Philippe Boch

Preparation of α- and β-tricalcium phosphate ceramics, with and without magnesium addition

Abstract α- and β-tricalcium phosphate (TCP) ceramics were prepared using various processing routes. The high-temperature limit of the stability domain of β-TCP is increased by magnesium addition (up to 5 wt% of equivalent MgO). Impregnation of pre-sintered, porous skeletons of TCP with a Mg-containing solution is an easy way to prepare Mg-TCP. Mg-chloride and Mg-sulfate lead to anionic residues which can combine with TCP, whereas Mg-acetate and Mg-nitrate lead to materials free from anionic residues. Reaction-sintering of hydroxylapatite and dicalcium phosphate mixtures impregnated with a solution of either Mg-acetate or Mg-nitrate is, therefore, an interesting route for preparing Mg-TCP ceramics with a controlled α-toβ ratio.

Microstructural aspects in a polymer-modified cement

Scanning electron microscopic observations of polymer-free and polymer-modified cements have shown that the polymer particles are partitioned between the inside of hydrates and the surface of anhydrous cement grains. Differential thermal analysis, thermogravimetric analysis, and porosimetry experiments show the retardant effect of the polymer and provide information on changes in porosity.

Densification and thermal conductivity of low-sintering-temperature AlN materials

Abstract The aim of the study was the sintering of aluminium nitride at rather low temperatures (≤1650°C). Various sintering aids were studied, in particular AlSi- and Ca-based oxides. CaCO3 and 3CaO-3SiO2-Al2O3 are the most efficient additives, because quantities as low as 0·5 wt% are enough to yield sintered AlN substrates with no open porosity. TEM observations showed that low concentrations of CaCO3 or CaSiO3 lead to location of the secondary phases at the triple points, whereas high concentrations lead to wetting of the grains. Thermal diffusivity of sintered materials was determined by a laser flash method. The highest conductivity (70 W m−1 K−1) is obtained with CaCO3 additions. These additions promote liquid formation, which cleans the surface of the AlN grains from oxygen and thereby prevents oxygen from entering the AlN lattice. The CaAl2O4 compound exhibits a lower thermal conductivity than the CaAl4O7 and Ca3Al10O18 compounds. The Ca2SiAl2O7 compound and the 27R AlN polytype have a strong negative influence on the thermal conductivity of sintered materials.

Influence of organic components on properties of tape-cast aluminum nitride substrates

Abstract The influence of organic phases on the properties of green tapes and sintered substrates made of aluminum nitride was studied. Substrates were processed by tape casting, thermocompression, burning out of organic components and sintering. Cracking during drying and delamination during thermocompression are sensitive to the inorganic to organic and binder to plasticizer ratios. The porosity of green tapes and the apparent density decrease when the organic content increases. The apparent density increases whereas the porosity and the tensile strength of green tapes decrease when the plasticizer content increases.

Sintering behaviour of CuO-doped SnO2

Abstract The sintering behaviour of CuO-doped SnO 2 ceramics was studied. Additive-free SnO 2 materials do not densify after heat treatment to 1400°C, whereas materials doped with 0.5–0.75 wt.% CuO densify to 98% of theoretical. Copper oxides (CuO/Cu 2 O) experience reduction and re-oxidation phenomena. There is no evidence of high vaporisation of SnO 2 , nor of liquid formation at low temperature ( 2 system. This suggests that CuO acts by grain-boundary mechanisms.

Sintering of TCP–TiO2 biocomposites: influence of secondary phases

TCP-TiO2 ceramic biocomposites with various alpha-to-beta TCP ratios can be prepared by quenching the alpha phase. The presence of dopants (Ca, P, or Na) leads to the precipitation of secondary phases, which decreases the densification of titania. In the system TCP-TiO2, there is a eutectic with a composition of 63 wt% TCP-37 wt% TiO2 at a temperature of 1380 degrees C.

Speciation of cadmium in cement. Part I. Cd2+ uptake by C-S-H

Abstract Hazardous cadmium can be trapped in C-S-H, the main “phase” in Portland cement. Two kinds of Cd-containing calcium silicates were synthesized: Cd/Ca silicate hydrates prepared by coprecipitation and Cd-exchanged C-S-H. In Cd-exchanged C-S-H, chemical and structural studies (ICP-AES, XRD, extended X-ray absorption fine structure (EXAFS), and nuclear magnetic resonance (NMR)) show that up to 30 wt.% Cd uptake is possible, with little structural change. XRD patterns and 29Si magic angle spinning (MAS) NMR spectra are similar in Cd-free and Cd-containing C-S-H. Cd-EXAFS shows that the majority of Cd2+ atoms are homogeneously distributed within the C-S-H structure, although 113Cd cross-polarization (CP) MAS NMR cannot discriminate between Cd atoms in main layer location and Cd atoms in interlayer location. In Cd/Ca silicate coprecipitates, the structure is nearly amorphous and the silicate species do not polymerize to a C-S-H-like structure.

Tribological and interfacial phenomena in Al2O3/SiC and SiC/SiC couples at high temperature

Abstract The dry-friction behaviour of two ceramic-to-ceramic tribological couples is shown to be very sensitive to temperature. This is associated with modifications in the rheological properties of the wear debris which form the ‘third body’. One ‘velocity accommodation mechanism’ is mainly involved here. It is the formation of small rolls, composed of agglomerated debris. These rolls act as minute roller-bearings, which leads to a decrease in the coefficient of friction, and therefore a reduction of the cracking of the wear track. Rolls develop at medium temperature only, and their morphology depends on various parameters, in particular the loading force and the sliding distance.

Micropore size analysis by NMR in hydrated cement.

The understanding of the microstructure of cement remains incomplete. Especially, the progressive setting of the material is still unclear. Micropore size distribution (microstructure) has been investigated by both standard proton nuclear magnetic relaxation (1H-NMR) and field-cycling relaxation in C3S hydrated paste. The non-exponential decay was interpreted as a distribution of discrete relaxation rates. The attribution of T1 is supported by both a spectral and a dispersion curve analyses. These experiments allow us to follow the structuration of the material during setting.

Mullitization and densification of (3Al2O3 + 2SiO2) powder compacts by microwave sintering

Abstract Reaction sintering of (3Al 2 O 3 + 2SiO 2 ) powder compacts was studied using either a conventional electric furnace or a 2·45 GHz microwave furnace. Special attention was paid to temperature measurement within the microwave furnace. The reality of a microwave effect that accelerates the kinetics and therefore, decreases the temperatures of mullitization and densification, remains uncertain. In any case, such an effect does not exceed ≈ 50 °C, which is in the order of the temperature gradient between the core and the surface of microwave heated specimens.