V.I. Putlayev

Pb3+ radiation defects in Ca9Pb(PO4)6(OH)2 hydroxyapatite nanoparticles studied by high-field (W-band) EPR and ENDOR

W-band pulsed EPR and ENDOR investigations of X-ray irradiated nanoparticles of synthetic hydroxyapatite Ca(9)Pb(PO(4))(6)(OH)(2) are performed. It is shown that in the investigated species lead ions probably replace the Ca(1) position in the hydroxyapatite structure.

Nitrogen-containing species in the structure of the synthesized nano-hydroxyapatite

Synthesized by the wet chemical precipitation technique, hydroxyapatite (HAp) powders with the sizes of the crystallites of 20–50 nm and 1 μm were analyzed by different analytical methods. By means of electron paramagnetic resonance (EPR) it is shown that during the synthesis process nitrate anions from the reagents (byproducts) could incorporate into the HAp structure. The relaxation times and EPR parameters of the stable axially symmetric NO32− paramagnetic centers detected after X-ray irradiation are measured with high accuracy. Analyses of high-frequency (95 GHz) electron-nuclear double resonance spectra from 1H and 31P nuclei and ab initio density functional theory calculations allow suggesting that the paramagnetic centers and nitrate anions as the precursors of NO32− radicals preferably occupy PO43− site in the HAp structure.

Resorbable calcium phosphates based ceramics

Resorbable monophase ceramics and ceramic composites based on calcium pyrophosphate (CPP, Ca2P2O7), tricalcium phosphate (TCP, Ca3(PO4)2), and hydroxyapatite (HAp, Ca10(PO4)6(OH)2) were prepared via thermal treatment of a HAp (Ca/P = 1.67) and CPP (Ca/P=1) mixture. High-temperature solid-state reaction between HAp and CPP leading to TCP formation was studied within the range 700–800 °C. The metastable α-TCP phase was observed in this range as a product of the solid-state reaction. The reaction had been completed before sintering of the powders started. The microstructure of CPP/TCP composites was found to be duplex-like, consisting of large CPP grains with smaller TCP grains among CPP ones. In the case of the monophase ceramics with starting HAp/CPP ratio corresponding to TCP, grain size was less than 300 nm.

Resorption of Ca 3 – x M 2 x (PO 4 ) 2 (M = Na, K) Calcium Phosphate Bioceramics in Model Solutions

The resorbability of bioceramics in the Ca3(PO4)2–CaNaPO4–CaKPO4 system is evaluated in an approach involving thermodynamic assessment of solubility and investigation of the dissolution kinetics in model media, in particular in citric acid solutions. Thermodynamic calculation indicates high solubility of the Ca5Na2(PO4)4, α-CaМPO4, β-CaKPO4, and β-СаK0.6Na0.4PO4 phases. Investigation of the dissolution kinetics of ceramics has made it possible to identify two distinct types of behavior of resorbable materials in weakly acidic solutions: with fast resorption kinetics in the case of the phases based on nagelschmidtite solid solutions and α-CaМPO4 disordered high-temperature solid solutions, and with a nearly constant, relatively low dissolution rate and a high solubility limit in the case of β-СaK1 – xNax-based phases.

Biocompatible Ceramics for Implants Based on Calcium Phosphates

Hydroxyapatite (HAp), tricalcium phosphate (TCP) and calcium pyrophosphate (CPP) are known to be among calcium phosphates used in clinical medicine due to their biocompatibility. HAp and other complex calcium phosphate salts are the end-products of the biological mineralization process. Calcium pyrophosphate Ca 2 P 2 O 7 (β-CPP) is one of the intermediate products involved in this process. The biological response with respect to new bone formation is quite similar for CPP and HAp. Sintered CPP has better biological responses, and, thus, a great potential as a biodegradable bone substitute. The rate of biodegradation depends on: (i) material texture (porosity type and level), (ii) quality of biodegradation phase (phase composition, grain size, properties of grain boundaries). Several sources for CPP phase in ceramics can be assumed. CPP phase may come from frit (CaO-P 2 O 5 , Ca/P=0.2-0.75) used as a sintering additive. Ceramics can be fabricated from powder of CPP with Na 4 P 2 O 7 as sintering additive, via interaction between H3PO4 or (NH4)2HPO4 and porous HAp at high temperature after soaking it in the former solutions. The aim of the present work was focused on fabrication of multiphase ceramics with enhanced resorptivity due to presence of CPP phase and investigation of processes leading to formation of the multiphase ceramics based on HAp and CPP originated from CaHPO 4 . Ceramic materials have been made from mixtures of powders of stoihiometric HAp (Ca/P=1.67) and monetite (CaHPO 4 , Ca/P=1). Powders of HAp and monetite were synthesized by means of wet chemical precipitation from aqueous solutions of Ca(NO 3 ) 2 *4H 2 O and (NH 4 ) 2 HPO 4 at 60 °C and pH=9 for HAp and pH=4-5 for monetite. Component ratio HAp:monetite was varied from 0:100 to 100:0 % with a step of 20%. Powders of raw materials and the mixtures were tested by means of XRD, TG, DTG, SEM, dilatometry. Linear shrinkage, density and microstructure of samples of ceramic materials sintered at 900, 1000, 1100°C with isothermal holding during 6 hours were tested. Complicated consequence of phase transformations took place during heating the the mixtures from 20 to 1200 C. The CPP (Ca/P=1, converted from CaHPO 4 at 400-500°C ) reacts with HAp (Ca/P=1.67) causing additional weight loss in the region of 600-1050°C due to solid state reaction leading to TCP (Ca/P=1.55) formation. Linear shrinkage of HAp at 1100°C after 6 hours was found to be about 21%; for Ca 2 P 2 O 7 formed from monetite, and for the mixtures - less than 11%. Resulted ceramics with the phase composition of HAp, CPP and TCP, i.e. with a different content of degradable phase and different ratio of CPP/TCP, can be treated as a biocompatible bioactive material with a tunable rate and limit of biodegradation.

Internal oxidation in solid solutions Bi2.1‑xPbxSr2-yCa1-zRy+zCu2O8+d (R = Y, Nd or La)

Bi 2-x Pb x Sr 2 CaCu 2 O 8+d solid solutions with substitution of Sr or Ca by Y, Nd or La were fabricated at 760 - 79°C in N 2 -flow. The as-synthesized solutions were oxidized in air under isothermal condition and tested with XRD, TGA chemical analysis, TEM, XANES. Effect of rare-earth element content on oxidation kinetics and microstructure of the product is discussed.

Magnetic characterization on the melt processed YBa2Cu3O7 and Bi1.8Pb0.2Sr2CaCu2Ox superconductors

Abstract Field and temperature dependence of the magnetization, remanent magnetic moment relaxation and AC susceptibility studies have been performed for the quench and melt growth (at different solidification speeds) processed YBa 2 Cu 3 O 7 (123) species with different microstructures, and for the Bi 1.8 Pb 0.2 Sr 2 CaCu 2 O 8 sample, prepared by crystallization from the glass. The Bi-containing superconductor reveals granular behavior with intragrain J c ∼ 10 6 A/cm 2 and intergrain J c ∼ 10 3 A/cm 2 at 4.5 K with possible enhanced critical current density in grain agglomerations. YBa 2 Cu 3 O 7 samples exhibit a weak link free character with a magnetically estimated J c of about 10 5 A/cm 2 at 10 K and 10 3 –10 4 A/cm 2 at 77 K. Peculiarities in magnetization loops and AC susceptibility curves have been attributed to the contributions to the magnetic response from the superconducting areas with different extensions. The temperature behavior of the flux creep activation energy is consistent with the assumption that in the Bi-containing superconductor there is a large amount of pinning centers with only a low pinning potential, while in the 123 material there is a wide distribution in the pinning energy. The highest J c and pinning characteristics have been obtained for an YBa 2 Cu 3 O 7 sample, prepared using the highest 123 phase crystallization speed (100 K/h).