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Preparation of Bi-2223/Bi-2310 and Bi-2212/BiFeO 3 composites has been performed so as to obtain some active composites where the superconductive properties of Bi-2223 or Bi-2212 can be modified using a property of the insulating phase. In both cases superconductive percolation was obtained, but only in the case of a Bi-2223 (or Bi-2212)/Bi-2310 composite was no (or very weak) chemical reaction observed during sintering. Since the superconductive percolation threshold was obtained in this case for a concentration of Bi-2223 or Bi-2212 lower than 20%, a special composite model had to be considered. The Bi-2212/BiFeO 3 system is very interesting due to the ferroic properties of BiFeO 3 . Superconductive percolation was achieved with this mixture too, but with more difficulties because of a chemical reaction occurring between the two compounds during sintering. A study of the properties of these composites has been performed and will be briefly described in what follows. The good quality of the Bi-2223(or Bi-2212)/Bi-2310 composite enables us to expect some future applications.

The influence of MgO addition on the structure and properties of mullite prepared through reaction sintering of Algerian kaolin and Al 2 O 3 was investigated. The raw powders were wet ball milled, dried and cold compacted using a uniaxial press. The green compacts were sintered 8 hours at 1600 and 1650°C. The microstructure of samples was characterized using a scanning electron microscope. Mechanical and thermal properties were characterized using Vicker's hardness tester, a universal testing machine and a dilatometer. It was found that the increase of MgO content from 0 to 3 wt-% increased the hardness of samples sintered 8 hours at 1600°C from 1039 to 1316.57 HV. Also, the increase of MgO content in samples sintered 8 hours at 1600 and 1650°C increased the compressive strength up to a maximum then decreased it. For a sintering temperature of 1600°C, the increase of MgO content up to 2 wt-% increased the flexural strength, but a further increase of MgO to 3 wt-% decreased it again, while for a sintering temperature of 1650°C, the increase of MgO content from 0 to 3 wt-% increased the flexural strength from 103.45 to 472.25 MPa. Amongst MgO containing samples, the increase of MgO content increased the coefficient of thermal expansion; however, it remained lower than the coefficient of thermal expansion of the sample without MgO addition.

Kaolin DD1 (from Djebel Debbagh, Guelma, Algeria), kaolin Tamazarte KT (from Jijel, Algeria) and magnesium oxide MgO mixtures with relative weight ratios of 59/29/12 have been studied in this work in order to obtain cordierite ceramics with the stoichiometric formula (2MgO•2Al2O3•5SiO2), by mixing raw materials abundant in Algeria. Thermogravimetric analysis and differential thermal analysis were carried out on powder from room temperature to 1300 • C, at heating rates from 10 to 50 • Cmin −1. The phases and their transformations were determined by X-ray diffraction for the mixture of kaolin (59 wt.% DD1 + 29 wt.% KT) and 12 wt.% of magnesium oxide powder treated at different temperatures between 900 and 1250 • C for 2 h. The activation energies of α-cordierite formation calculated from isothermal data were around 372 kJmol −1. The average Avrami parameter is found to be around 1.95 for each heating rate.

Thermal reactions and sintering behavior of kaolin DD3 (Djebel Debbagh, Algeria) and CaO mixtures to obtain dense anorthite ceramics were investigated. Mixed powders were uniaxially pressed and fired between 850 and 1150• C. Firing the pressed specimens yielded a dense anorthite ceramics. The sintered density increased with increase of CaO content and reached the maximum value of 2.57 g/cm 3 for the composition containing 10 wt% CaO and fired at 1150• C. Their coefficient of linear expansion of the sintered samples at 1100• C decreases with the addition of CaO. X-ray diffraction experiments carried out on the samples containing varied amount of CaO and fired at the temperatures higher than 1000• C for 2 h showed the presence of only anorthite phase.