Abstract
Objective. This study reports real time neutron diffraction on the Empress II glass-ceramic system.
Methods . The commercial glass-ceramics was characterized by real time neutron diffraction, 31 P and 29 Si solid-state MAS-NMR, DSC and XRD.
Results. On heating, the as-received glass ceramic contained lithium disilicate (Li 2 Si 2 O 5 ), which melted with increasing temperature. This was revealed by neutron diffraction which showed the Bragg peaks for this phase had disappeared by 958 °C in agreement with thermal analysis. On cooling lithium metasilicate (Li 2 SiO 3 ) started to form at around 916 °C and a minor phase of cristobalite at around 852 °C. The unit cell volume of both Li-silicate phases increased linearly with temperature at a rate of +17 × 10 −3 Å 3 .°C −1 . Room temperature powder X-ray diffraction (XRD) of the material after cooling confirms presence of the lithium metasilicate and cristobalite as the main phases and shows, in addition, small amount of lithium disilicate and orthophosphate. 31 P MAS-NMR reveals presence of the lithiorthophosphate (Li 3 PO 4 ) before and after heat treatment. The melting of lithium disilicate on heating and crystallisation of lithium metasilicate on cooling agree with endothermic and exotermic features respectively observed by DSC. 29 Si MAS-NMR shows presence of lithium disilicate phase in the as-received glass-ceramic, though not in the major proportion, and lithium metasilicate in the material after heat treatment. Both phases have significantly long T 1 relaxation time, especially the lithium metasilicate, therefore, a quantitative analysis of the 29 Si MAS-NMR spectra was not attempted.
Significance. The findings of the present work demonstrate importance of the commercially designed processing parameters in order to preserve desired characteristics of the material. Processing the Empress II at a rate slower than recommended 60 °C min −1 or long isothermal hold at the maximal processing temperature 920 °C can cause crystallization of lithium metasilicate and cristobalite instead of lithium disilicate as major phase.
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Introduction
Lithium aluminosilicate glass-ceramics (GCs) have been studied extensively and have many important technological applications . Lithium silicate glasses have been shown to phase separate into silica rich droplet and lithia rich matrix phases on heat treatment. Adding phosphate to this system increases the number and volume fraction of the silica-rich droplets and P 2 O 5 has been used as a nucleating agent in a number of glass-ceramic system. Presence of network modifiers such as the Li + ion promotes formation of a separate phosphate phase due to the incompatibility of the PO 4 units in the silicate structure. The separate orthophosphate phase, charge balanced by lithium, may crystallise at low temperatures, e.g. to a phase such Li 3 PO 4 , catalysing heterogeneous nucleation of higher temperature phases such as lithium metasilicate by an epitaxial growth process. As a crude rule epitaxial growth is possible if at least one of the lattice parameters (within an integer multiple ±15%) of each of the two phases are integer multiples of one another. Two of the lattice parameters fit this criteria for the Li 3 PO 4 –Li 2 Si 2 O 5 unit cells: <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='aLi3PO4=aLi2Si2O5+8%’>aLi3PO4=aLi2Si2O5+8%aLi3PO4=aLi2Si2O5+8%
a Li 3 PO 4 = a Li 2 Si 2 O 5 + 8 %
and <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='cLi2Si2O5=3cLi3PO4−3%’>cLi2Si2O5=3cLi3PO4−3%cLi2Si2O5=3cLi3PO4−3%
c Li 2 Si 2 O 5 = 3 c Li 3 PO 4 − 3 %
. The lithium disilicate crystal phase mentioned above gives rise to the excellent mechanical properties described below which are extremely important in dental applications for restorative materials used to replace oral calcified tissue. The high fracture toughness and flexural strength arise from the high aspect ratio of the interlocking Li 2 Si 2 O 5 crystals .
Empress II is an extrudable and machinable lithium disilicate dental glass-ceramic developed by Ivoclar Vivadent AG, Lichtenstein. Empress II exhibits superior mechanical and chemical properties compared to the first generation, leucite-based, Empress system . The fracture toughness (400 MPa) and flexural strength (3.3 MPa m 1/2 ) of Empress II is equivalent, or superior to, human dentin (160 MPa and 1.6–2.6 MPa m 1/2 respectively) . Table 1 gives the approximate chemical composition of the Empress II system . Glass-ceramic ingots are provided to dental surgeries pre-ceramed and consist of 70 vol.% lithium disilicate in a glassy matrix. The billets can be extruded to complex shapes at 920 °C in a custom built press-furnace . Previous studies have shown that in addition to lithium disilicate, other phases which crystallise in this system are lithium metasilicate, lithiophosphate (lithium orthophosphate) and cristobalite . It is claimed that lithium phosphate does not nucleate formation of silicate phase in Empress II. The Li 3 PO 4 and Li 2 Si 2 O 5 phases were found on treatment at high temperature, above 600 °C, whereas low temperature treatment, below 600 °C, produces only lithium metasilicate phase . Addition of Al 2 O 3 suppresses formation of cristobalite and lithium metasilicate after the recommended firing cycle . The purpose of this study is to examine in detail the crystallisation of Empress II with temperature using thermal analysis and real time neutron diffraction to heat the GC system above the dissolution temperature of any crystalline phases followed by cooling.
