Christine Bogicevic

Preparation of transparent oxyapatite ceramics by combined use of freeze-drying and spark-plasma sintering

Lanthanum silicate oxyapatites, ion-conducting materials presenting a strong aversion against densification, have been obtained in the form of dense transparent ceramics, by combining the beneficial use of freeze-drying and spark plasma sintering methods.

Demineralized dentin 3D porosity and pore size distribution using mercury porosimetry.

OBJECTIVES The objectives of this study were to assess demineralized dentin porosity and quantify the different porous features distribution within the material using mercury intrusion porosimetry (MIP) technique. We compared hexamethyldisilazane (HMDS) drying and lyophilization (LYO) (freeze-drying) in sample preparation. METHODS Fifty-six dentin discs were assigned into three groups. The control (CTR) group discs were superficially acid-etched (15s 37% H(3)PO(4)) to remove the smear layer and then freeze-dried whereas LYO and HMDS groups samples were first totally demineralized using EDTA 0.5M and then freeze-dried and HMDS-dried respectively. MIP was used to determine open porosity and pore size distribution of each pair of samples. Field emission scanning electron microscopy (FESEM) was used to illustrate the results. RESULTS The results showed two types of pores corresponding either to tubules and micro-branches or to inter-fibrillar spaces created by demineralization. Global porosity varied from 59% (HMDS-dried samples) to 70% (freeze-dried samples). Lyophilization drying technique seems to lead to less shrinkage than HMDS drying. FESEM revealed that collagen fibers of demineralized lyophilized samples are less melted together than in the HMDS-dried samples. SIGNIFICANCE Demineralized dentin porosity is a key parameter in dentin bonding that will influence the hybrid layer quality. Its characterization could be helpful to improve the monomers infiltration.

Dielectric evidences of core-shell-like effects in nanosized relaxor PbMg1∕3Nb2∕3O3

Temperature and frequency dependence of dielectric permittivity in nanotextured ceramics of PbMg1∕3Nb2∕3O3 has been performed in a wide range of grain sizes and shown to display strong variation. In particular, frequency relaxation follows the Vogel Fulcher law for the high grain sizes, whereas for intermediate sizes, two Debye processes are evidenced. This behavior as well as a diminution of the maximum of the real dielectric constant emax could be explained within the mechanism of relaxation based on the existence of polar nanoregions with a core-shell structure for each grain.

Original Reaction Sequence of Pb(Yb1/2Nb1/2)O3-PbTiO3: Consequences on Dielectric Properties and Chemical Order

The solid solution [Pb(Yb1/2Nb1/2)O3]1−x-[PbTiO3]x was synthesized with , using several high-temperature techniques as well as room-temperature mechanosynthesis. The high-temperature synthesis reveals a reaction path involving the synthesis first of the end-members before the solid solution. The density and dielectric constant measured on the ceramics prepared from these powders indicate the crucial role of the synthesis technique in the subsequent properties. Mechanosynthesis results in ceramics with higher density and dielectric constant. Identical optimized sintering conditions were then applied to all investigated compositions and the resulting dielectric properties and chemical orders were compared. All polar orders (antiferroelectricity, ferroelectricity, and relaxor behavior) were evidenced. The 1 : 1 chemical order on the B-site of Pb(Yb1/2Nb1/2)O3 results in the formation of a double perovskite Pb2YbNbO6, and the superstructures in the X-ray diagrams signing the existence of this order persist up to 30% PbTiO3. The underlying mechanism for substitution of Yb or Nb by Ti is presented.

Synthesis dependent characteristics of Sr{sub 1-x}Mn{sub x}TiO{sub 3} (x=0.03, 0.05, 0.07 and 0.09)

Sr1-xMnxTiO3 (where x=0.03, 0.05, 0.07 and 0.09) was synthesized via different routes that include solid-state, oxalate precipitation and freeze drying. In oxalate precipitation technique, compositions corresponding to 3 and 5 mol% doping of Mn were monophasic whereas the higher compositions revealed the presence of the secondary phases such as MnO, Mn3O4 etc., as confirmed by high resolution X-ray diffraction (XRD) studies. The decomposition behavior of the precursors prepared using oxalate precipitation method corresponding to the above mentioned compositions was studied. Nanopowders of compositions pertaining to 5 to 9 mol% of Mn doping were obtained using freeze-drying technique. The average crystallite size of these nanopowders was found to be in the 35 to 65 nm range. The microstructural studies carried out on the sintered ceramics, fabricated using powders synthesized by different routes established the fine grained nature ( < 1 mu m) of the one obtained by freeze drying method. Raman scattering studies were carried out in order to complement the observations made from XRD regarding the phase purity. The dielectric properties of the ceramics obtained by different synthesis routes were studied in the 80-300 K temperature range at 100 kHz and the effect of grain size has been discussed