Peculiarities in thermal evolution of precipitated amorphous calcium phosphates with an initial Ca/P ratio of 1:1.

Thermal evolution of amorphous calcium phosphate (ACP) powder from a fast nitrate synthesis with a Ca/P ratio of 1:1 were studied in the range of 20-980 °C. The powder consisted of amorphous dicalcium phosphate anhydrate (CaHPO) after heating to 200 °C. CaHPO gradually condensed to amorphous calcium pyrophosphate CaPO (CPP) between 200 to 620 °C.

Structural and compositional features of amorphous calcium phosphate at the early stage of precipitation.

Precipitates formed at an early stage (during the first 6 h) of the hydroxyapatite crystallization of a solution were studied. A nitrous synthesis was used (0.583M (NH(4))(2)HPO(4) and 0.

Thermal impurity reactions and structural changes in slightly carbonated hydroxyapatite.

Lattice and surface impurity reactions and structural changes induced by them in slightly carbonated hydroxyapatite (SCHA) treated at 25-1100 degrees C were comprehensively studied. The SCHA was processed by a conventional wet synthesis at a high possible temperature(96 degrees C) using ammonium containing parent reagents. IR-spectroscopy, XRD, TG-DTA technique and mass spectrometric thermal analysis (MSTA) were employed for characterization of the samples.

Preparation and characterization of biphasic calcium phosphate ceramics of desired composition.

A modified processing route for fabricating dense and porous biphasic calcium phosphate (BCP) ceramics of desired and reproducible phase composition (hydroxyapatite (HA)/beta-tricalcium phosphate (beta-TCP) ratio) has been developed. The principal idea of the route was combining a precipitation and a solid phase methods. First, a nonstoichiometric (slightly carbonated calcium-deficient) HA (CdHA) precipitate was synthesized by mixing a calcium carbonate (CaCO(3)) water suspension with an orthophosphoric acid (H(3)PO(4)) solution in abundance (related to the amount resulting in a stoichiometric HA) under definite conditions, and a powder of the precipitate was prepared and calcinated in air (860 degrees C, 1.

Porous calcium phosphate ceramic granules and their behaviour in differently loaded areas of skeleton.

Two kinds of calcium phosphate ceramic (CPC) granules of high porosity (50 +/- 5%) and improved (for such materials) compressive strength (10-25 MPa) consisted of hydroxyapatite (PHA) and a mixture of hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP) in 60 HA/40 beta-TCP composition (PCPC) were developed. A comparative study of in vivo behavior of the materials implanted into an almost unloaded (greater trochanter of femur) and loaded (distal methaphysis of femur) zones in the skeleton of rabbits was performed. Significant activating influence of loading on the processes of new bone formation and reconstruction in macropores of both materials during all periods of implantation (up to 6 months) was observed.

Nonstoichiometric hydroxyapatite granules for orthopaedic applications.

A new method for the preparation of nonstoichiometric hydroxyapatite (HA) "dense" and porous granules, round in form and up to 8 mm in sizes designed for application in orthopaedic surgery has been developed. The "dense" granules' porosity was up to 32% and they only contained micropores. They differed from that kind of granules by increased values of compression strength (up to 48 MPa).

Sintering peculiarities for hydroxyapatite with different degrees of crystallinity.

It has been shown that reduction of sintering temperature of hydroxyapatite is connected to the use of a powder, the particles of which consist of crystalline and amorphous-like constituents. Shrinkage of the pressings made of the powder starts at the middle temperatures (600-700 degrees C) and is realized by a mechanism of a mutual sliding of the particles. If the firing temperature of the pressings increases, the second stage, realized by a diffusion mechanism (over approximately 900 degrees C), begins.

Preparation and properties of inhomogeneous hydroxyapatite ceramics.

Inhomogeneous ceramics of hydroxyapatite (HA) were prepared by sintering briquettes in which an inhomogeneous distribution of density was made by pressing HA powder into a die with rough walls. The resulting sample of such a ceramic is a hard thin shell with a loose core, and it is characterized by an inhomogeneous macro- and microstructure. It is a sintered conglomerate from HA grains containing grain boundary macropores in contact with the surface and micropores located inside the grains, part of which are also associated with the free surface.

Possibilities for strengthening hydroxyapatite ceramics.

Some physics during heating and sintering of powder pressings of hydroxyapatite (HA) under conventional (usual) conditions have been studied. It is revealed that heating and firing of the pressings of a middle-dispersity powder are accompanied by release of gases. The gas release hinders and can stop the shrinkage (sintering).

Inorganic phase composition of remineralisation in porous CaP ceramics.

The formation of bone tissue in pores, and around, a block made of partially dehydroxylated porous hydroxyapatite ceramic and implanted into a rat's femur using X-ray diffraction (XRD), infrared spectroscopy (IRS) and histological methods were investigated. Structural characteristics and composition of new bone tissue formed three months after implantation, in fact, do not differ from those of the normal femur. A model of osteogenesis based on dissolution-precipitation reactions is given, taking into account peculiarities of structure, deficiency and impurity composition for the material of the implant.

Phase and structural changes in hydroxyapatite coatings under heat treatment.

Hydroxyapatite (HA) crystallites of smaller size than those formed during the spraying process are found in HA coatings on titanium as a result of the crystallization of the amorphous phase (approximately 630 degrees C) when the coatings are vacuum-heat-treated in the temperature interval 100-1000 degrees C. As the annealing temperature increases within the 630-1000 degrees C range, the size of the crystallites increases, and at 1000 degrees C reaches the size of those formed during the process of spraying. At the same time, at 800 degrees C and above, HA transforms into other calcium phosphate phases (alpha-tricalcium phosphate, beta-tricalcium phosphate, tetracalcium monoxide diphosphate).

Periodic crystallization effect in the surface layers of coatings during plasma spraying of hydroxyapatite.

A new effect of the amorphous component of periodic crystallization in the surface layers of coatings during hydroxyapatite (HA) plasma spraying is revealed. The effect is caused by significant temperature increases of surface layers due to plasma heating in the process of coating. Under prevalent spraying conditions, the crystallite sizes of the crystallite phase are considerably less than those in dense HA.

Further studies on the plasma-sprayed amorphous phase in hydroxyapatite coatings and its deamorphization.

Plasma-sprayed hydroxyapatite coatings contain a quite large amount of amorphous phase. Infrared analysis shows that the plasma-sprayed amorphous phase is an oxyapatite and the coating predetermined as a hydroxyapatite is proved to be an oxyhydroxyapatite with a small quantity of hydroxyapatite. Heat treatment promotes the transformation of amorphous oxyapatite into a crystalline hydroxyapatite structure and reduces the dissolution rate of the coatings.

Amorphous phase and morphological structure of hydroxyapatite plasma coatings.

It is shown that under standard conditions of plasma spraying hydroxyapatite powder reaches the substrate in the form of molten drops. On striking the surface, the molten drop splatters, cools rapidly and solidifies. Consequently, a pancake particle surrounded by tiny spherical or differently shaped particles is formed on the substrate.