J. Vogel

The structure of CaO-Na2O-MgO-P2O5 invert glass

Abstract The short and intermediate range order of invert glasses of the system CaO–Na2O–MgO–P2O5 have been examined by using X-ray and neutron diffraction and 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The depolymerization of the phosphate chains with decreasing P2O5 mol% content from 39.4 to 28.7 and simultaneous addition of MgO from 0 to 22.8 mol% are quantitatively described by the increase in the concentration of Q1 and Q0 groups. With progressive depolymerization the structure of the glasses becomes more compact indicated by the shortening of the distances between the MeOn (Me stands for the metal ion) polyhedra and between the decreasingly long PO4 chains, which correlates with the increase in packing density, w. These changes have as a consequence that the glass transition temperature, Tg, and the chemical durability continuously increase. Two effects could be a factor contributing to the glass-forming tendency of the high network modifier (>60 mol%) compositions of the invert glasses: (i) The Mg–O co-ordination number, which does not significantly change in dependence on composition, was found to be smaller in the glasses (NMgO∼5) than in the related crystal structures (NMgO=6). (ii) In the pyrophosphate region, a disproportionation of the Q1 species in the glass melt to Q0 and Q2 is indicated by the NMR results. In addition, the concentration of the Q0 and Q1 sites towards orthophosphate composition deviates from the chemical ordering model which can be explained by the occurrence of unreacted MgO. A small angle scattering effect indicating the presence of heterogeneities of electron density between 0.8 and 1.0 nm in diameter is observed for the samples, with the exception of the glass near the pyrophosphate stoichiometry. Additional information on their nature is gained from examinations of heat treated samples.

NMR study of phosphate glasses and glass ceramic structures

Abstract 31P and 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR) measurements were used to investigate P2O5CaONa2O(Al2O3) and P2O5CaONa2OMgO(Al2O3) glasses and glass ceramics. The structures of these materials were analyzed as a function of MgO and Al2O3 content. The change of Q[2] group content during heat treatment is discussed. The structure of an unknown complex phosphate phase was investigated.

Measurements of chain length distributions in calcium phosphate glasses using 2D 31P double quantum NMR.

31P double quantum (DQ) NMR is used for improved studies of the structure of phosphate glasses. The common Qn notation of the network forming tetrahedral phosphate units with n bridging oxygen atoms is extended to Qn,jkl indicating the Q type of the bonded adjacent units by the additional superscripts j,k and l. It will be shown that: (i) Q1 units have different isotropic chemical shifts depending on whether they form diphosphate anions in the amorphous state (Q1,1) or end groups of chains (Q1,2) and (ii) that even Q2 units possess different isotropic chemical shifts such that in the DQ dimension Q2,11 can be distinguished from Q2,12 and middle groups of longer chains or rings (Q2,22). Based on this result the opportunity for measuring chain length distributions in the amorphous state is discussed and first results for binary calcium phosphate glasses are presented. The advantages and also the limitations of this novel approach will be discussed.

Structural study of magnesium polyphosphate glasses

Abstract Magnesium polyphosphate glasses with molar ratios, y=n(MgO)/n(P2O5), ranging from 1.0 to 1.9 have been examined by X-ray and neutron scattering and 31P magic angle spinning nuclear magnetic resonance spectroscopy to extract information on their short-range, intermediate-range and submicroscopic structure. The depolymerization of the PO4 chains with rising MgO content is quantitatively described by the increasing concentration of Q1 and Q0 groups determined by NMR. In the pyrophosphate region the Q1 sites disproportionate to Q0 and Q2 groups, whereas there is no disproportionation of the Q2 sites at metaphosphate composition. The shortening of the real-space distances, rm, indicates that the structure of the glasses becomes more compact with progressive depolymerization which is due to the increasing connection of the MgOn polyhedra by sharing the non-bridging oxygen atoms. The Mg–O co-ordination sphere was found to change not significantly in dependence on composition. Heterogeneities about 1 nm in diameter exist in the glasses with MgO content exceeding 46.6 mol% indicated by a weak small angle X-ray scattering.

31P MAS and 2D exchange NMR of crystalline silicon phosphates

Abstract 31P MAS NMR has been used to determine 31P chemical shift tensors of crystalline silicon phosphates. The chemical shift data of five polymorphs of SiP2O7 (hexagonal, tetragonal, cubic and two monoclinic forms) are presented besides those of Si3(PO4)4. Results of 31P 2D exchange NMR on a phase mixture of different silicon diphosphates are shown. This novel approach allows an unambiguous identification of the various phases even in multicomponent mixtures as occurring in ceramics.

Control of phase formation processes in glass-ceramics for medicine and technology

Abstract The control of phase separation processes in silicate glasses makes possible the control of nucleation and crystallization of base glasses. Ti(III,IV)-species influence the phase formation in SiO 2 –Al 2 O 3 –MgO glasses to form glass-ceramics based on s- and α-quartz solid solutions. Additions of Na 2 O, K 2 O and F in a specific composition range and heat treatment of the glass result in an new glass-ceramic with curved micas. The structure of these crystals shows a high tetrahedral rotation. Glasses of the SiO 2 –Al 2 O 3 –MgO–Na 2 O–K 2 O–P 2 O 5 –F system show a double phase separation. Apatites and micas grow as a result of controlled in situ crystallization. The material can be machined and has bioactive properties. Phosphate glasses show no glass-in-glass phase separation. However, the high supersaturation is cancelled by a thermal treatment of the glass and a primary crystal is formed. Apatite crystals grow in the primary phase.

Optimisation of the synthesis of glass-ceramic matrix biocomposites by the response surface methodology

Abstract In this work a Bioverit®III glass-ceramic matrix composite, containing Yttria-stabilised zirconia (Y-PSZ) particles as toughening phase, was prepared, with the aim of improving the glass-ceramic mechanical properties. In order to prepare the composite, a pressureless sintering process has been optimised. The aim of the work is to determine the best processing conditions, i.e. time and temperature of sintering. For this purpose traditional methods (thermal, morphological and mechanical analysis) have been supported by the statistical method called ”Response Surface Methodology” (RSM). On the basis of these studies two sintering series have been performed, so that we could determine the region where the best process conditions could be found.

Nuclear magnetic resonance investigations of aluminum containing phosphate glass-ceramics

Abstract Glasses and glass-ceramics of the system P 2 O 5 –CaO–Na 2 O–{Al 2 O 3 } have been studied by 31 P and 27 Al and two-dimensional correlation nuclear magnetic resonance. The basic glasses have a chain-like structure including amorphous diphosphate and longer chains having at least four units. The Q 1 /Q 2 group ratios depend on both the content of network-modifying oxides and aluminum oxide. Crystallization of the glasses involves the conversion of glassy metaphosphate sites into crystalline pyro- and orthophosphate sites. 27 Al correlation experiments do not detect the presence of Al–O–Al bonds in the glasses and in the glass-ceramics.