Browsing by Author "Hristov V."

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    A study on the technological parameters' influence on the properties, polishing and microstructure of corundum ceramic discs for water taps
    (2015-01-01) Djambazov S.; Yoleva A.; Hristov V.; Ivanov N.
    Some of corundum ceramics properties like high chemical resistance, hardness and mechanical strength make it an extremely suitable material for water tap gaskets and regulators. Solid fragments that normally get into the water, like sand, rust and limestone, cannot damage corundum ceramics. The latter stability allows these products to remain perfectly smooth even when exposed to varying levels of pressure and extreme water temperature fluctuations. The corundum ceramics properties depend mainly on the purity of the starting powder, the production method and the microstructure. This work investigates how the pressure applied during the semidry pressing, the firing temperature and the source materials composition (percentage of Al2O3) affect the apparent density, the coefficient of abrasion, the polished surface (the degree of polishing) and the microstructure of corundum ceramic water taps discs. Four types of a corundum powder were used as a source material for the process of semidry pressing. Five different pressures (100 MPa 150 MPa, 200 MPa, 250 MPa, 350 MPa) and three firing temperature values (1600°C, 1650°C, 1680°C) were applied in addition to the subsequent procedures of lapping and polishing in the course of production of corundum ceramic discs for water taps. The fired corundum samples underwent characterization aimed to determine the apparent density (using the method of Archimedes), the coefficient of abrasion (determined by the loss of mass in percentage points), as well as the degree of surface polishing. Using SEM and optical microscopy we proved that the technological parameters determine to a large extent the appearance of a microstructure which in turn affects the properties and the degree of polishing of corundum ceramics. Water taps of a homogeneous structure having the highest density, the smallest pores, the lowest coefficient of abrasion and the highest degree of polishing were made from a starting powder containing 98 % of Al2O3 and a glass phase containing CaO, MgO and SiO2. A firing temperature of 1650°C and pressing pressure 350 MPa were applied.
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    Organic/inorganic bioactive materials part I: Synthesis, structure and in vitro assessment of collagen/silicocarnotite biocoatings
    (2009-10-19) Hristov V.; Radev L.; Samuneva B.; Apostolov G.
    The silicocarnotite, as an inorganic part of the coatings, has been synthesized using a polystep sol-gel method. The chemical composition of the prepared silicocarnotite sol is described as 58.12 CaO, 29.42 P2O5, 12.45 SiO2 (wt%), where Ca/P+Si = 1,67. The acid soluble type I collagen, as an organic part of the obtained coatings, was mixed with silicocarnotite powder in a weight ratio of 25:75 and 75:25 weight ratio without cross-linkage. The acidity of the obtained mixture was readjust with 25% NH4OH to pH = 9.0. The mixture was then dried at 37°C for 12 h. The growth of B-type carbonate containing hydroxyapatite (B-type CO3HA) in which CO32+ → PO43- on the surface of collagen/silicocarnotite coatings soaked in 1.5 simulated body fluid (1.5 SBF) was observed. The nucleation of B-type CO3HA was estimated on the obtained coatings after 3 days immersion in 1.5 SBF. The negatively charged carboxylate groups from the collagen surface may be responsible for the HA deposition. This was confirmed by the ``red shift`` of carboxylate groups of collagen molecules in the FTIR spectra. After soaking in 1.5 SBF, the morphology of prepared coatings and HA formation was observed by SEM. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2009.
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    Organic/inorganic bioactive materials part II: In vitro bioactivity of Collagen-Calcium Phosphate Silicate/Wollastonite hybrids
    (2009-10-19) Radev L.; Hristov V.; Samuneva B.; Ivanova D.
    In the present study, novel hybrid materials of Collagen (C) and Calcium Phosphate Silicate/Wollastonite (CPS/W) were synthesized. The CPS/W ceramic was prepared via polystep sol-gel method. The dissolution test of CPS/W ceramic was filled with TRIS-HCl buffer. FTIR depicts that hydroxyl carbonate apatite (OHCO3HA) was observed after 3 days of immersion in TRIS-HCl buffer. Biohybrids of C-CPS/W were produced from diluted hydrochloric acid collagen type I and ceramic powder with different ratios of C and CPS/W equal to 25:75 and 75:25 wt.%. The synthesized hybrids were characterized by FTIR, XRD and SEM. FTIR depicts a ``red shift`` if amide I could be attributed to the fact that the collagen prefers to chelate Ca2+ from partial dissolution of CPS/W ceramic. The growth of B-type carbonate containing hydroxyapatite (B-CO3HA) on the C-CPS/W hybrids soaked in 1.5SBF was observed. The negatively charged carboxylate groups from the collagen may be responsible for hydroxyapatite (HA) deposition. This fact was confirmed by the ``red shift`` of carboxylate groups of collagen in FTIR spectra. The formation of HA was observed by FTIR, XRD and SEM. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2009.
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    Organic/inorganic bioactive materials part IV: In vitro assessment of bioactivity of gelatin-calcium phosphate silicate/wollastonite hybrids
    (2010-04-01) Radev L.; Hristov V.; Fernandes M.H.V.; Salvado I.M.M.
    Biohybrids consisting of gelatin (G) and calcium phosphate silicate/wollastonite (CPS/W) have not been prepared so far. In this work our results are focused on the possibility of obtaining G-CPS/W bioactive hybrids in vitro. XRD, FTIR, SEM/EDS techniques were employed to characterize the synthesized hybrid materials. FTIR shows that before immersion in 1.5 SBF the ``red shift`` of COO- band for pure G is observed. The presence of this bond could be attributed to the formation of COO-Ca2+ via non-biomimetic route. After immersion in 1.5 SBF, FTIR shows the presence of A- and B-type carbonate containing hydroxyapatite (A/B-CO3HA). ESD and FTIR show that small amount of calcite (CaCO3) are present after in vitro test in 1.5 SBF for 3 days. XRD reveals that CO3HA and small amounts of CaCO3 can be detected after in vitro test. SEM results obtained for immersed samples show that hydroxyapatite (HA) particles fully covered the surface of the hybrids by a layer composed of spherulites. At higher magnification, very small elongated crystallites could be observed. © 2010 Versita Warsaw and Springer-Verlag Wien.
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    Sol-gel bioactive glass-ceramics Part I: Calcium phosphate silicate/ wollastonite glass-ceramics
    (2009-04-08) Radev L.; Hristov V.; Michailova I.; Samuneva B.
    In this work we present experimental results about synthesis, structure evolution and in vitro bioactivity of new calcium phosphate silicate/ wollastonite (CPS/W) glass-ceramics. The samples obtained were synthesized via polystep sol-gel process with different Ca/P+Si molar ratio (R). The structure of the materials obtained was studied by XRD, FTIR spectroscopy and SEM. XRD showed the presence of Ca15 (PO4)2(SiO4)6, β-CaSiO3 and α-CaSiO3 for the sample with R=1.89 after thermal treatment at 1200°C/2h. The XRD results are in good agreement with FTIR analysis. SEM denotes that apatite formation can be observed after soaking in simulated body fluid (SBF). © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2009.
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    Sol-gel bioactive glass-ceramics Part II: Glass-ceramics in the CaO-SiO2-P2O5-MgO system
    (2009-04-08) Radev L.; Hristov V.; Michailova I.; Samuneva B.
    Ceramics, with basic composition based on the CaO-SiO2- P2O5-MgO system with different Ca+ Mg/P+Si molar ratio (R), were prepared via polystep sol-gel technique. The structure of the obtained ceramic materials has been studied by XRD, FTIR spectroscopy, and SEM. X-ray diffraction showed the presence of akermanite and HA for the sample with R = 1.68 and Mg substituted β-TCP and silicocarnotite for the sample with R = 2.16, after thermal treatment at 1200°C/2 h. The obtained results are in good agreement with FTIR. In vitro test for bioactivity in static condition proved that the carbonate containing hydroxyapatite (CO3HA) can be formed on the surface of the synthesized samples. CO3 HA consisted of both A- and B-type CO32- ions. SEM micrographs depicted different forms of HA particles, precipitated on the surface after soaking in 1.5 simulated body fluid (SBF). © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2009.