Search Results

You are looking at 101 - 110 of 299 items for :

  • "Sintering" x
  • Refine by Access: All Content x
Clear All

Abstract  

Porous ceramics based on Ca3(PO4)2 (TCP) was obtained by sintering of a mixture of hydroxyapatite (HAp) and brushite (CaHPO4·2H2O) powders. The main interaction of HAp + CPP (Ca2P2O7) → TCP type in the composite sample HAp/brushite takes place at temperatures higher than 700 °C and leads to rather uniform porous microstructure. We have suggested that CPP-like phases (and especially K2CaP2O7) undergo partial decomposition accompanied by evaporation of P2O5. The role of KCl—the by-product of solution synthesis of HAp and brushite powder precursors, consists in (i) it replace in part Ca in CPP phase making last one more reactive, (ii) it switches sintering of the ceramics in liquid-phase regime, (iii) at higher temperature it evaporates and, thus, contributes to formation of pores in the ceramics.

Restricted access

Abstract  

Bismuth germanate ceramic powders were synthesized for the first time by the polymeric precursor method (Pechini’s method). Differential thermal analysis and thermogravimetric techniques were used to study the decomposition of the resin precursor, which indicated a suitable calcination temperature at 600 °C. It was observed that the mass loss occurs in two main stages that are associated with two exothermic reactions. The crystalline phases of the powders were inspected by the X-ray diffraction technique after thermal treatment between 300 and 600 °C. Single phase Bi4Ge3O12 ceramic bodies were obtained after sintering at 840 °C for 10 h. The sintered ceramics presented a luminescence band emission centred at around 530 nm when excited with X-rays and UV radiation.

Restricted access

Abstract  

Scheelite (calcium tungstate)is the product of one of the processing methods of wolframite by its roasting with calcium oxide or limestone or its fusion with calcium chloride, followed by acid processing of calcium tungstate with the formation of tungstic acid. Scheelite occurs in contact metamorphic deposits, hydrothermal veins and pegmatites. The present work illustrates a thermal analysis study of synthesis of scheelite by sintering of wolframite with calcite and sintering of tungsten oxide with calcite or calcium oxide using a derivatograph. The reaction products were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA curve of sintering of wolframite with calcite shows the beginning of the reaction at 560C with the formation of scheelite. The intensive formation of scheelite is represented by the medium and wide endothermic peak at 740C. This is followed directly by a large and sharp endothermic peak at 860C, representing the dissociation of unreacted calcite. The DTA curve of tungsten trioxide shows three thermal effects. The sharp exothermic peak at 320C represents the oxidation of tungsten oxide content of lower valency. The endothermic peaks at 750 and 1090C are related to polymorphic changes of tungsten trioxide. The beginning of its sublimation is observed at temperature higher than 800C. The DTA curves of sintering of tungsten trioxide with calcite or calcium oxide indicate that the intensive formation of scheelite takes place by endothermic reactions at 660 and 545C respectively. The medium and small endothermic peaks at 520 and 730Con the DTA curve of tungsten trioxide with calcium oxide represent the dehydration of calcium oxide and the loss of carbon dioxide due to some carbonatization of calcium oxide with carbon dioxide from air, respectively. The produced scheelite is colorless in thin sections, has distinct cleavage (101), crystallizes in the tetragonal system in the form of tabular crystals and is optically positive.

Restricted access

The problems of the preparation of High Temperature Superconductors by sintering and melting are examined. Results of the thermal properties of the total substitution of Yttrium by a rare earth element are reported. The process of fabrication of HTSC after high temperature decomposition is indicated. The results of first characterisation are also reported.

Restricted access

Summary  

The Iron Age ceramic technology used in the manufacturing of cooking pots was studied by thermo-FTIR spectroscopy analysis. The pottery was excavated at Tel Hadar on the eastern shore of the Sea of Galilee. The results demonstrate that the cooking pots were manufactured using noncalcareous or slightly calcareous raw material proceeds from soil. The firing was at about 750-850C and the cementation to ceramic was obtained by low temperature sintering of the clay. The use of soil raw material composed of smectitic (montmorillonitic) clay enabled the low temperature sintering. The clay from soil is relatively poorly crystallized and rich in natural iron oxide, both of which induce earlier sintering. Most of the cooking pots were tempered with broken pieces of large calcite crystals that were added to the clayey raw material from an additional source. Alternative tempering with limestone particles composed of polycrystalline calcite is inappropriate as it brings about earlier and intense decarbonation during the firing, which causes defects in the pots.

Restricted access

Abstract  

Powellite (calcium molybdate) is an essential industrial product used as additive material to steel and for smelting of ferromolybdenum. Powellite often occurs as a secondary mineral and as pseudomorph after molybdenite in the oxidation zone of molybdenite deposits. The present work reports a thermal analysis study of synthesis of powellite by sintering of molybdite (molybdenium oxide) with calcite or calcium oxide using a derivatograph. The reaction products were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA curve of sintering of molybdite with calcite shows the beginning of the reaction at 480C with the formation of powellite. The intensive formation of powellite is represented by the medium and wide endothermic peak at 630C. This is followed by a small endothermic peak at 790C, representing the melting of unreacted molybdite. This is followed directly by large and sharp endothermic peak at 880C, representing the dissociation of unreacted calcite. The wide and large endothermic peak at 1155C represents the boiling of unreacted molybdite with appreciable vaporization. The DTA curve of sintering of molybdite with calcium oxide shows a medium and wide endothermic peak at 525C representing the intensive formation of powellite and also the dehydration of calcium oxide. The small endothermic peak at 730C represents the loss of carbon dioxide due to some carbonatization of calcium oxide with carbon dioxide from air. The medium endothermic at 790C represents the melting of unreacted molybdite. The produced powellite is yellow in thin sections, has indistinct cleavage, crystallizes in the tetragonal system in the form of tabular crystals and is optically positive.

Restricted access

Abstract  

The composite powders 90 vol.% Al2O3–5 vol.% YAG–5 vol.% ZrO2 were produced by doping commercial alumina powders with zirconium and yttrium chloride aqueous solutions. Both a nanocrystalline transition alumina and a pure α-phase powder were used as starting materials. The obtained materials were characterized by DTA-TG, XRD and dilatometric analyses and compared to the respective biphasic systems developed by the same procedure. Pressureless sintering at 1500 °C for 3 h was able to consolidate the doped powders in fully dense bodies, characterized by a very fine and homogeneous dispersion of the second phases into the micronic alumina matrix.

Restricted access

Abstract  

Waste compromises environmental preservation as well human health in many countries. Recycling is an alternative that sometimes represents the only economical activity for a significant population in the big cities. Almost 3% of waste materials in Brazil are vitreous. Ceramic production adding waste glass is possible with advantages of costs reduction associated to decrease on firing temperatures and to the raw material itself. At present paper up to 80 mass% of waste glass was added to clay. The sintering temperature decreased linearly and the shrinkage increased with glass content, an effect more pronounced for high glass amount.

Restricted access

Ageing of wet-synthesized oxide powders

Role of surface carbonation, effect on sintering, restoration

Journal of Thermal Analysis and Calorimetry
Authors:
Laura Montanaro
,
K. Belgacem
,
P. Llewellyn
,
F. Rouquerol
,
F. Merlo
, and
Paola Palmero

Abstract  

Wet chemical synthesis of precursor oxide ceramics is a method to obtain small particulate powders. Such powders are far more prone to ageing in air than more traditional precursors. Thermogravimetric analysis is used to highlight the species responsible for the ageing of ceramic precursors. Indeed water and carbon dioxide are observed to evolve from aged powders. Ceramics obtained from aged precursors can reach a very low final density with respect to the theoretical value. A large degree of the original sintering properties can be recovered after washing the aged powders with ethanol in a basic medium.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors:
K. B. Povarova
,
A. G. Padalko
,
A. A. Drozdov
,
N. K. Kasanskaya
,
N. L. Korenovskii
,
O. A. Skachkov
,
A. N. Veselov
,
A. E. Morozov
, and
I. O. Bannykh

Summary Thermostable high-temperature structural alloys based on the refractory (melting temperature T m=2060°C) RuAl intermetallic (IM) with an ordered B2 crystal structure (CsCl type) are developed. This IM surpass other aluminides (NiAl, TiAl and Ni3Al) used as the base for the development of high-temperature alloys and matrices of high-temperature composites (CM) intended for hot parts of supersonic engines, which serve at the temperatures exceeding operation temperatures of modern nickel-base superalloys. The differential barothermal analysis was used to develop the basic technological process of barothermal reaction sintering to produce near-net shape billets from RuAl-based structural materials.

Restricted access