James E. Dickinson

Dielectric constants of crystalline and amorphous spodumene, anorthite and diopside and the oxide additivity rule

The dielectric constants and dissipation factors of LiAlSi2O6, CaAl2Si2O8 and CaMgSi2O6 in both the crystalline (α-spodumene, anorthite, and diopside) and amorphous forms were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: spodumene κ′11=7.30 tan δ= 0.0007 κ′22=8.463 tan δ= 0.0002 κ′33 =11.12 tan δ= 0.0007 anorthite κ′a*=5.47 tan δ= 0.0009 κ′b*=8.76 tan δ= 0.0010 κ′c*=7.19 tan δ= 0.0013 diopside κ′11=9.69 tan δ= 0.0016 κ′22= 7.31 tan δ= 0.0007 κ′33=7.29 tan δ= 0.00019 LiAlSi2O6κ′=8.07 tan δ= 0.047 amorphous CaAl2Si2O8κ′=7.50 tan δ= 0.0024 amorphous CaMgSi2O6κ′=8.89 tan δ= 0.0021 amorphous The dielectric properties of a spodumene glass, progressively crystallized at different conditions, were also determined. As the crystallization temperature was increased from 720 to 920° C, κ′ increased from 6.22 to 6.44. The dissipation factor, tan δ, remained constant at 0.020. Similarly, as the crystallization time at 750° C increased from 0.5 hr to 6.0 hr, κ′ increased from 6.28 to 6.35. The deviations of the measured dielectric polarizabilities as determined from the Clausius-Mosotti equation from those calculated from the sum of oxide polarizabilities according to αD(mineral, glass) =σ αD(oxides) are +7.4% for α-spodumene, +1.2% for diopside, and +28.0, +19.6 and +15.9% for amorphous spodumene, anorthitie and diopside compositions, respectively. Positive deviations in α-spodumene and anorthite are consistent with lower than normal apparent cation bond valence sums and are believed to be evidence for loosely bonded “rattling” Li and Ca ions. Diopside, with Ca and Mg ions having normal bond valence sums, exhibits no abnormal deviation from additivity. Larger positive deviations in amorphous SiO2, LiAlSi2O6, CaAl2Si2O8 and CaMgSi2O6 are postulated to arise from a combination of loosely bonded cations and disordered O= ions where the oxygen dielectric polarizability increased from its normal value of 2.0 Å3 in well-behaved oxides to 2.2–3.0 Å3 in the amorphous phases.

Energetics and structural changes associated with phase separation and crystallization in lithium silicate glasses

Abstract The liberation of the enthalpy of vitrification of Li 2 Si 2 O 5 glass as a function of annealing time at 500 and 520°C has been studied by high temperature drop solution calorimetry to relate the energetics of crystallization and concomitant structural changes in the glass. Enthalpy is liberated such that partially crystallized glasses behave like mechanical mixtures of glass and crystal. 29 Si MAS-NMR spectroscopy corroborates this observation. There is good agreement between four independent determinations of the degree of crystallization as a function of time, for glasses annealed at 500 and 520°C, namely by density measurements, X-ray intensity measurements, optical microscopy and calorimetry. Preliminary calorimetric and spectroscopic results on phase separation and crystallization in Li 2 Si 4 O 9 glass show that the enthalpy difference between optically clear and phase-separated glass is almost zero, the phase-separated glass being 2.3±2.6 kJ/mol more stable in energy. This lack of a difference is consistent with macroscopic phase separation representing coarsening of the size of heterogeneous local environments already present in the clear glass.

Laser texturing of doped borosilicate glasses

We describe a novel process of laser-assisted fabrication of surface structures on doped oxide glasses with heights reaching 10 - 13% of the glass thickness. This effect manifests itself as a swelling of the irradiated portion of the glass, and occurs in a wide range of glass compositions. The extent of such swelling depends on the glass base composition. Doping with Fe, Ti, Co, Ce, and other transition metals allows for adjusting the absorption of the glass and maximizing the feature size. In the case of bumps grown on borosilicate glasses, we observe reversible glass swelling and the bump height can increase or decrease depending on whether the consecutive laser pulse has higher or lower energy compared with the previous one. To understand the hypothetical mechanism, which includes laser heating of glass, glass melting, and directional flow, we explored density, refractive index, fictive temperature, and phase separation dynamics.

Multi-nuclear NMR studies of borosilicophosphate glasses and microfoams

Abstract Glasses in the ternary SiO2–B2O3–P2O5 system have a compositional region encompassed by 40–75 mol% SiO2, 10–40 mol% B2O3 and 10–35 mol% P2O5, in which glass-ceramics containing BPO4 crystallites can be formed. This system is also interesting in that all of the constituents are traditional glass formers. Nuclear magnetic resonance (NMR) spectroscopy has been used to examine the cation speciation and hydrogen evolution in this multi-glass former system. Spectroscopic results from 31P and 11B magic angle spinning (MAS) NMR show formation of the BPO4 crystalline phase in all heat-treated compositions, with structural differences between glasses that form traditional glass-ceramics and those that form the novel gas-ceramics. NMR studies of the microstructure of glasses with similar compositions, but different sources of glass formers (BPO4, NH4H2P2O4, Si, BN) have been performed to examine differences in the formation of the gas-ceramics. These data show that crystallization of BPO4 and a coordination change of boron, from trigonal to tetrahedral, accompany the nucleation of the hydrogen filled voids. In addition, the NMR results are consistent with hydrogen dissolved in the glasses as molecular H2, with little evidence for hydroxyl groups associated with the glass-forming cations.

Raman spectroscopic study of glasses on the join diopside-albite

Abstract Raman spectroscopy has been used to study the structural variation of a series of glasses along the join diopside-albite. The spectra show that these compositions can be characterized by a smooth and continuous variation in silicate speciation as a function of composition. However, the presence of three discrete bands in the low frequency region of intermediate composition glasses at 642 cm −1 , 580 cm −1 , and 470 cm −1 , which change only in relative intensity and not in frequency, argues that the change in speciation is accompanied by the formation of molecular clusters. The 642 cm −1 and 470 cm −1 bands suggest structures similar to the end-member components Di and Ab, while the 580 cm −1 band is interpreted to have a structure similar to a disilicate. There is no evidence for significant Si/Al mixing on tetrahedral sites with non-bridging oxygens, nor is there any evidence that Ca and/or Mg charge-balance Al in preference to Na. These observations should be taken into account to more accurately model the entropy of mixing of diopside-albite solutions.

Laser-written high-contrast waveguides in glass

Laser writing of waveguides in bulk glasses opens the opportunity for creating three-dimensional photonic devices. In order to become practical, the numerical aperture (NA) of these waveguides should be significantly higher than currently achievable of 0.1 - 0.15. One reason is that with higher NAs one can decrease the bending radii of the embedded photonic devices without significant loss penalty and make them compact. Thus, femtosecond-laser-written waveguides in glasses do not allow bending radii smaller than 15 - 20 mm. In order to overcome this limitation, we propose to fabricate waveguides in phase-separable and leachable glass where the index contrast is determined by the difference between the refractive indices of the unprocessed glass and of the leached porous glass. We show that we can achieve the NA = 0.25 prior to optimization. Surface and sub-surface treatment with a nanosecond ultraviolet (UV) laser produces a similar effect with even higher NA = 0.35. Applications may include a range of tightly packed embedded and three-dimensional photonic devices in bulk glass like directional couplers, splitters, interferometers, etc.