Role of impurities on the spark plasma sintering of ZrCx–ZrB2 composites

The study of the spark plasma sintering (SPS) of ZrCx and ZrCx–ZrB2 composites was carried out considering the effect of experimental parameters such as the applied load, the temperature and the heating time. In addition, the role of the main impurities, detected in the raw materials, on the sintering behaviour has been explored by Transmission Electron Microscopy (TEM). The analyses of monoliths and composites showed up the liquid phase formation from silica impurities and the complete structural reorganisation of free carbon into the graphite form during the sintering treatment. It is also shown that within composites, the plastic strain is preferentially accommodated by ZrB2 crystals

Reinvestigation of Phase Transitions in Na0.5Bi0.5TiO3 by TEM. Part II: Second Order Orthorhombic to Tetragonal Phase Transition

International audienceNBT presents an orthorhombic to tetragonal second order phase transition that occurs near 320 °C. It corresponds to the so-called antiferroelectric−paraelectric phase transition. A model is presented in which the diffuse phase transition is achieved by the progressive canceling of the antiphase octahedra tilting prevailing within a−b+a− orthorhombic structure, whereas the in-phase tilting is maintained. This transformation gives finally rise to the a0a0c+ octahedra tilting system of the tetragonal phase. Electron diffraction experiments show that a long-range ordering occurs within the tetragonal phase. It probably develops as P42/mnm ordered nanoregions disseminated within a P4/mbm disordered matrix. The order is still visible in the temperature domain of the cubic phase

A transmission electron microscopy study of the A-site disordered perovskite Na0.5Bi0.5TiO3

International audienceA transmission electron microscopy study of Na0.5Bi0.5TiO3 (NBT) crystals shows two types of ferroelectric domains characterized by interface boundaries lying in the {110}C and {100}C planes. A one-dimensional {100}C modulated texture is also observed locally. The electron diffraction study reveals the presence of (001)T tetragonal platelets a few cells thick within the R3c matrix. They develop within the three basal {100}C planes of the prototype phase and represent relics of the high-temperature tetragonal phase. The loss of symmetry compared to the average structure of NBT can thus clearly be attributed to the existence of these tetragonal platelets. The {100}C modulated texture corresponds to a modulation of strain induced by the coexistence of two types of octahedra tilting systems: aaa for the rhombohedral matrix and a0a0c+ for the tetragonal platelets. These (001)T platelets indirectly intervene in the relaxation process classically encountered at about 230 C, in marked contrast to the behaviour of Pb(Mg1/3Nb2/3)O3

The structuraloriginoftheantiferroelectricpropertiesandrelaxorbehavior of Na0.5Bi0.5TiO3

International audienceThis workpresentsastudyofNa0.5Bi0.5TiO3 (NBT) bytransmissionelectronmicroscopyinthe20–370 1C temperaturerange.Aneworthorhombicintermediatephasebetweentherhombohedralandthe tetragonalphasesisproposedtoaccountfortheoccurrenceof 12 (oee) superstructurespots.Thephase transition fromtherhombohedraltotheorthorhombicphaseoccursviaamodulatedphaseformedby rhombohedral blocksandorthorhombicsheets.Itisshownthattheselatterrepresentrhombohedral (010)twinplanes.Themodulatedphaseisproposedtoexplaintheantiferroelectricandrelaxor behaviorsofNBT

Na-Bi long rage ordering Na0.5 Bi0.5 TiO3

At ambient temperature, Na0.5Bi0.5TiO3 (NBT) presents a rhombohedral structure with the polar R3c space group. In the temperature domain of 200-300°C, it turns to an antiferroelectric Pnma orthorhombic phase before transforming into a paraelectric tetragonal phase at 320°C. The Curie-Weiss law is fulfilled above 400°C attesting of the non-polar character of the tetragonal phase. Up to know, refinements of in-situ temperature powder neutron diffraction carried out on this tetragonal phase were conducted with the polar P4bm space group, which is now admitted as the reference space group for this phase. However, investigations performed by infra-red and raman spectrometry clearly indicate an ordering tendency within NBT relying on A cations (Na and Bi), the ordered structure being then described by the polar P42nm space group. However, such an ordering process was never confirmed by any diffraction experiment and both retained space groups (P4bm and P42nm) are polar, while the NBT is paraelectric at that temperature and then non polar. The first aim of this communication is to reinvestigate the high temperature tetragonal phase by electron diffraction to state upon a possible ordering process taking place in NBT and finally reconsider the space group of the tetragonal phase. The second objective was to propose a crystallographic model to account for the orthorhombic to tetragonal phase transitio

Na-Bi long rage ordering Na0.5 Bi0.5 TiO3

At ambient temperature, Na0.5Bi0.5TiO3 (NBT) presents a rhombohedral structure with the polar R3c space group. In the temperature domain of 200-300°C, it turns to an antiferroelectric Pnma orthorhombic phase before transforming into a paraelectric tetragonal phase at 320°C. The Curie-Weiss law is fulfilled above 400°C attesting of the non-polar character of the tetragonal phase. Up to know, refinements of in-situ temperature powder neutron diffraction carried out on this tetragonal phase were conducted with the polar P4bm space group, which is now admitted as the reference space group for this phase. However, investigations performed by infra-red and raman spectrometry clearly indicate an ordering tendency within NBT relying on A cations (Na and Bi), the ordered structure being then described by the polar P42nm space group. However, such an ordering process was never confirmed by any diffraction experiment and both retained space groups (P4bm and P42nm) are polar, while the NBT is paraelectric at that temperature and then non polar. The first aim of this communication is to reinvestigate the high temperature tetragonal phase by electron diffraction to state upon a possible ordering process taking place in NBT and finally reconsider the space group of the tetragonal phase. The second objective was to propose a crystallographic model to account for the orthorhombic to tetragonal phase transitio

A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-)carbide powders

International audienceZirconium oxycarbide powders with controlled composition ZrC0.94O0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100 MPa). The densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (Ea) from the densification rate law. It was concluded that at low macroscopic applied stress (25 MPa), an intergranular glide mechanism (n 6 2) governs the densification process, while a dislocation motion mechanism (nP 3) operates at higher applied load (100 MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687-774 kJ mol1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for densification

Étude des mécanismes de réduction carbothermique d'oxydes métalliques du groupe IV par MET (Apport de précurseurs de type polymères de coordination)

Cette thèse est dédiée à l'étude des mécanismes réactionnels mis en jeu lors de la formation d'oxycarbures élaborés par différentes voies de synthèse. Elle revisite tout d'abord la littérature qui était jusque là plutôt consacrée à des approches thermochimiques et cinétiques. Ce travail original concerne une étude structurale et chimique menée principalement par microscopie électronique à transmission (MET) et par diffraction des rayons X (DRX) sur des échantillons obtenus à des degrés d'avancement variables de réactions. La première partie de cette étude est consacrée aux voies classiques de transformation carbothermique mettant en jeu des dioxydes (TiO2, ZrO2, HfO2) réduit en oxycarbures au contact de noir de fumée. Nous montrons que les mécanismes réactionnels mis en jeux lors de la déstabilisation des phases parents sont intrinsèquement liés à la structure des dioxydes et que la taille initiale des cristallites d oxydes n a pas d influence sur celle des oxycarbures formés. Le site de nucléation de l'oxycarbure ainsi que les mécanismes de croissance ont également été mises en évidence et discutés. Une seconde partie du manuscrit est consacrée à une étude originale visant à synthétiser des carbures à partir de polymères de coordination (MOF pour Metal-Organic Frameworks). La transformation progressive des précurseurs en oxycarbure est étudiée en détail et nos résultats montrent que les mécanismes mis en jeu lors de cette transformation s'apparentent beaucoup à ceux mis en évidence dans le cadre des réactions carbothermiques classiques.This thesis is dedicated to the study of the reactional mechanisms involved in the formation of oxycarbides by different synthesis routes. It reconsiders the literature that used to deal with thermochemical and kinetical approaches. This original work deals with a structural and chemical study mainly led by transmission electron microscopy (TEM) and X-ray diffraction (XRD) on samples obtained at different advancement rates. The first part of this study is about the classical carbothermal reduction transformations of dioxides (TiO2, ZrO2, HfO2) reducted by carbon black into oxycarbide. We highlight that the reactional mechanisms involved into the parent phases destabilization are intrinsically linked to the dioxides structure and that the initial size of oxide particles has no influence on the ones of the oxycarbides. The nucleation site of the oxycarbide and the growth mechanisms have also been highlighted and discussed. A second part of the manuscript is dedicated to an original study aiming to synthesize carbide from Metal-Organic Frameworks (MOF). The progressive transformation of the precursors into oxycarbides is very likely to the ones highlighted in the field of classical carbothermal reactions.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Transmission electron microscopy study of the reaction mechanisms involved in the carbothermal reduction of anatase

International audienceThe carbothermal reduction of anatase in titanium oxycarbide was studied by transmission electron microscopy (TEM). This study emphasizes that the reaction proceeds through complex solid-gas equilibria involving three main steps. During the first step, the carbonmonoxide- enriched atmosphere prevailing within the furnace provokes the direct transformation of anatase into the Magne'li phase. This transformation is accompanied by abnormal oxide grain growth showing a high number of stacking defects. The ordering tendency of such defects and their progressive increasing density lead to the final Ti3O5 compound. The second step of the reaction concerns the destabilization of Ti3O5 and carbon black in a high pCO atmosphere to form the oxycarbide. The titanium oxycarbide (TixOyCz) nucleates in the carbon black aggregates, giving rise to a first generation of defect-free faceted crystals (automorphous habit) characterized by a constant chemical composition. During the third step, Ti3O5 is missing and the primary carbide enters a maturation step characterized by carbon enrichment, attested by an increase in its cell parameter. This third step is characterized by the recrystallization phenomenon of the primary oxycarbide into the secondary one with an abrupt change in stoichiometry. The latter is characterized by rounded crystals (xenomorphous habit) bearing either a high density of dislocations or geometrical internal porosity linked to the migration of vacancies through the Kirkendall effect. Its further enrichment in carbon is assumed to be assisted by dislocation motion