Method For The Preparation Of Glasses And Glass Ceramics With SiO2 As Main Crystal Phase

Abstract

The invention relates to a method for the preparation of a glass ceramic or a glass, which comprise cerium ions and are suitable in particular for the preparation of dental restorations, the fluorescence properties of which largely correspond to those of natural teeth.

The invention also relates to a glass ceramic and a glass which can be obtained using the method according to the invention, the use thereof as dental material and in particular for the preparation of dental restorations, as well as a glass-forming composition which is suitable for use in the method according to the invention.

Claims

1. Method for the preparation of a glass ceramic with SiO.sub.2 as main crystal phase or a glass which comprises nuclei for the crystallization of SiO.sub.2, which comprises a step in which a melt of a starting glass which comprises cerium ions is exposed to reducing conditions.

2. Method according to claim 1, in which the melt of the starting glass is reacted with at least one reducing agent.

3. Method according to claim 2, in which the melt of the starting glass is formed from a glass-forming composition which comprises SiO.sub.2, Li.sub.2O, a cerium compound and at least one reducing agent.

4. Method according to claim 2, in which the at least one reducing agent is a compound which comprises at least one oxidizable carbon atom.

5. Method according to claim 4, in which the at least one reducing agent is an acetylacetonate.

6. Method according to claim 2, in which the at least one reducing agent is a reducing gas.

7. Method according to claim 1, in which the starting glass comprises 0.1 to 7.0 wt.-% cerium ions, calculated as CeO.sub.2.

8. Method according to claim 1, in which the starting glass further comprises terbium ions.

9. Method according to claim 1, in which the starting glass comprises 0 to 11.0 wt.-% alkaline earth metal oxide.

10. Method according to claim 1, in which the starting glass comprises at least one of the following components in the given quantities: TABLE-US-00012 Component wt.-% SiO.sub.2 58.0 to 92.0 Li.sub.2O 2.0 to 10.0 CeO.sub.2 0.1 to 7.0 Tb.sub.4O.sub.7 0 to 2.0 Me.sup.I.sub.2O 0 to 13.0 Me.sup.IIO 0 to 11.0 Me.sup.III.sub.2O.sub.3 0 to 10.0 Me.sup.IVO.sub.2 0 to 21.0 P.sub.2O.sub.5 0 to 7.0 Me.sup.V.sub.2O.sub.5 0 to 6.0 Me.sup.VIO.sub.3 0 to 6.0 fluorine 0 to 5.0, wherein Me.sup.I.sub.2O is selected from Na.sub.2O, K.sub.2O, Rb.sub.2O and/or Cs.sub.2O, Me.sup.IIO is selected from MgO, CaO, SrO and/or ZnO, Me.sup.III.sub.2O.sub.3 is selected from Al.sub.2O.sub.3, B.sub.2O.sub.3, Y.sub.2O.sub.3, La.sub.2O.sub.3, Ga.sub.2O.sub.3 and/or In.sub.2O.sub.3, Me.sup.IVO.sub.2 is selected from ZrO.sub.2, GeO.sub.2, TiO.sub.2 and/or SnO.sub.2, Me.sup.V.sub.2O.sub.5 is selected from V.sub.2O.sub.5, Ta.sub.2O.sub.5 and/or Nb.sub.2O.sub.5 and Me.sup.VIO.sub.3 is selected from WO.sub.3 and/or MoO.sub.3.

11. Method according to claim 1 for the preparation of a glass with nuclei which are suitable for forming low quartz, cristobalite or a mixture thereof.

12. Method according to claim 1 for the preparation of a glass ceramic which has low quartz, cristobalite or a mixture thereof as main crystal phase.

13. Method according to claim 1 for the preparation of a glass ceramic which comprises 5.0 to 50.0 wt.-% SiO.sub.2 as crystal phase.

14. Method according to claim 1, in which the starting glass is subjected to at least one heat treatment in the range of from 700 to 950 C.

15. Method according to claim 1, in which the glass ceramic or the glass are present in the form of a powder, a blank or a dental restoration.

16. Glass ceramic with SiO.sub.2 as main crystal phase or glass which comprises nuclei for the crystallization of SiO.sub.2, which is obtainable using the method according to claim 1.

17. Glass ceramic with SiO.sub.2 as main crystal phase or glass which comprises nuclei for the crystallization of SiO.sub.2, which have a fluorescence intensity at 420 nm and/or 541 nm which is at least 1.5 times the corresponding fluorescence intensity of a reference sample, wherein the reference sample is obtainable by melting a starting glass with the composition: 74.3 wt.-% SiO.sub.2, 7.7 wt.-% Li.sub.2O, 3.4 wt.-% K.sub.2O, 1.8 wt.-% MgO, 3.3 wt.-% CaO, 4.0 wt.-% Al.sub.2O.sub.3, 3.6 wt.-% P.sub.2O.sub.5, 1.5 wt.-% CeO.sub.2 and 0.4 wt.-% Tb.sub.4O.sub.7 on a scale of 200 g from suitable raw materials in a platinum-rhodium crucible at 1650 C. for 1 h, pouring 30 g of the glass melt into a pre-heated mould in order to produce a glass block, and converting the glass block into a glass ceramic by successive temperature treatments at 530 C. for 20 min and 800 C. for 30 min, wherein the heating rates between the temperature treatments are 30 K/min in each case.

18. (canceled)

19. Method of using the glass ceramic or the glass according to claim 16 as dental material.

20. Method according to claim 19, in which the glass ceramic or the glass are shaped by pressing or machining to form a desired dental restoration.

21. Method according to claim 4, in which the at least one reducing agent is selected from the group consisting of organic salts, carbohydrates and cereal flours.

22. Method according to claim 6, in which the at least one reducing agent comprises hydrogen or hydrogen and nitrogen.

23. Method according to claim 7, in which the starting glass comprises 1.0 to 4.0 wt.-% cerium ions, calculated as CeO.sub.2.

24. Method according to claim 8, in which the starting glass comprises 0.05 to 2.0 wt.-% terbium ions, calculated as Tb.sub.4O.sub.7.

25. Method according to claim 8, in which the starting glass comprises 0.3 to 0.7 wt.-% terbium ions, calculated as Tb.sub.4O.sub.7.

26. Method according to claim 9, in which the starting glass comprises 1.0 to 7.0 wt.-% alkaline earth metal oxide.

27. Method according to claim 13 for the preparation of a glass ceramic which comprises 10.0 to 30.0 wt.-% SiO.sub.2 as crystal phase.

Description

EXAMPLES

[0084] A total of 12 glasses and glass ceramics according to the invention with the compositions given in Table I were prepared by melting corresponding starting glasses, followed by heat treatments according to Table II for controlled nucleation and crystallization, wherein in Table I the oxidation states of the given oxides refer to the oxidation states of the raw materials used for melting the starting glasses. The following meanings apply in Table II

T.sub.g glass transition temperature
T.sub.S and t.sub.S temperature and time used for melting
T.sub.N and t.sub.N temperature and time used for nucleation
T.sub.C and t.sub.C temperature and time used for crystallization
T.sub.sint and t.sub.sint temperature and time used for sintering.

Examples 1-10: Use of a Reducing Cerium Compound as Reducing Agent

[0085] To prepare glasses and glass ceramics using a cerium compound as reducing agent, first starting glasses corresponding to the compositions given in Table I were melted from a mixture of usual raw materials in the platinum crucible at the temperature T.sub.s given in Table II for the period t.sub.s, wherein cerium(III) acetylacetonate was used as raw material for the given Ce.sub.2O.sub.3 content. By pouring the starting glasses into water, glass frits were prepared which were dried in the drying oven at 150 C.

[0086] In the Examples 1 to 8, the dried glass frits were then melted a second time at the temperature T.sub.s for the period t.sub.s for homogenization. The obtained glass melts were then poured into pre-heated graphite moulds to produce glass blocks. The glass blocks were then converted to nuclei-containing glasses and glass ceramics by thermal treatment. The thermal treatments used for controlled nucleation and controlled crystallization are given in Table II.

[0087] In the Examples 9 and 10, the dried glass frits were ground in a Retsch mill to a particle size of <90 m. The glass powder was pressed to form a blank and then sintered. The sintering step also brought about a crystallization of the glass accompanied by the formation of a dense glass ceramic shaped body in addition to the compaction of the material. The thermal treatment used for the sintering is given in Table II.

Example 11: Use of an Organic Compound as Reducing Agent

[0088] To prepare glasses and glass ceramics using an organic compound as reducing agent, a starting glass corresponding to the composition given in Table I was first melted from a mixture of usual raw materials with the addition of 1.5 wt.-% saccharose in the platinum crucible at the temperature T.sub.s given in Table II for the period t.sub.s. By pouring the starting glass into water, a glass frit was prepared which was dried in the drying oven at 150 C. and then melted a second time at the temperature T.sub.s for the period t.sub.s for homogenization. The obtained glass melt was then poured into pre-heated graphite moulds in order to produce glass blocks.

[0089] The glass blocks were then converted to nuclei-containing glasses and glass ceramics by thermal treatment. The thermal treatments used for controlled nucleation and controlled crystallization are given in Table II.

Example 11a: Repetition of Example 11 without Addition of a Reducing Agent (Comparison)

[0090] Example 11 was repeated without addition of saccharose. The fluorescence of the sample thus obtained is much lower than in Example 11.

Example 12: Use of Forming Gas as Reducing Agent

[0091] To prepare glasses and glass ceramics using forming gas as reducing agent, a starting glass corresponding to the composition given in Table I was first melted from a mixture of usual raw materials in the platinum crucible at the temperature T.sub.s given in Table II for the period t.sub.s. By pouring the starting glass into water, a glass frit was prepared which was dried in the drying oven at 150 C. The dried glass frit was melted again at the temperature T.sub.S and about 3 l/min forming gas (95% N.sub.2, 5% H.sub.2) was passed through the glass melt for 60 min by means of a quartz glass dip tube. The dip tube was then removed from the melt and the melt surface was rinsed with forming gas for about 30 min in order to prevent a reoxidation. The glass melt was then poured into pre-heated steel moulds to produce glass blocks.

[0092] The glass blocks were then converted to nuclei-containing glasses and glass ceramics by thermal treatment in a normal oven atmosphere. The thermal treatments used for controlled nucleation and controlled crystallization are given in Table II. No effects of the melting under forming gas on the crystallization and/or crystalline structure were observed.

Example 13: Fluorescence Measurements

[0093] Platelets were sawn out of the blocks obtained after nucleation with the aid of a suitable saw (Buehler Isomet 5000) and the surface was polished with an APEX grinding wheel (0.5 m). The platelets were then subjected to a temperature treatment according to Table II in a Programat furnace (Ivoclar Vivadent AG) for the crystallization. In a further processing step, the platelets were ground to the dimensions 17.9 mm15.9 mm2 mm and the surface was polished with an APEX grinding wheel (0.5 m).

[0094] To measure the fluorescence properties, a fluorescence spectrometer of the FL1039 type (Horiba Jobin Yvon GmbH) with an excitation monochromator and an emission monochromator was used. The excitation of the samples was carried out by means of a 450 W xenon lamp. The emission intensity was determined using a photomultiplier detector (PMT) of the PMT 1424M type (Horiba Jobin Yvon GmbH) as pulses per second (counts per second, cps). The calibration of the excitation monochromator was carried out by means of an integrated silicon photodiode. The emission monochromator was calibrated via the position of the water Raman peak. The linearity of the detector in the measurement range was ensured via device-specific correction data sets. The linearity of the excitation intensity was ensured during the determination of the excitation spectra via a mathematical correction of the measured emission intensity via the lamp intensity (division of the measured signal by the reference signal of the integrated silicon photodiode which directly determines the lamp intensity). To protect the detector and in order not to reach the saturation range, a 5% Neutral Density Filter was used in the emission beam path.

[0095] The samples were clamped in a solid sample holder in the right-angle mode. To prevent reflections of the excitation light, the samples were rotated by 30 relative to the excitation beam, with the result that only diffusely scattered emission light was detected. All samples were measured using identical spectrometer settings (gap widths 1 nm (excitation monochromator) and 1.5 nm (emission monochromator), scan range 372 to 700 nm, increment 1 nm, integration time 1 s, excitation wavelength 366 nm).

[0096] FIG. 1 shows, for the glass ceramic sample obtained according to Example 1, the emission spectrum at an excitation wavelength of 366 nm as well as excitation spectra for emission at 430 nm and 541 nm. The emission spectrum displayed a broad maximum at 420 nm, which is to be attributed to the 5d.fwdarw.4f transition of Ce.sup.3+. The corresponding excitation spectrum for emission at 430 nm displayed an excitation maximum at 340 nm. Furthermore, the emission spectrum displayed maxima at 483, 541, 585 and 619 nm, which are to be attributed to the transitions .sup.5D.sub.4.fwdarw..sup.7F.sub.6, .sup.7F.sub.5, .sup.7F.sub.4 and .sup.7F.sub.3 of Tb.sup.3+. The associated broad excitation spectrum for emission at 541 nm displayed an excitation maximum at 310 nm. The fluorescence emissions shown in the emission spectrum of FIG. 1 are perceived by the human eye overall as white-blue fluorescence.

[0097] FIG. 2 shows emission spectra obtained at an excitation wavelength of 366 nm for the sample according to Example 11, which was prepared under reducing conditions with the addition of saccharose, and the corresponding reference sample of the same composition according to Example 11a which was melted under normal conditions in oxygen-containing atmosphere. The broad emission maximum of Ce.sup.3+ at about 420 nm and the emission bands of Tb.sup.3+ at 483, 541, 549, 585 and 619 nm can be seen. A comparison of the spectra shows a clear rise in the intensities of the individual emission bands due to the melting under reducing conditions for the sample according to Example 2. A comparison of the total light emission, determined by calculation of the surface integral under the emission curves over the range of from 375 to 700 nm (total measurement range), shows a rise by a factor of 2.0.

[0098] FIG. 3 shows the emission intensities of the samples according to Examples 1-6, 9, 11 and 12.

TABLE-US-00010 TABLE I Example 1 2 3 4 5 6 7 8 9 10 11 12 Composition wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% SiO.sub.2 74.7 72.9 71.0 71.2 72.6 70.8 78.3 85.5 77.4 60.9 74.3 75.1 Li.sub.2O 7.6 7.7 7.1 7.0 6.6 6.8 6.8 9.0 7.6 3.6 7.7 9.2 Na.sub.2O 0.5 2.2 K.sub.2O 3.4 3.5 3.6 3.2 2.3 3.2 2.0 3.4 3.6 3.4 Cs.sub.2O 4.9 MgO 1.8 1.8 2.2 4.1 0.6 1.7 1.5 1.8 2.9 1.8 3.1 CaO 3.3 3.3 3.7 2.9 2.8 3.3 2.6 3.3 SrO 4.0 ZnO 3.3 Al.sub.2O.sub.3 3.8 4.1 5.6 3.7 2.6 3.3 2.0 3.7 4.2 4.0 3.8 Y.sub.2O.sub.3 3.3 La.sub.2O.sub.3 1.6 GeO.sub.2 15.1 TiO.sub.2 0.2 ZrO.sub.2 1.0 2.0 Ce.sub.2O.sub.3* 1.4 1.7 2.7 2.8 2.6 1.9 1.5 1.9 2.8 2.0 CeO.sub.2 0.9 1.5 1.5 P.sub.2O.sub.5 3.6 3.6 3.7 3.3 3.4 3.4 2.8 3.2 4.3 3.6 4.7 V.sub.2O.sub.5 0.1 Ta.sub.2O.sub.5 0.7 Tb.sub.4O.sub.7 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.4 F 0.1 *used as cerium(III) acetylacetonate

TABLE-US-00011 TABLE II Example 1 2 3 4 5 6 7 8 9 10 11 12 T.sub.g/ C. 487 469 505 504 488 495 494 482 464 506 473 T.sub.s/ C. 1650 1650 1650 1650 1680 1680 1650 1680 1650 1680 1650 1600 T.sub.S/min 120 120 120 120 120 120 120 120 180 120 60 120 T.sub.N/ C. 520 520 530 520 510 520 510 530 530 490 T.sub.N/min 30 30 30 30 60 20 30 30 30 60 T.sub.C/ C. 800 800 800 810 830 860 760 900 800 740 T.sub.C/min 30 30 30 30 30 10 30 30 30 30 T.sub.Sint/ C. 890 920 t.sub.Sint/min 15 15 Main low low cristo- cristo- low cristo- low low low low low low crystal quartz quartz balite balite quartz balite quartz quartz quartz quartz quartz quartz phase Secondary Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 Li.sub.3PO.sub.4 CsAlSi.sub.5O.sub.12 Li.sub.3PO.sub.4 Li.sub.2Si.sub.2O.sub.5 Li.sub.3PO.sub.4 Li.sub.2Si.sub.2O.sub.5 Li.sub.3PO.sub.4 Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 crystal Li.sub.3PO.sub.4 Li.sub.3PO.sub.4 tridymite Li.sub.3PO.sub.4 SiO.sub.2 Li.sub.3PO.sub.4 Li.sub.2Si.sub.2O.sub.5 cristo- Li.sub.3PO.sub.4 Li.sub.3PO.sub.4 phase Li.sub.2Si.sub.2O.sub.5 balite tridymite Li.sub.3PO.sub.4 diopside