For the purpose of this study, two lithium magnesium phosphate glass compositions were produced, one comprised 30 mol% Li2O, 20 mol% MgO and 50 mol% P2O5 (LMP) and the other 30 mol% Li2O, 15 mol% MgO, 5 mol% Al2O3 and 50 mol% P2O5 (LMAP). Considering the nominal O/P ratio of 3.0 and 3.1, respectively, the composition of the glasses can be classified as close to metaphoshphate.
For each glass composition, high purity Li2CO3, MgO, Al2O3 and NH4H2PO4 powders were carefully mixed and then loaded in corundum crucibles (Degussitt AL23) hold at 673 K in a chamber furnace. Stepwise loading of the powder was required due to excessive degassing by decomposition of ammonium dihydrogen phosphate. After that, degassing temperature was increased to 1373 K in the case of LMP and to 1523 K in the case of LMAP. After heating for 1 h melts were quenched by pouring on a brass plate. Glasses were crushed and re-melted under the same conditions to improve chemical homogeneity. Clear and bubble-free glasses without any signs of crystallization were obtained by this procedure.
Bulk composition was analyzed using inductively coupled plasma optical emission spectrometry, ICP-OES (715-ES Varian/Agilent). 30-50 mg of glass was dissolved in 10 % HCl for each analysis. Samples were taken from three different regions of the glass body to check material homogeneity (Tab. 1.1). Analyses revealed that both glasses are homogenous and compositions are close to the nominal values except for aluminum. Contamination by 2.5 mol%
Al2O3 from the corundum crucible has to be acknowledged.
Table 1.1:Nominal and measured composition of starting materials in mol% normalized to 100.
Li2O MgO Al2O3 P2O5 O/P
LMP 30 20 0 50 3.00 nominal
28.88 ± 0.17 19.32 ± 0.11 2.53 ± 0.08 49.26 ± 0.11 3.06 OES
LMAP 30 15 5 50 3.10 nominal
29.33 ± 0.21 14.44 ± 0.07 7.48 ± 0.19 48.74 ± 0.1 3.17 OES Notes. Errors represent one standard deviation.
1.2.2 Hydrous and compacted glasses
Hydrated glasses with water contents up to 8 wt% were prepared in noble metal capsules.
For the LMP series Au was chosen as the capsule material, while Au90-Pt10 was used for the LMAP series due to higher experimental temperature required for these glasses. In order to achieve a homogeneous initial water distribution, glass powder and distilled water were alternately added to the capsule. The powder-water mixture was compacted with a steel piston between loading steps to minimize air bubble inclusions. After loading, capsules were sealed by arc-welding. Weld seams of capsules were tested for leakage by heating in a drying oven at 373 K for at least one hour.
Syntheses were performed in an internally heated pressure vessel (IHPV) at 500 MPa and 1323 K for LMP and 1423 K for LMAP for 15 h, using Argon as pressure medium. In each run, two capsules were placed in the hot zone of a normal quench sample holder (controlled by a K-type thermocouple), pressurized and heated up to the desired p-T conditions. Detailed description of apparatus and procedures are given by Berndt et al. [60].
Samples were isobarically quenched to preserve pressure-induced structural changes and to avoid water loss from the hydrated glasses. The produced glasses had cylindrical shape with a length of 30 mm and a diameter of 6 mm. All glass cylinders were transparent and contained neither bubbles nor crystals. For further measurements, ~5 mm thick slices were cut out from both ends of the cylinders, to check for homogeneity of water distribution within the glass samples. Due to a lack of material, no additional chemical analyses were performed on hydrous glasses. However, because the capsule represents a closed system, no significant change in bulk composition is expected.
1.2.3 Karl-Fischer titration
The total water content of hydrated glasses was determined by pyrolysis and subsequent Karl-Fischer titration (KFT). For this purpose, ca. 15 mg of each glass were filled into small platinum containers and were heated up rapidly to 1573 K within 4 minutes. To prevent explosive release of H2O during heating, the upper end of the Pt container has been folded down. A detailed description of the KFT is given in Behrens et al. [61]. Results are shown in Tab. 1.2. Note that 0.1 wt% H2O is added to the measured water content to account for unextracted water during analyses [62].
Table 1.2: Sample characterization and spectroscopic data of NIR measurements.
Notes. I and II in the sample name refer to pieces cut from both ends of the synthesized glass body. Subscript “start” indicate the glass melted at ambient pressure, subscript “dry” refers to the glass after high pressure synthesis.
The number in the sample name indicates the nominal water content. Water contents were measured by KFT, except for data marked by * based on MIR spectroscopy using the calibration presented in this paper. Fictive temperatures Tf of glasses were determined by the first upscan of DTA and glass transition temperatures Tg are averages from three following upscans of DTA. Uncertainties are ± 2 K. Peak positions (± 5 cm-1) and absorbances (± 0.002) of NIR combination bands were determined after linear baseline corrections, see text for details.
cH2Ot, KFT Tf Tg ρ d Peak position A5200 A4500
[wt%] [K] [K] [g/l] [mm] [cm-1] [cm-1] [mm-1] [mm-1]
LMPstart n.a. 677 676 2486 ± 2
LMPdry 0.059* 683 677 2544 ± 2 /
LMP1-I 1.24 ± 0.05 626 625 2539 ± 2 0.265 / 4445 0 0.018
LMP1-II 0.97 ± 0.04 0.270 / 4445 0 0.022
LMP2-I 2.63 ± 0.04 588 587 2530 ± 3 0.275 / 4445 0 0.015
LMP2-II 2.74 ± 0 04 0.275 / 4445 0 0.034
LMP4-I 3.55 ± 0 05 539 539 2506 ± 2 0.267 4441 0.003 0.066
LMP4-II 4.14 ± 0 05 0.264 / 4441 0 0.045
LMP6-I 5.85 ± 0.05 503 499 2474 ± 3 0.260 5208 4443 0.005 0.043
LMP6-II 5.64 ± 0 05 0.257 5208 4443 0.006 0.051
LMP8-I 7.94 ± 0.07 459 459 2432 ± 2 0.250 5208 4448 0.011 0.063
LMP8-II 8.08 ± 0 07 0.253 5208 4448 0.011 0.061
LMAPstart n.a. 694 695 2500 ± 2
LMAPdry 0.047* 695 685 2578 ± 3 /
LMAP1-I 0.90 ± 0.05 638 638 2579 ± 2 0.296 / 4461 0 0.019
LMAP1-II 0.89 ± 0.05 0.298 / 4461 0 0.019
LMAP2-I 1.77 ± 0.05 606 603 2584 ± 2 0.298 / 4458 0 0.038
LMAP2-II 2.36 ± 0.06 0.301 / 4458 0 0.044
LMAP4-I 3.61 ± 0.07 559 558 2564 ± 2 0.299 / 4445 0 0.036
LMAP4-II 4.50 ± 0.04 0.304 5206 4445 0.002 0.041
LMAP6-I 4.89 ± 0.06 514 513 2523 ± 2 0.295 5206 4448 0.006 0.054
LMAP6-II 5.86 ± 0.06 0.294 5206 4448 0.011 0.052
LMAP8-I 7.70 ± 0.07 470 467 2479 ± 2 0.307 5196 4458 0.023 0.081
LMAP8-II 8.10 ± 0.08 0.300 5196 4458 0.028 0.074
1.2.4 Differential thermal analysis
The glass transition temperature, Tg, was determined by differential thermal analysis in air using 15 – 20 mg of glass pieces or powdered glass placed in Pt-crucibles (thermobalance TAG 24, Setaram, Caluire, France). The same measurement routine and data evaluation was applied to hydrous borate glasses, and Tg values were found to be in perfect agreement with isokom temperatures (T12) at which viscosity equals 1012 Pa·s [63]. For each sample four heating and cooling cycles with 10 K min-1 were applied. The maximum temperature did not exceed Tg by more than 50 K. The first cycle represents the fictive temperature Tf of the glasses, since the cooling history of the samples reflects the status of quenching after IHPV synthesis. The following three cycles were used for the determination of Tg. Definition of Tf and Tg is based on the onset of the endothermic step in the DTA curve according to Mazurin [64, 65]. The average Tg values for all investigated glasses are included in Tab. 1.2. Both, LMP and LMAP show a continuous decrease of Tg with increasing water content, similar to borate glasses but less pronounced as found for silicate and aluminosilicate glasses [66-68].
1.2.5 IR spectroscopy
IR spectra were measured on both-side polished sections using a FTIR spectrometer Bruker IFS 88. The spectrometer is coupled with an IR microscope Bruker IR scope II, equipped with a mercury-cadmium-tellurium (MCT) detector. Absorption spectra in the mid-infrared (MIR) were recorded to investigate fundamental OH stretching vibrations. In these measurements a KBr beam splitter and a globar light source were used. Spectra were recorded from 600 to 6000 cm-1 with a spectral resolution of 2 cm-1. For each sample and background (air) measurement 50 scans were accumulated.
In addition, MIR spectra of KBr pressed pellets (2 mg glass powder + 198 mg KBr) of LMP were collected in the range of 370 – 4000 cm-1 using a FTIR spectrometer (Bruker Vertex 80v) with a globar light source, a KBr beam splitter and a pyroelectric deuterated L-alanine doped triglycerine sulfate (DLaTGS) detector. A pure KBr pellet was used as reference. The spectral resolution was 2 cm-1 and 32 scans for each spectrum were accumulated.
Near-infrared (NIR) spectra were recorded to study water speciation in the glasses using the same set-up as for MIR measurements on polished sections, but with a tungsten light source and a CaF2 beam splitter. The spectral resolution was 4 cm-1. For each spectrum 100 scans were accumulated. On each sample at least three spectra were recorded to check reproducibility and water distribution in the glass. The thickness of the samples was determined by a digital micrometer (Mitutoyo Absolute) with a precision of ± 2µm.
1.2.6
27Al and
31P MAS NMR spectroscopy
MAS-NMR spectra were recorded on a Bruker ASX 400WB spectrometer at room temperature. Measurements were collected at 104.27 MHz (27Al) and 161.97 MHz (31P) respectively, using a standard Bruker 4 mm probe with rotor speed of 12.5 kHz. Na2HPO4 (diso. = 1.4 ppm) for 31P and AlCl3 (1M) for 27Al were processed as reference standards. For the
27Al MAS-NMR spectra a single pulse duration of 0.6 µsec was applied to ensure homogenous excitation. For 31P a pulse duration of 4 µsec was chosen. The recycle delay was 0.1 sec (27Al) and 480 sec (31P), respectively. 16000 scans were accumulated for 27Al spectra and 24 scans for 31P.