Carlos Paucar

Effect of Some Physical-Chemical Variables in the Synthesis of Hydroxyapatite by the Precipitation Route

The synthetic hydroxyapatite is a very useful material for numerous applications in medicine as a biomaterial. One of the most economic manufacturing process is the precipitation route. In the present work, synthetic hydroxyapatite was prepared using the precipitation route, starting with aqueous solutions of calcium nitrate (Ca(NO3).4H2O ) and ammonium phosphate (H2(PO4)NH4). The effects of physical-chemical variables such as pH, temperature, time of agitation, ageing time and heat treatment of the mixture were evaluated. The characterization of the samples obtained in different conditions made possible to conclude about the optimal values of the studied variables for the synthesis of this material in laboratory conditions.

The effect of slurry preparation methods on biaxial flexural strength of dental porcelain

STATEMENT OF PROBLEM One-step and incremental mixing procedures are currently used to produce dental ceramic pastes. In the ceramic industry, high quality is obtained using one-step mixing, but in dentistry, the best method has not been yet determined. PURPOSE The purpose of this study is to evaluate the effects of 2 mixing techniques on the biaxial flexural strength and microstructure of dental porcelain. MATERIAL AND METHODS Feldspathic porcelain discs (2 × 15 mm in diameter) were produced and divided according to the ceramic paste preparation method, powder-liquid incremental mixing group (n=50) or one-step mixing, as a control group (n=50). Specimens were tested for biaxial flexural strength and characterized using porosimetry, relative humidity, SEM/EDS, XRD, and FT-IR analyses. Statistical analysis was conducted using Weibull statistics. The Weibull modulus, characteristic strength and relative humidity were compared between groups, using Students t-test and Mann-Whitney U test (α=.05). RESULTS The powder-liquid incremental mixing group showed significantly higher values (SD) of Weibull modulus (6.74 (0.70), P<.001) and characteristic strength (79.87 (2.01) MPa, P<.001) when compared to the one-step mixing group (4.94 (0.94) and 75.95 (2.61) MPa). Significantly lower mean (SD) relative humidity values (P=.009) were found for powder-liquid incremental mixing group (20% (0.5%)) compared to one-step mixing group (22% (1%)). XRD spectra showed that the one-step mixing group produced higher amounts of the amorphous phase. CONCLUSIONS Specimens produced by the incremental mixing technique showed higher biaxial flexural strength than one-step mixing.

On the magnetic behavior of polycrystalline RBaCo205+δ synthesized by solid state and wet chemical routes

In this paper, polycrystalline RBaCo2O5+δ (R = La, Y, Eu) samples were obtained through solid state reaction and polymeric precursor method. The crystalline quality of the samples was carefully evaluated by means of X-ray diffraction. Depending on the R cation, the main phases were characterized in a tetragonal P4/mmm, orthorhombic Pmmm or trigonal R-3c structure. The oxygen content, being a fundamental parameter in the studied system, was determined by iodometric tritration. While no stoichiometric oxygen content was determined, the value of 5 ended up being close to 0.5 for all the samples studied. VSM magnetization measurements were taken according to the zero field cooling protocol. The M(T) curves clearly shows the characteristic magnetic behavior of the cobaltate with stoichiometry 112. The temperature at which the paramagnetic-ferromagnetic-antiferromagnetic transition occurs varies with the cation R.

Comportamiento termoluminiscente de la fase vitroceramica li2o.al2o3.2sio2xceo2(x=0.5, 1, 2, 10, 20) para aplicaciones en dosimetría de rayos x

Polvos vitreos de aluminosilicatos de litio con adicion de CeO2 fueron sintetizados mediante la gelificacion de formiatos de aluminio y litio, con tetraetilortosilicato y oxido de cerio para obtener compactos tipo Li2O.Al2O3.2SiO2 dopados con 0.5%, 1%, 2%, 10% y 20% molar de CeO2. Las muestras fueron sinterizadas isotermicamente e irradiadas con rayos X para ser analizadas mediante la generacion y el modelado (metodo de Muntoni) de sus curvas termoluminiscentes. Todas las composiciones exhibieron una respuesta termoluminiscente mostrandose la composicion Li2O.Al2O3.2SiO2.20CeO2, como la mas prometedora para aplicaciones dosimetricas, abriendo la posibilidad de emplear estos sistemas para el desarrollo de sensores radiologicos.

Influencia del tipo de dispersion mecanica en la sintesis de mullita cristalina.

La dispersion mecanica de las materias primas, es una etapa primordial en el proceso de sintesis de mullita por la ruta quimica de los hidroxihidrogeles. Este trabajo pretendio evaluar dos metodos de dispersion mecanica; el primero usando un rotor-stator y el segundo un molino de perlas, buscando obtener mullita bien cristalizada a menores temperaturas y tiempos de los requeridos por la sintesis convencional. Ambos metodos de dispersion generan un efecto de alta cizalla logrando una excelente mezcla de los materiales de partida. Finalmente se caracterizo el producto mediante DRX, SEM, analisis quimico y cono pirometrico equivalente para evaluar su refractariedad.