ReviewStructural relaxation and viscosity of Al2O3 doped magnesium phosphate glasses
Introduction
Amorphous magnesium phosphate (AMP) is an important material for various bio-applications, mostly used as a biomaterial for bioresorbable orthopedic implants. In [1] the AMP was studied with respect to the utilization for bone void filling applications. The mineralization of AMP was shown to mimic the hydroxyapatite-type characteristic morphology on the substrate surface, and to promote the proliferation and differentiation of the osteoblast-like cells. Recently, a biodegradable, non-exothermic, self-setting orthopedic cement based on AMP was developed [2] by incorporation of the water soluble biocompatible polymer (polyvinyl alcohol, PVA), which enables a controlled growth of the final phase via the biomimetic process. The cement composition with 15% of PVA in aqueous medium exhibited clinically relevant setting times, mechanical properties and biodegradability. In another application, the composite film of nanostructured amorphous magnesium phosphate and polylactic acid was spin-coated on the surface of the biodegradable AZ31 magnesium alloy [3], significantly improving corrosion resistance and biomineralization capability. This indicates the great potential of the AMP composites in the protective and bioactive coating applications on biodegradable orthopedic magnesium alloys.
Understanding the structural relaxation processes and viscosity behavior is the key factor for the successful practical application of all glassy materials. These processes not only determine the long-term stability of the materials’ mechanical properties, but they are also closely related to the glass-forming tendency and manufacturing procedures. Unfortunately, since these properties can be tedious to measure, literature usually lacks the data needed for considering the manufacturing/processing side of the novel biomaterials. In the present study, the relaxation and viscosity behaviors of the Al2O3-doped magnesium phosphate glasses will be reported, where the Al2O3 component was added in order to improve the chemical resistivity and anti-corrosive properties (it is well known that the tetrahedral Al ions cross-link with the neighboring phosphor chains by the formation of AlPO4 species that strengthen the glass network [4, 5]). In addition, the utilization of the correlation coefficients between the composition and various physical quantities will be demonstrated, showing the strengths and also the possible weaknesses of this approach.
Section snippets
Experimental
Magnesium phosphate glasses doped with Al2O3 were prepared by the common melt-quenching procedure. The glasses were prepared by melting the p.a. ammonium dihydrogen phosphate (NH4H2PO4) and magnesium carbonate (MgCO3) in alumina crucibles [6]. Appropriate quantities of these compounds were mixed in an agate mortar and placed into alumina crucible. In the first step, the crucible was heated up to 700 °C in an electrical furnace at slow heating rate to remove the water, ammonia and carbon dioxide
Results
The Raman spectra were recorded in the range of (100 – 1500) cm−1; the data were corrected by the Böse-Einstein population factor [10] (see Eq. (1)) after the baseline subtraction: where Iexp and Icor are the observed and corrected Raman intensities, respectively, ν and ν0 are the Raman shift ant the wavenumber of the excitation laser, respectively, and h, k, c, and T represent Planck's constant, Boltzmann's constant, the speed of light, and thermodynamic
Discussion
The relation between the relaxation motions and the viscous flow process has been the subject of debate for a long time – see e.g. [34], [35], [36]. In general, the activation energies of these phenomena are often considered close or even similar. In the present work, we have tested this hypothesis for the three Al2O3-doped magnesium phosphate glasses. The activation energies of viscous flow Eη, volume structural relaxation Δh*V and enthalpy structural relaxation Δh*H are for the studied
Conclusions
The structural relaxation and viscosity behaviors were studied using TMA and DSC for the Al2O3-doped magnesium phosphate glasses. Volume relaxation behavior (measured via cyclic TMA experiments performed in the glass transition region) was described by the TNM and TNMa models in combination with the viscous creep equation. Cyclic enthalpy relaxation measurements were evaluated in terms of the TNM model. The viscosity was described by the MYEGA, AM and VFT models in the (107 - 1011) Pa•s range.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by The Slovak Grant Agency for Science under grant No. VEGA 2/0091/20, VEGA 1/0064/18, APVV SK-PL-18-0062 and the project centre for Functional and Surface Functionalized Glass (CEGLASS), ITMS code is 313011R453, operational program Research and innovation, co-funded from European Regional Development Fund.
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