Turbine engine blades are subjected to extreme conditions characterized by significant and simultaneous excursions in both stress and temperature. These conditions promote thermo-mechanical fatigue (TMF) crack growth which can significantly reduce component design life beyond that which would be predicted from isothermal/constant load amplitude results. A thorough understanding of the thermo-mechanical fatigue crack behavior in single crystal superalloys is crucial to accurately evaluate component life to ensure reliable operations without blade fracture through the use of "retirement for cause" (RFC). This research was conducted on PWA1484, a single crystal superalloy used by Pratt & Whitney for turbine blades. Initially, an isothermal constant amplitude fatigue crack growth rate database was developed, filling a void that currently exists in published literature. Through additional experimental testing, fractography, and modeling, the effects of temperature interactions, load interactions, oxidation and secondary crystallographic orientation on the fatigue crack growth rate and the underlying mechanisms responsible were determined. As is typical in published literature, an R Ratio of 0.7 displays faster crack growth when compared to R = 0.1. The effect of temperature on crack growth rate becomes more pronounced as the crack driving force increases. In addition secondary orientation and R Ratio effects on crack growth rate were shown to increase with increasing temperature. Temperature interaction testing between 649°C and 982°C showed that for both R = 0.1 and 0.7, retardation is present at larger alternating cycle blocks and acceleration is present at smaller alternating cycle blocks. This transition from acceleration to retardation occurs between 10 and 20 alternating cycles for R = 0.1 and around 20 alternating cycles for R = 0.7. Load interaction testing showed that when the crack driving force is near KIC the overload size greatly influences whether acceleration or retardation will occur at 982°C. Semi-realistic spectrum testing demonstrated the extreme sensitivity that relative loading levels play on fatigue crack growth life while also calling into question the importance of dwell times. A crack trajectory modeling approach using blade primary and secondary orientations was used to determine whether crack propagation will occur on crystallographic planes or normal to the applied load. Crack plane determination using a scanning electron microscope enabled verification of the crack trajectory modeling approach. The isothermal constant amplitude fatigue crack growth results fills a much needed void in currently available data. While the temperature and load interaction fatigue crack growth results reveal the acceleration and retardation that is present in cracks growing in single crystal turbine blade materials under TMF conditions. This research also provides a deeper understanding of the failure and deformation mechanisms responsible for crack growth during thermo-mechanical fatigue. The crack path trajectory modeling will help enable "Retirement for Cause" to be used for critical turbine engine components, a drastic improvement over the standard "safe-life" calculations while also reducing the risk of catastrophic failure due to "chunk liberation" as a function of time. Leveraging off this work there exists the possibility of developing a "local approach" to define a crack growth forcing function in single crystal superalloys.

The primary goals of the research in this dissertation are to perform a systematic study to identify and understand the fundamental cause of prolonged processing time during transient liquid phase bonding of difficult-to-bond single crystal Ni-base materials, and use the acquired knowledge to develop an effective way to reduce the isothermal solidification time without sacrificing the single crystalline nature of the base materials. To achieve these objectives, a multi-scale numerical modeling approach, that involves the use of a 2-D fully implicit moving-mesh Finite Element method and a Cellular Automata method, was developed to theoretically investigate the cause of long isothermal solidification times and determine a viable way to minimize the problem. Subsequently, the predictions of the theoretical models are experimentally validated. Contrary to previous suggestions, numerical calculations and experimental verifications have shown that enhanced intergranular diffusivity has a negligible effect on solidification time in cast superalloys and that another important factor must be responsible. In addition, it was found that the concept of competition between solute diffusivity and solubility as predicted by standard analytical TLP bonding models and reported in the literature as a possible cause of long solidification times is not suitable to explain salient experimental observations. In contrast, however, this study shows that the problem of long solidification times, which anomalously increase with temperature is fundamentally caused by departure from diffusion controlled parabolic migration of the liquid-solid interface with holding time during bonding due to a significant reduction in the solute concentration gradient in the base material. Theoretical analyses showed it is possible to minimize the solidification time and prevent formation of stray-grains in joints between single crystal substrates by using a composite powder mixture of brazing alloy and base alloy as the interlayer material, which prior to the present work has been reported to be unsuitable. This was experimentally verified and the use of the composite powder mixture as interlayer material to reduce the solidification time and avoid stray-grain formation during TLP bonding of single crystal superalloys has been reported for the first time in this research.

The field of low-dimensional quantum magnets has received lot of attention owing to the possibility of studying phenomena associated with the quantum nature of matter. Many materials that realize low-dimensional spin arrangements in their structure have been synthesized in the past twenty years due to the emergence and development of crystal growth techniques. These materials have been studied using various experiments in order to explore the wide range of interesting properties predicted theoretically for low-dimensional systems. In this pursuit, novel properties have been observed and many open questions have been raised. One such property that is typically observed in many low-dimensional quantum magnets is heat transport via magnetic excitations. Large magnitudes of magnetic heat conductivity has been found experimentally in materials belonging to this class in addition to the conventionally known phononic heat conduction, and interesting theoretical predictions like the divergence of heat conductivity in certain spin models exist, that have stimulated research in this field. This experimental work mainly deals with the crystal growth and heat transport properties of low-dimensional quantum magnets that include one-dimensional (1D) spin chain systems Sr$_2$CuO$_3$ and SrCuO$_2$, two-dimensional (2D) Heisenberg antiferromagnet La$_2$CuO$_4$, and a five-leg spin ladder La$_8$Cu$_7$O$_{19}$, with a view to understand propagating low-energy magnetic excitations and their interaction amongst themselves, other quasiparticles and impurities present in the systems. These interactions result in scattering processes that govern the magnitude and temperature dependence of heat conductivity. In spite of considerable theoretical and experimental work in the field of heat transport, a complete understanding of the scattering mechanisms is lacking. The work tries to add to the experimental knowledge about magnetic heat transport in such systems and presents cases which motivate the need for theoretical understanding of aspects of heat transport. The focus of this work was twofold. One part focusses on the single crystal growth using the travelling-solvent floating zone (TFSZ) method of materials which realize low-dimensional spin systems in their structure. The TFSZ method is indispensable for growing large single crystals of extraordinary purity, which can be used for investigations using neutrons and other techniques like heat conductivity measurements that probe anisotropic properties. The other part deals with the experimental results on heat transport and other thermodynamic properties of these materials. In order to study the behaviour of the magnetic heat conductivity at high temperatures, and the effect of small amount of magnetic and non-magnetic impurities on the heat transport of 2D Heisenberg antiferromagnet La$_2$CuO$_4$, single crystals of pure La$_2$CuO$_4$, and Ni- and Zn-doped versions, La$_2$Cu$_x$Ni$_{1-x}$O$_4$ and La$_2$Cu$_x$Zn$_{1-x}$O$_4$ for $x$ = 0.001 and 0.003, were grown using the TFSZ method. Heat transport in the pure compound was experimentally investigated for the first time up to very high temperatures of 813 K using two methods, namely the steady state method for low temperatures and the dynamic flash method for measuring high temperature conductivity. Analysis of the magnon mean-free path using empirical models based on semi-classical theories, and qualitative comparison to theoretical calculations seems to suggest that scattering between magnons might play an important role in addition to scattering of magnons with phonons and defects, and that the spin-spin correlation length could be crucial in limiting the mean free path of magnons at high temperatures. These experimental results and indications of probable scattering mechanisms based on non-rigorous analyses and comparisons, strongly motivate the need for theoretical studies. Heat conductivity measurements on the Ni- and Zn- doped versions of La$_2$CuO$_4$ are still incomplete and inconclusive, and hence have not been reported in this work. Heat transport experiments on Ni- and Ca-doped Sr$_2$CuO$_3$ were performed, with a motivation to investigate the role of disorder induced by impurities lying within the spin chains (Ni) and those lying outside the spin chains (Ca), on the heat transport in this system. In both the cases, the magnetic heat transport is observed to be strongly suppressed upon doping. Empirical analysis of the data seems to suggest that in the temperature regime of 100-300 K, the temperature dependence of the mean-free path of magnetic excitations for the Ni- and Ca-doped samples can be described by scattering with defects (Ni and Ca impurities) and phonons alone. However, surprisingly, a strong increase of phononic conductivity is observed perpendicular and parallel to the spin chains of the Ni-doped compounds compared to the pure compounds, whose explanation seems to lie in the existence of an additional dissipative scattering mechanism present in the pure compounds that is lifted upon doping, possibly due to the presence of a spin gap in the doped compounds. The effect of Ni on the Sr$_2$CuO$_3$ and SrCuO$_2$ was also investigated by studying the low energy regime of the spin excitation spectrum using other microscopic probes like nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS). Large single crystals of SrCu$_x$Ni$_{1-x}$O$_2$, with $x$ = 0.01 were grown and used in these experiments that observed the presence of a spin gap in the Ni-doped sample. Further theoretical investigations are however required to understand the possible role of the spin gap in influencing the spin-phonon scattering mechanism, and its relevance to the observed enhancement in phononic conduction. Although we observe that in the case of both 1D and 2D systems, a semi-classical kinetic model for heat transport along with empirical models of scattering processes describe the temperature dependence of the measured heat conductivity surprisingly well in the temperature regime up to 300 K and 800 K respectively, interpretations based on these analyses must be treated as only preliminary, and as a step towards understanding microscopically the scattering mechanisms involved in low-dimensional systems such as the ones discussed in this work. In the direction of exploratory research towards synthesis of novel low-dimensional materials, two cuprate compounds were synthesized in the form of single crystals using the floating zone method for the first time, namely, a five leg $S=tfrac{1}{2}$ antiferromagnetic spin ladder compound La$_8$Cu$_7$O$_{19}$ and an insulating delafossite LaCuO$_{2}$. A bulk 3D antiferromagnetic ordering is observed in La$_8$Cu$_7$O$_{19}$. Heat conductivity of La$_8$Cu$_7$O$_{19}$ is observed to be purely phononic and no contribution from magnetic excitations seem to exist, although the measurements indicates that there is a large anisotropy in heat transport. However, detailed diffraction experiments using x-rays and neutrons indicate that both the crystal and magnetic structures are complicated, and that the details of the structure prevent La$_8$Cu$_7$O$_{19}$ from being a perfect realization of a five-leg spin ladder.

No description available.

We have synthesized dimethyl derivatives of the GFP chromophore (p-HOBDI) and several of its derivatives, and their photochemistry and photophysics were investigated using various steady-state and time-resolved techniques as follows. We first consider the effect of the £]-barrel on the optical properties of the GFP chromophore (p-HOBDI) experimentally by selective variation of the protonation state of chromophores and different solvents. Each of these forms shows a complex solvatochromic behavior and is governed by both polar and acid/base properties of the solvents. In contrast to their solution behavior, some O-alkyl GFP chromophore (alkoxy-BDI) derivatives exhibit large fluorescent enhancement in the solid state. The color of the crystalline BDI is tuned by substituent-mediated crystal packing, showing the potential applications in optoelectronic devices. Using femtosecond polarization-sensitive infrared (IR) spectrosceopy of the C=O stretching mode of the HOBDI, we have then discovered a near complete twisting around the ethylenic bridge between the phenolate and imidazolidinone groups upon electronic excitation. Cis/trans isomerization induced by the rotation around the bridge is thought to be responsible for the behavior of blinking in fluorescent protein; however, the mechanism of the thermal reverse isomerization is more problematic. Thus we synthesized BDI derivatives with decreasing para-donating ability, HO, CH3O, CH3, H, and Cl, and used a Hammett plot for the rate study. With a positive â value, we conceived, for the first time, a novel nucleophilic addition/elimination mechanism. Finally, the GFP chromophore falls into the general category of hydroxyarene photoacids, which exhibit high excited-state acidities but neutral ground states. A hydroxyl substituent at the meta position shows enhanced charge transfer and greater acidity in the excited state. As a result, we have demonstrated that the fast quenching of the excited state by internal conversion to the ground state is much slower in meta- than in para-HOBDI derivatives. This allows studies of this ultrafast intermolecular ESPT that competes with isomerization. The photoinduced dynamics of the meta isomer of GFP chromophore was further investigated using femtosecond transient absorption and fluorescence upconversion spectroscopies.

In this dissertation the work done in the projects KME-410/502 will be presented.The overall objective in these projects is to evaluate and develop tools for designingagainst fatigue in single-crystal nickel-base superalloys in gas turbines. Experimentshave been done on single-crystal nickel-base superalloy specimens in order toinvestigate the mechanical and fatigue behaviour of the material. The constitutivebehaviour has been modelled and veried by FE-simulations of the experiments.Furthermore, the microstructural degradation during long-time ageing has been investigatedwith respect to the material's yield limit. The eect has been includedin the constitutive model by lowering the resulting yield limit. Moreover, the fatiguecrack initiation of a component has been analysed and modelled by using acritical plane approach in combination with a critical distance method. Finally, asan application, the derived single-crystal model was applied to all the individualgrains in a coarse grained specimen to predict the dispersion in fatigue crack initiationlife depending on random grain distributions. This thesis is divided into three parts. In the rst part the theoretical framework,based upon continuum mechanics, crystal plasticity, the critical plane approachand the critical distance method, is derived. This framework is then used in thesecond part, which consists of six included papers. Finally, in the third part, detailsof the used numerical procedures are presented.

Σκοπός της παρούσας διπλωματικής εργασίας είναι η μελέτη της επίδρασης της γωνίας πρόσπτωσης της ηλιακής ακτινοβολίας στη λειτουργία φωτοβολταϊκών κυττάρων, σε περιοχές μεσαίου γεωγραφικού πλάτους. Συγκεκριμένα μελετήθηκε η επίδραση της γωνίας αυτής στη λειτουργία δύο ΦΒ πλαισίων ενός μονοκρυσταλλικού πυριτίου (sc-Si) και ενός δισεληνοϊδιούχου χαλκού (CIS). Τα πλαίσια τοποθετήθηκαν σε ταράτσα κτηρίου του Τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας υπολογιστών του Πανεπιστημίου Πατρών. Οι μετρήσεις ξεκίνησαν τον Οκτώβριο του 2009 και ολοκληρώθηκαν το Σεπτέμβριο του 2010. Τα ΦΒ πλαίσια είχαν νότιο προσανατολισμό δεδομένου ότι τοποθετήθηκαν σε τόπο του βόρειου ημισφαιρίου με γεωγραφικό πλάτος 38ο 32’ . Κατά την πειραματική διαδικασία οι μετρήσεις γίνονταν σε πολλαπλές κλίσεις κάθε φορά και για τα δύο πλαίσια . Μετρήθηκαν το ρεύμα βραχυκύκλωσης, η τάση ανοιχτοκυκλώσεως, η θερμοκρασία πλαισίου και η ηλιακή ακτινοβολία. Υπολογίστηκε ο βαθμός απόδοσης , ο συντελεστή ποιότητας και η γωνία πρόσπτωσης, ανά κλίση ΦΒ πλαισίου. Υλοποιήθηκε αλγόριθμος για τον θεωρητικό υπολογισμό της βέλτιστης γωνίας κλίσης έτσι ώστε η γωνία πρόσπτωσης να γίνεται ελάχιστη ανά χρονική στιγμή (μέγιστης παραγόμενης ενέργειας ) και έγινε σύγκριση της με τα πειραματικά αποτελέσματα. Διαπιστώσαμε πειραματικά την εξάρτηση της παραγόμενης ισχύος από την γωνία πρόσπτωσης της ηλιακής ακτινοβολίας . Επιπλέον μέσω της πειραματικής διαδικασίας υπολογίστηκε η βέλτιστη κλίση πλαισίων ανά εποχή για τόπους γεωγραφικού πλάτους ίδιου με τον τόπο των πειραματικών μετρήσεων . / -

Cette thèse vise à trouver des approches possibles pour l’amélioration des propriétés électromécaniques de monocristaux piézoélectriques à base de KNN. La TSSG et la SSSG sont entreprises afin de faire croître des monocristaux La conclusion de l'aspect de croissance cristalline est: (1) Pour chaque élément pris individuellement, leurs coefficients de ségrégation reposent fortement sur leurs concentrations initiales dans la solution liquide. (2) La compétition entre éléments occupant le même site du réseau est démontrée. (3) Le très faible coefficient de ségrégation de Li dans la matrice KNN est responsable de l'apparition d'une phase secondaire présentant la structure bronze de tungstène quadratique. (4) Les régions optiquement laiteuses observées dans les monocristaux diminuent la réponse électrique et peuvent être réduites par traitement thermique et refroidissement lent. Dans la deuxième partie, nous avons utilisé trois approches pour améliorer le comportement piézo/ferroélectrique des monocristaux à base de KNN. La Ta ou Sb substitution indique qu'une réponse électromécanique améliorée est obtenue lorsque la transition orthorhombique-quadratique est à proximité de la température ambiante. Le traitement thermique sous atmosphère d'O2 pur a conduit au doublement de la valeur du coefficient piézoélectrique et des paramètres ferroélectriques d'un monocristal de (K,Na,Li) (Ta,Nb,Sb)O3. Son coefficient piézoélectrique à la température ambiante, qui constitue un record mondial à l’heure actuelle vis-à-vis de ce qui est reporté dans la littérature internationale, vaut 732 pC/N. La troisième approche consiste au dopage des monocristaux de (K,Na,Li)(Ta,Nb)O3 avec Mn. / The thesis aims to find possible approaches for improved electromechanical properties in KNN-based piezoelectric single crystals. Both submerged-seed and top-seeded solution growth techniques were employed to produce single crystals. Conclusions from the crystal growth aspect are: (i) For individual elements, segregation coefficients highly rely on the initial concentration in the liquid solution. (ii) A competition between elements occupied on the same lattice site was found. (iii) The very low Li segregation coefficient in the KNN matrix is responsible for the occurrence of a secondary phase with the tetragonal tungsten bronze structure. (iv) Observed optically-cloudy regions in as-grown crystals decrease the electrical response and can be reduced by thermal treatment with slow cooling. In the second part, we used three approaches to enhance the piezoelectric and ferroelectric behavior of KNN-based single crystals. Ta or Sb substitutions indicates that enhanced electromechanical response is achieved when the orthorhombic-tetragonal phase transition is near room temperature. Thermal treatment in pure O2 atmosphere resulted in a twofold increase of the piezoelectric coefficient and ferroelectric parameters of a (K,Na,Li)(Ta,Nb,Sb)O3 single crystal. The highest room-temperature piezoelectric coefficient in annealed KNN-based single crystals of 732 pC/N was obtained. The third approach, doping with Mn ions in (K,Na,Li)(Ta,Nb)O3 single crystals, is also presented.

Les propriétés en fatigue à grand et très grand nombre de cycles des superalliages base Ni monogranulaires ont été étudiées à 1000°C sous trois fréquences (0.5, 70 et 20000 Hz) et quatre rapports de charge (-1, 0.05, 0.3, 0.8), en prenant le CMSX-4 comme alliage de référence.Dans un premier temps, le régime de fatigue à très grand nombre de cycles a été étudié à 20 kHz. A R = -1, la fissure fatale s’amorce à chaque fois depuis un pore de fonderie en interne et se propage suivant les plans cristallographiques {111}, pour des durées de vie entre 106 et 109 cycles, et ce malgré la présence d’autres défauts tels que les eutectiques ou carbures. Le procédé d’élaboration, en impactant directement la taille des pores de fonderie, contrôle la durée de vie alors que la microstructure a peu d’influence. A R = 0.8, le fluage contrôle la durée de vie en fatigue gigacyclique, et rend alors ce régime sensible à la microstructure et à la composition chimique de l’alliage. Les essais à très longues durées de vie (> 109 cycles) ont également été caractérisés et ont permis de proposer un mécanisme d’amorçage dans ce régime de fatigue.Dans un second temps, l’influence de la fréquence d’essai sur les endommagements dépendants du temps tels que l’oxydation et le fluage, et leur interaction, a été étudiée. A R ≤ 0, diminuer la fréquence d’essai revient à inhiber la criticité des défauts internes en favorisant l’endommagement en surface par oxydation. A R ≥ 0.8, le fluage est l’endommagement conduisant à la rupture des éprouvettes quelle que soit la fréquence de sollicitation ou la durée de vie. Les rapports de charge intermédiaires présentent une interaction importante entre les endommagements en oxydation, fatigue et fluage en fonction de la fréquence de sollicitation et des conditions de chargement. A partir de ces observations, un modèle d’endommagement a été proposé afin de prédire la durée de vie du CMSX-4 et présente des résultats satisfaisants par rapport aux données expérimentales. / High and very high cycle fatigue properties of Ni-based single crystal superalloys have been studied at 1000°C using three frequencies (0.5, 70, 20000 Hz), four stress ratios (-1, 0.05, 0.3, 0.8) and CMSX-4 alloy as reference.Firstly, the very high cycle fatigue regime (VHCF) has been studied at 20 kHz. At R = -1, fatal crack always initiates between 106 and 109 cycles from an internal casting pore et then propagates in a crystallographic mode, despite the presence of other metallurgical defects such as eutectics and carbides. The elaboration process controls VHCF life under these conditions as it directly affects casting pore size, whereas microstructure has no relevant influence. At R = 0.8, creep controls VHCF life and makes this regime sensitive to microstructure and alloy’s chemical composition. Long term tests (> 109 cycles) have also been characterized and have enabled to propose a crack initiation mechanism operating in the VHCF regime.Secondly, the influence of frequency on time-dependent damages such as oxidation and creep, and their interaction, has been studied. At R ≤ 0, reducing frequency inhibits the harmfulness of internal critical defects by promoting oxidation surface damage. At R ≥ 0.8, creep damage leads to specimens rupture for all frequency test and loading conditions. Intermediate stress ratios present an important interaction between oxidation, fatigue and creep damage according to the frequency and loading conditions. Based on these observations, a damage model that predicts CMSX-4 fatigue life has been proposed and presents satisfying results in comparison to experimental data.

Les solutions solides appartenant au système quasi-ternaire BaTiO3-CaTiO3-BaZrO3 (BCTZ) sont des candidates prometteuses pour les piézoélectriques sans plomb. Ce travail de thèse expérimental est consacré à la cristallogenèse et à la caractérisation de différents monocristaux dans le système BCTZ : BaZrO3, CaTiO3 ainsi que les solutions solides Ba1-xCaxTi1-yZryO3 présentant des teneurs en zirconium (Zr) et en calcium (Ca) proches de celles de la composition Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 (BZT-50BCT) où les performances piézoélectriques sont exacerbées.Les monocristaux de CaTiO3 et de BaZrO3 ont été obtenus à la fois depuis une solution à haute température par la méthode du flux et à partir de leur phase fondue par la technique de la zone flottante optique. Dans le cas de CaTiO3 obtenu dans un four à image à 1975°C, l'aluminium (Al), le magnésium (Mg) et le baryum (Ba) ont été détectés comme étant les principales impuretés. Les spectres Raman de CaTiO3 sont en bon accord avec les spectres référencés dans la littérature. La croissance cristalline de BaZrO3 est beaucoup plus difficile à cause de son point de fusion très élevé (2700°C). Le flux BaB2O4 a été utilisé avec succès pour faire croitre des cristaux d’environ 150-200μm à 1350°C, soit à la moitié de son point de fusion. Des boules de BaZrO3 de taille centimétrique ont également été obtenues à partir de la phase fondue en four à image. Les impuretés majoritaires telles le strontium (Sr), l’hafnium (Hf), le calcium (Ca) et le titane (Ti) ont été détectées par GDMS et SIMS dans une gamme de concertation atomique de 0.3-0.5%. L’énergie de gap optique est d'environ 4,8 eV et souligne la grande qualité des cristaux de BaZrO3 obtenus en four à image. Les propriétés diélectriques à basse température de BaZrO3 confirment l'absence de transition de phase structurelle. Les études de Raman révèlent que même si BaZrO3 n'a pas de transition de phase à basse température, il présente une transition de phase cubique-quadratique sous haute pression à 11GPa à température ambiante.Dans la deuxième partie de cette thèse, des monocristaux centimétriques de BCTZ ont été obtenus avec succès par la croissance en flux. Les profils de concentrations en Ca et Zr le long des boules indiquent que leurs coefficients effectifs de ségrégation dépendent fortement de leur concentration initiale dans la solution liquide. Ceux-ci évoluent considérablement au cours du processus de cristallogenèse, rendant ainsi la croissance de BCTZ très délicate en vue d’obtenir des compositions constantes et proches de celles de la région de convergence de phases. De plus, une décomposition spinodale a été mise en évidence, indiquant la coexistence de deux solutions solides de compositions proches au sein des cristaux de BCTZ. Les propriétés diélectriques et piézoélectriques des cristaux obtenus ont été déterminées et présentent des caractéristiques allant du ferroélectrique classique au relaxeur. Les mesures diélectriques montrent notamment une double boucle d'hystérésis (PE) anormale qui disparaît après polarisation. / Solid solutions belonging to BaZrO3–BaTiO3–CaTiO3 (BCTZ) pseudo-ternary system are promising candidates for lead-free piezoelectrics. This thesis aims at growing and characterizing various single crystals of the BCTZ system: the end members BaZrO3 and CaTiO3 as well as Ba1-xCaxTi1-yZryO3 solid solution compounds with zirconium (Zr) and calcium (Ca) contents close to Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 composition (BZT-50BCT) where high piezoelectric performances are expected.CaTiO3 and BaZrO3 single crystals were both grown from high temperature solution by the flux method and from the melt by the optical floating zone technique. In the case of CaTiO3 grown with a mirror furnace at 1975°C, aluminum (Al), magnesium (Mg) and barium (Ba) as main impurities were detected. The Raman spectra of CaTiO3 are in good agreement with the spectra referenced in the literature. The growth of BaZrO3 was more challenging because of its very high melting point (2700°C). BaB2O4 flux was successfully used to produce 150-200 μm-sized BaZrO3 crystals at half its melting point (1350°C) and bulk centimeter-sized BaZrO3 boules were grown from the melt. Sr, Hf, Ca and Ti were detected by GDMS and SIMS as main impurities in the range of 0.3-0.5 at.%. The optical band gap is found to be ~4.8 eV and indicates the high quality of the BaZrO3 crystals grown with mirror furnace. Low temperature dielectric properties of BaZrO3 are displayed and confirmed the absence of structural phase transition. Raman investigations reveal that even though BaZrO3 does not have any phase transition at low temperatures, it exhibits a high-pressure phase transition from cubic to tetragonal at 11GPa at room temperature.In the second part, BCTZ centimeter-sized single crystals have been successfully grown by the top seeded solution growth technique. Ca and Zr content profiles throughout the as-grown boules indicate that their effective segregation coefficients are highly dependent on their initial concentration in the liquid solution. Concentrations evolve substantially during the crystal growth, making the BCTZ crystal growth a tricky issue when a narrow compositions range is targeted, as in the vicinity of the phase convergence region. Furthermore, spinodal decomposition was observed, indicating the coexistence of two solid solutions with close compositions in BCTZ crystals. Dielectric and piezoelectric properties were measured for some crystals, which were found to display a variety of behavior form relaxor to pure ferroelectric. In addition, an abnormal double-like PE hysteresis loop was observed, that was associated to an irreversible effect disappearing upon poling.