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Thực trạng suy thoái rừng hiện nay và giải pháp

This research investigates the perspectives of the novice lecturers in the Faculty of English Language Teacher Education (FELTE) regarding Teaching Practicum’s impacts on their classroom management skills. Using survey research, data was gathered through questionnaires distributed among the whole population of 29 participants, followed by semi-structured interviews to gain a deeper insight into participants’ experience. The study’s results showed that novice lecturers generally made considerable progress in classroom management skills, especially in attention-drawing and response encouragement, creation of a motivating environment as well as physical and emotional interactions with students. On the other hand, it was discovered that inadequate duration and inappropriate timing generally impeded novice lecturers’ gains during their Teaching Practicum. However, some challenges related to supervisor and school choice turned out to be significant factors from which participants could derive considerable benefits, which is opposed to previous research. At the end of the study, some recommendations were suggested for future research on this topic.

In order to improve fire performance of thermoplastic polyurethane (TPU) material, halogen-free flame retardants (triphenylphosphate- TPP and diamonium hydrogen phosphate-DAP) were studied in an attempt to obtain UL-94V ratings. The fire behaviors and thermal stability properties were evaluated using UL-94 vertical test and thermogravimetric analysis (TGA). The UL- 94V results showed that V-0 ratings were achieved at 5 wt% of DAP or 7.5 wt% loading of TPP. The incorporation of these flame retardant (FR) increases the flame retardant properties as well as the amounts of charred residues protecting the mixture from further degradation. This assertion could be accepted when observing that the char residual of TPU/DAP mixture at 500–600oC was much higher than that of neat TPU. The char layer limited the amount of fuel available and insulate the underlying composite material from the flame and, thus, make further degradation more difficult. The mechanism of flame retardants was also discussed in this study.

The five-level VIENNA rectifier is very well suited to the short-circuit switch fault. However, this type of fault causes a distortion in the input phase current, one flying- capacitor to be discharged to zero and the other to be charged until corresponding output capacitor voltage. There is a big difference between two output capacitor voltages and decreases the apparent frequency of input converter voltage. Therefore, the proposed fault diagnosis is necessary to prevent further failures, and to analyze the impact of fault on system performance. This paper analyzed the behavior of the ON- fault in the single phase five-level VIENNA rectifier and proposed a fault diagnosis and integrated reconfiguration control. The effectiveness of the proposed fault diagnosis and reconfiguration control methods are verified through simulation results.

A series of poly(ether ester) thermoplastic elastomers (TPEEs) are synthesized by a one-pot, two-step method: (1) transesterification of dimethyl-2,6-naphthalenedicarboxylate with 1,4-butanediol (BDO) as chain extender (CE), followed by (2) low-pressure melt polycondensation with poly(tetramethylene ether glycol) as a soft segment in the presence of Ti(OBu)4 as a catalyst. In order to design phosphorous-containing flame-retardant TPEEs, hydroxyl-terminated isobutylbis(hydroxypropyl)phosphine oxide (IHPO) is integrated into the polymer backbone as the second CE, modulating the IHPO content up to 30% with respect to BDO. The resultant TPEEs are systematically characterized using various spectral, thermal, and mechanical analyses. An increase in phosphorus content in the polymer backbone enhances the flame retardancy of TPEE, adapting them as promising halogen-free self-extinguishing thermoplastic elastomers without losing their elastomeric properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45478.

This work is an exploration and an evaluation of new variants of multi-level AC/DC topologies (PFC) considering their global reliability and availability: electrical safety with an internal failure and post-failure operation. They are based on a non-differential AC and centre tap connection that led to symmetrical arrangement cells in series. These topologies permit an intrinsic active redundancy between cells in a same group and a segregation capability between the two symmetrical groups of cells. More again, they are modular and they can be paralleled and derived to any number of levels. Only single low-voltage (600V) transistor pear cell is used avoiding the short-circuit risk due to an unwanted control signal. Comparisons, taking into account losses, distribution losses, rating and stresses (overvoltage and over-temperature) during the post-operation are presented. Results highlight the proposed 5-level Double-Boost Flying Capacitor topology. This one was patented at the beginning of thesis, as a solution with the best compromise. On the theoretical side, we show that the reliability calculation based only on a "first fault occurrence" criterion is inadequate to really describe this type of topology. The inclusion of fault tolerance capability is needed to evaluate the overall reliability law (i.e. including a second failure). The adaptation of theoretical models with constant failure rate including overvoltage and over-temperature dependencies exhibit an increasing of the reliability over a short time. This property is an advantage for embedded systems with monitoring condition. Local detection and rapid diagnosis of an internal failure were also examined in this work. Two methods are proposed firstly, by a direct flying caps monitoring and secondly, by a realtime and digital synchronous demodulation of the input sampled voltage at the switching frequency (magnitude and phase). Both techniques have been integrated on FPGA and DSP frame and evaluated on a AC230V-7kW DC800V - 31kHz lab. set-up. We put forward the interest of the second method which only uses one input voltage sensor. Finally, we propose in this dissertation a new generic X-level PFC Vienna using, in 5-level version, half transistors and drivers for identical input frequency and levels. At the cost of a slight increase of losses and density losses, this topology appears very attractive for the future. A preliminary lab. set-up and test were also realized and presented at the end of the thesis.; Ce travail vise une exploration et une évaluation de nouvelles variantes de topologies multiniveaux AC/DC non réversibles (PFC) du point de vue de leur sûreté de fonctionnement : recherche d'une grande sécurité électrique sur destruction interne et maintien d'une continuité de fonctionnement. Elles sont caractérisées par une connexion AC non différentielle, un partitionnement cellulaire en série et symétrique autour d'un point milieu. Cette organisation permet d'exploiter la redondance active série entre les cellules d'un même groupe et l'effet de ségrégation topologique qui apparaît entre les deux groupes de cellules. Les structures étudiées sont modulaires et peuvent être parallélisées et étendues à un nombre quelconque de phases. Elles ne possèdent que des cellules mono-transistors basse-tension (Si et SiC 600V max) performantes et intrinsèquement tolérantes aux imperfections de la commande et aux parasites donc naturellement sécurisées. Les comparaisons prenant en compte les pertes, la répartition des pertes, le dimensionnement et le report de contraintes sur défaut interne mettent en avant la structure PFC Double- Boost Flying Cap. à 5 Niveaux, brevetée en début de thèse, comme une solution ayant le meilleur compromis. Sur le plan théorique nous montrons que le seul calcul de la fiabilité basé uniquement sur un critère d'occurrence au premier défaut est inadapté pour décrire ce type de topologie. La prise en compte de la tolérance de panne est nécessaire et permet d'évaluer la fiabilité globalement sur une panne effective (i.e. au second défaut). L'adaptation de modèles théoriques de fiabilité à taux de défaillance constant mais prenant en compte, au niveau de leurs paramètres, le report de contrainte en tension et l'augmentation de température qui résulte d'un premier défaut, permet de chiffrer par intégration et en valeur relative, le gain obtenu sur un temps court. Ce résultat est compatible avec les systèmes embarqués et la maintenance conditionnelle. Un prototype monophasé à 5 niveaux, à commande entièrement numérique et à MLI optimisée reconfigurable en temps réel a été réalisé afin de valider l'étude. Il permet une adaptation automatique de la topologie de 5 à 4 puis à 3 niveaux par exemple. Ce prototype a également servi de banc de test d'endurance du mode de défaillance sur claquage - avalanche de transistors CoolMos™ et diodes SiC, volontairement détruits individuellement dans des conditions d'énergie maîtrisée et reproductibles, afin de prouver expérimentalement le maintien du service sur plusieurs centaines d'heures au prix d'un derating de 30% maximum en puissance seulement. La détection et le diagnostic rapide de défauts internes ont également été traités dans ce travail. D'une part, par la surveillance directe et le seuillage des tensions internes (tensions flottantes) et d'autre part, par une détection harmonique de la fréquence de base (amplitude et phase) en temps réel. Ces deux techniques ont été intégrées numériquement et évaluées sur le prototype, en particulier la seconde qui ne requiert qu'un seul capteur. VI Enfin, nous proposons dans ce travail une nouvelle variante PFC Vienna multicellulaire expérimentée en fin de mémoire, utilisant deux fois moins de transistors et de drivers pour les mêmes performances fréquentielles au prix d'un rendement et d'une répartition des pertes légèrement moins favorables que la structure brevetée.

PurposeThis study aims to discuss and test the combined role of perceived risk, objective knowledge and certainty as moderators in the satisfaction‐loyalty relationship.Design/methodology/approachThe authors use survey data of 387 Vietnamese consumers in a food context. A structural equation modeling (SEM) approach for moderator analysis with latent constructs is used to test the hypotheses.FindingsPerceived risk is a barrier in the forming of loyalty with a negative moderating effect on the satisfaction‐loyalty relationship. However, the satisfaction‐loyalty relationship is stronger when objective knowledge and certainty increase.Research limitations/implicationsThe object and setting are limited to one product category in one market. In addition, other moderators (e.g. situation and ambivalence) can be added. The nature of causality is problematic due to the use of survey design.Practical implicationsCustomer management based on satisfaction is not sufficient to keep customers' loyalty, especially in the situations of highly perceived risk and uncertainty. Marketing strategies, which reduce consumers' risks, consolidate their confidence and educate them with relevant knowledge, may be effective strategies to increase their loyalty.Originality/valueThe study fills several gaps in the present literature. First, it overcomes some shortcomings of previous studies of moderators in the satisfaction‐loyalty relationship by testing the combined role of three important moderators. Second, it tests the moderator effect of objective knowledge and adds an extra explanation to previous studies. While some previous studies suggest a negative moderator effect of subjective knowledge, this paper argues for and confirms a positive moderator effect of objective knowledge on this relationship. Finally, it uses SEM for moderator analysis with latent constructs.

A new 5-level reconfigurable PFC topology with low energy short-failure capability and fast fault-diagnosis is proposed. It includes a 2-stacked low energy 3-level Fly-Cap Boost through two input low frequency rectifiers. 600V–30kHz devices in series pear stack allow one short-fault for transistor and up to two short-faults for diodes. Firstly, authors shows for the first time the low ageing drift and the great stability of the parasite short-failure resistance (Rsc) of failed CoolMos™ transistors and SiC diodes inside well known epoxy molded package (TO220/TO247/TO3P cases). Secondly, two fault-diagnosis strategies are presented: one directly monitors the voltages across the flying capacitors to detect and to localize the faulty switch by means of two sensors; other is based on a harmonic detection (magnitude and phase-shift) at the switching frequency through only one voltage sensor at the input of the PFC. All these strategies are analyzed and simulated with the efficient PD-PWM technique that exhibits a lower THD of the leg-leg input voltage for three-phase application. The design and the most important features are highlighted thanks to a digital control frame and a single-phase mock-up rated to AC115V–DC400V — 4kW–4×31kHz.