitivity analyses and falsification tests. Overall, our findings contribute to the literature on the determinants of FDI and provide new evidence on the real effect of recentralisation reform.Publisher PDFPeer reviewe

The dark side of formal and informal governance

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Value synergy and value asymmetry in relationship marketing programs

Prior research advocates a positive, linear association between relationship investments and relationship performance. Our study challenges this conventional wisdom and advances the extant literature by investigating the potential curvilinear effects of suppliers' different relationship marketing programs (i.e., social, financial, and structural) on dyadic perceptions of relationship value. From an analysis of 113 buyer-supplier dyads, we found that social programs enhance relationship value synergy, but their effect on relationship value asymmetry between suppliers and buyers follows a U-shaped curve. On the other hand, we observe a positive and increasing returns-to-scale effect of financial programs on relationship value synergy and its inverted U-shaped association with supplier's relationship value asymmetry. Interestingly, structural programs increase relationship value synergy and have a stronger effect on increasing relationship value for the supplier than for the buyer. In addition, we find that structural programs are more effective in creating value in long-term relationships than in short-term relationships; therefore, as the relationship with a buying firm ages, managers should consider investing more in structural programs to develop their relationship. However, in long-term relationships, managers should avoid investing too much in financial programs because financial programs are less effective in increasing creation of relationship value as a relationship ages

Key determinants of relationship value gap: a conceptual framework

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Optimal connection of wind turbines to distribution grid to minimize power loss

This research aims to connect wind turbines to a distribution grid to minimize the power loss and to satisfy the grid’s normal operating condition. The proposed algorithm will determine optimal positions, optimal operation mode and wind turbine type. We must choose the best operation mode from available modes including the constant power factor mode and the constant voltage mode. According to the optimal operation mode, we decide the optimal setting data of wind turbine. This algorithm is coded in MATLAB software and implemented to IEEE 33-buses distribution grid. Noted that in this research, we tested two cases including the original IEEE 33-buses grid and its modification where the power system connected to this grid at multi-position. Results indicated that the proposed algorithm could determine the number of wind turbines, position, optimal operation mode, wind turbine type and the priority order of wind turbine installation to minimize power loss. Moreover, results were also compared to that of other algorithms

The impacts of parental migration on left-behind children in the northern provinces of Việt Nam: From the child’s perspective.

The world’s upward trend in people’s mobility has increased the number of left-behind children. While exploring the impacts of parental migration on these children, the vast body of research has largely focused on education and health and is based on adult’s views while only few captured children’s perspectives on migration. The emergent sociology of childhood in recent decades has advocated for considering children as active agents in constructing and determining their own lives. Building upon this research paradigm and aiming to fill the knowledge gap in existing literature, this study captures children’s experiences of parental migration by using a theoretical framework that positions the child’s agency at the centre, situated within the interrelations of social structure and culture. To stay true to children’s accounts, the study employs a qualitative approach with semi-structured interviews with 58 children and two focus group discussions with 11 children, all aged 10–15 years old in Cao Bằng and Bắc Kạn provinces of Việt Nam, a geographical area that has been largely absent in existing literature. Seeking to explore 3 issues: (a) the impact of parental migration on left-behind children from the child’s lens, (b) children’s exercise of their agency within the context of parents’ migration; and (c) their participation in migration decisions, the study finds that: 1) with the trade-off between financial gains and emotional well-being, left-behind children have dynamic yet ambivalent experiences of parental migration; 2) they have a limited voice in migration decisions but have some influence on decisions at home after the parents’ departure; 3) they exhibit agency at basic, tactical or strategic levels depending on the specific context to make sense of and living with their left-behind life; 4) social structure and cultural norms and practices are key factors that enable the parents’ migration to become an enabler, a barrier or both to children’s agency. While adding nuance to understandings of the impacts of parents’ migration on left-behind children, this study makes a theoretical contribution to the conceptualisation of ambivalence and agency and how they interact. The thesis offers suggestions for future research and policy to support left-behind children’s well-being