الفهرس 
Background, Literature Review and IntroductionOnly 14 pages are availabe for public view
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Abstract
The present study is concerned with studying the role of Cr2O3 in bioactive borate glasses. The characteristics of crystallized glasses as biodental fillers are found to depend upon the kind and quantity of the crystal phases formed in the host glass network.
Development of the studied glasses in terms of enhancing their bioactivity is based upon controlled precipitation of crystalline phases from the base glass by the effect of adding nucleating agents or by the effect of treating the glasses thermally or both. In this situation Cr2O3, Al2O3 and Fe2O3 are added as individual agent in order to induce bulk crystallization of the phases. It is found that Cr2O3 has a remarkable effect on the growth rates of apatite crystals. Chromium oxide (Cr2O3) was found to be more effective as agent than Al2O3 and Fe2O3. The result indicated that the larger amount of crystalline biocompatible apatite phases are highly concentrated in glass containing Cr2O3 compared with that of base glass and glasses containing Fe2O3 or Al2O3.
Analysis of FTIR and XRD spectra indicated that a major part of chromium ions exists as Cr6+ with tetrahedral environment (CrO4) in glasses containing 0.8-1.5 mol % Cr2O3. While at lower concentration, Cr3+ ions is the dominant and can act as a modifier resulting in creation of non-bridging bonds in the glass network. Also it is concluded that Cr3+ that acts as a modifier is present in glasses containing higher concentration of Cr2O3 (2 mol% Cr2O3). Glasses containing high concentration of Cr6+ between 0.8-1.5 mol% exhibited degeneracy with two closed distinct IR bands at 990 cm-1 and 1120 cm-1 that can be observed. The splitting in IR band was explained in terms of site symmetry and local lattice interaction on chromium ions that act as a former and as an agent or stimulator for crystallization. The observed splitting was found to be eliminated in the spectra of all other glasses (0, 0.5 and 2 mol% Cr2O3). This was attributed to presence of Cr3+ ions as a dominant species that occupy octahedral position and play the role of a glass modifier which can induce some degree of disorder in the network of the glasses.
X-ray diffraction presents microstructure changes that agree well with the changes reflected from IR spectra of glasses upon increasing Cr2O3 concentration. XRD spectra of both glasses containing 0.5 and 2 mol% Cr2O3 showed a clear broad hump which reflects that most of Cr2O3 can acts as a modifier resulting in an increase in the disorder in the host glass network. On the other hand, the broadness is totally disappeared in the spectra of glasses containing 0.8-1.5 mol% Cr2O3 which ensures the improved crystallinity of the some phases in glasses. The crystalline calcium borate CaB2O4 and fluorapatite Ca5(PO4)3F seemed to be the main bioactive phases product in the present glasses. The EDS analysis of the glass material exhibits P, Ca, Cr and F elements in various crystalline phases. The ratio of Ca/P (1.1) was found to be close to that of crystalline tricalcium phosphate (TCP). The value of Ca/Cr was found to close to that of crystalline CaCrO4. The presence of these phases (TCP & CaCrO4) is considered to be important, since they play the essential role in achieving bioactivity and make the material to be useful in the field of biodental and clinical applications. Presence of such types of crystals, in the region at which Cr2O3 plays a role of a glass former (0.8-1.5 mol%) makes the glass network more compact, harder and more resistance through forming P-O-P, P-O-Cr and B-O-Cr bonds. This was also evidenced from increasing glass density, hardness number and glass transition temperature (Tɡ) with increasing Cr2O3 concentrations. The value of hardness number of some of investigated glasses was found to be comparable with that of the commercial material which is already used as dental filler.
The SBF immersion experiment showed that bioglass containing crystalline apatite exhibit excellent in vitro bioactivity, since the obtained hydroxyapatite species are highly comparable to that of natural bone. Assessment of bioactivity through the in vitro test lend support that the studied bioborate glasses can be recommended as biodental materials valuable in branches of biodental applications such as dental fillers.