Anagnostis Stergiou

Effect of physical state and particle size distribution on dissolution enhancement of nimodipine/PEG solid dispersions prepared by melt mixing and solvent evaporation.

The physical structure and polymorphism of nimodipine were studied by means of micro-Raman, WAXD, DSC, and SEM for cases of the pure drug and its solid dispersions in PEG 4000, prepared by both the hot-melt and solvent evaporation methods. The dissolution rates of nimodipine/PEG 4000 solid dispersions were also measured and discussed in terms of their physicochemical characteristics. Micro-Raman and WAXD revealed a significant amorphous portion of the drug in the samples prepared by the hot-melt method, and that saturation resulted in local crystallization of nimodipine forming, almost exclusively, modification I crystals (racemic compound). On the other hand, mainly modification II crystals (conglomerate) were observed in the solid dispersions prepared by the solvent evaporation method. However, in general, both drug forms may appear in the solid dispersions. SEM and HSM microscopy studies indicated that the drug particle size increased with drug content. The dissolution rates were substantially improved for nimodipine from its solid dispersions compared with the pure drug or physical mixtures. Among solid dispersions, those resulting from solvent coevaporation exhibited a little faster drug release at drug concentrations lower than 20 wt%. Drug amorphization is the main reason for this behavior. At higher drug content the dissolution rates became lower compared with the samples from melt, due to the drug crystallization in modification II, which results in higher crystallinity and increased particle size. Overall, the best results were found for low drug content, for which lower drug crystallinity and smaller particle size were observed.

Characterization of the distribution, polymorphism, and stability of nimodipine in its solid dispersions in polyethylene glycol by micro-Raman spectroscopy and powder x-ray diffraction

In the present study, a series of solid dispersions of the drug nimodipine using polyethylene glycol as carrier were prepared following the hot-melt method. Micro-Raman spectroscopy in conjunction with X-ray powder diffractometry was used for the characterization of the solid structure, including spatial distribution, physical state, and presence of polymorphs, as well as storage stability of nimodipine in its solid formulations. The effect of storage time on drug stability was investigated by examination of the samples 6 months and 18 months after preparation. Confocal micro-Raman mapping performed on the samples showed that the drug was not uniformly distributed on a microscopic level. The presence of crystals of nimodipine with sizes varying between one and several micrometers was detected, and the crystal size seemed to increase with overall drug content. In samples examined 6 months after preparation it was found that the crystals existed mainly as the racemic compound, whereas after 18 months of storage mainly crystal conglomerates were observed.

Tailoring the Release Rates of Fluconazole Using Solid Dispersions in Polymer Blends

Formulations of the drug Fluconazole with different release characteristics were prepared by dispersing the active pharmaceutical ingredient (API) in various polymeric carriers, and especially in polymer blends. Fluconazole was tested as a model drug with low solubility in water. First solid dispersions in pure polymers were studied. Use of pure polyvinylpyrrolidone (PVP) as carrier even for high drug load (30 wt%) resulted in rapid release. The drug release rates decreased by increasing the API content. The dissolution rate enhancement was attributed to drug amorphization, particle size reduction, and possible improvement of the drug wetting characteristics. Hydroxypropyl methylcellulose (HPMC) gave solid dispersions, from which the release rates of the drug varied from immediate to sustaining. As the drug amount increased, the rates became higher. Similar behavior also was found when Chitosan was used as carrier, with much more controlled rates close to those for sustained release. These differences were mainly attributed to the limited solubility and swelling of HPMC and Chitosan in aquatic media. To study the effectiveness of polymer blends in adjusting the release rates of the drug, solid dispersions in PVP/HPMC and PVP/Chitosan miscible blends were studied. The release rates of Fluconazole were adequately adjusted by differentiating the weight ratio of the polymers in the blends. PVP/HPMC blends with high PVP content can be used for immediate release formulations but PVP/Chitosan blends are inappropriate for such formulations and can only be used for controlled release.

New Dy-Substituted Ba Hexaferrites With High Coercivity

Single-phase hexaferrite compounds were obtained in a novel series of Dy-substituted Ba hexaferrites with nominal formula (Ba1-xDyx)Omiddot5.25 Fe2O3 ( x=0-0.30). The samples were prepared by the chemical coprecipitation method from nitrate precursors and calcination at T= 900-1200degC for 2 h. Analysis of XRD powder patterns shows that all samples are M-type hexagonal ferrites, with Fe2O3 only impurities, indicating the incorporation of Dy in the hexaferrite structure. Single-phase compounds are obtained at T= 900-1100degC, for a x range up to 0.05-0.15. Fe2O3 is found from traces up to 20%, for higher temperature and x values. Magnetization at 1.8 T decreases with x, but coercivity remains high at values close to 400-440 kA/m in most cases, regardless of the x value and impurity content. The high coercivity values related to the presence of Dy may be attributed to microstructural effects such as grain growth inhibition or to variation of the magnetocrystalline anisotropy.

Non-stoichiometric barium ferrite particles for high-density magnetic recording

Abstract The organometallic precursor method was employed for the synthesis of β” ferrites, from which non-stoichiometric Ba ferrite particles have been prepared by ionic exchange. The magnetic grains are hexagonal platelets with diameters as low as 50 nm and aspect ratio of 1.5. After annealing, the sample diameter increases to 200 nm while the grains remain single-domain. The Curie temperature is 455°C. Annealed samples show specific magnetization up to 62 emu/g, coercive fields j H c close to 2 kOe, and an SFD factor 1− S ∗ of 0.55

Crystal structure of non-stoichiometric β″ K–Ba-Na mixed ferrite K0.30Na0.62Ba0.62Fe10.44Mg0.29O17

Abstract Crystals of non-stoichiometric K–Na-Ba ferrite, with β″-alumina structure type and chemical formula K 0.30 Na 0.62 Ba 0.62 Fe 10.44 Mg 0.29 O 17 , were prepared by a partial ionic exchange from a K–Na-β″-ferrite. The ion exchange was performed in a Ba(NO 3 ) 2 -KNO 3 eutectic melt at 650°C for 15 min. The study of a single crystal with X-ray diffraction (MoKα radiation), resulted in the R3m space group, with a = b =5.9436(7) A, c =35.743(7) A and Z =3. The structure determination was based on the β″-alumina structure model (Na 1+ x Al 11− x Mg x O 17 ), where the Na atom sites are occupied by (Ba, K, Na) and the Al ones by Fe. The investigation on the electron density of the conduction plane ( z =1/6) gave trigonal distribution around the middle position (0,0,1/6) for (K–Na-Ba) atoms. This distribution can be described by a partial occupation of two sites (18 h) with coordinates 0.056, 0.028, 1/6 and 0.26, 0.13, 1/6, by (K, Na, Ba) atoms with a consequent decrement of the site occupation factors. Refinement of the cation population parameters, and positional and anisotropic thermal parameters for all atoms, resulted in the final residual factor R 1 =0.057.

Crystal structure of nonstoichiometric mixed (Ba, K, Na) hexagonal ferrite Ba0.71Na0.64K0.31Fe10.55Mg0.29O17.30

Abstract Mixed hexagonal ferrite of Ba, Na and K was obtained by ionic exchange of a (Na,K)-β″-ferrite in a eutectic KNO3–Ba(NO3)2 melt at 650°C followed by annealing at 1000°C. The structure of a single crystal selected, with chemical formula K0.31Na0.64Ba0.71Fe10.55Mg0.29O17.30, space group P 6 and unit cell constants a=5.910(1) A, c=23.203(7) A, was studied by single crystal X-ray diffraction and refined to a final agreement factor of R=0.042. The main character of the structure was found to be of magnetoplumbite type. The structure was described as an intergrowth of M and rich β-alumina type structures, in a proportion 2:1. Mixed (Ba,Na) and (Ba,K) atoms were located on the mirror plane at BR and aBR positions respectively. Also, mixed (Na,K) atoms were located at two sites out of the mirror plane, inside the spinel block, partially substituting for oxygen atoms.

On The Effect Of Zinc Melt Composition On The Structure Of Hot‐Dip Galvanized Coatings

Zinc hot‐dip galvanizing is an effective method for the corrosion protection of ferrous materials. A way of improving the results is through the addition of various elements in the zinc melt. In the present work the effect of Ni, Bi, Cr, Mn, Se and Si at concentration of 0.5 or 1.5 wt.% was examined. Coupons of carbon steel St‐37 were coated with zinc containing the above‐mentioned elements and were exposed in a Salt Spray Chamber (SSC). The micro structure of these coatings was examined with SEM and XRD. In every case the usual morphology was observed, while differences at the thickness and the crystal size of each layer were induced. However the alloying elements were present in the coating affecting its reactivity and, at least in the case of Mn and Cr, improving corrosion resistance.

Structure Analysis Of Corrosion Resistant Thermal Sprayed Coatings On Low Alloy Steels

Metallic coatings have been proved to reduce the rate of corrosion of steel in various atmospheres. In this work the structure of Al, Cu‐Al and Zn thermal sprayed coatings is examined. The as formed coatings are extremely rough, and they are composed of several phases which increase corrosion resistance as it was determined Salt Spray Chamber tests.