Ceramics and glass ceramics exhibiting low or negative thermal expansion

Abstract

Ceramics and glass-ceramics have low and/or negative coefficients of thermal expansion. Crystalline phases of the formula AM.sub.2Si.sub.2-yGe.sub.yO.sub.7 (A=Sr and Ba and M=Zn, Mg, Ni, Co, Fe, Cu, Mn, with Sr, Ba and Zn necessarily having to be present) can be produced by conventional ceramic processes or by crystallization from glasses. The compositions form solid solutions, where the elements indicated as component M can be replaced by one another in virtually any concentration but the concentration of Zn must always be at least 50% of the sum of all components indicated under M. The stoichiometry of these silicates and also their structure can differ to a greater or lesser extent.

Claims

1. A material based on a crystal phase of composition Ba.sub.1-xSr.sub.xM.sub.2Si.sub.2-yGe.sub.yO.sub.7 with 0.1<x<1 and 0y2, where M comprises Zn as a constituent such that the material has a negative coefficient of thermal expansion or a coefficient of linear thermal expansion of <1.Math.10.sup.6K.sup.1.

2. The material as claimed in claim 1, wherein M contains at least one further constituent selected from the group consisting of Mg, Mn, Co, Ni, Fe and Cu.

3. The material as claimed in claim 1, wherein the crystal phase is present in a volume concentration of >50% and other phases present being other crystal phases or one or more glass phases of a different chemical composition.

4. The material as claimed in claim 1, wherein the crystal phase has been crystallized from a glass.

5. A glass-ceramic or ceramic product comprising the material as claimed in claim 1.

6. A method of producing the material as claimed in claim 1, comprising sintering amorphous powders, with densification by viscous flow commencing before crystallization of the crystal phase occurs at an end portion of the sintering.

7. A method of producing the material as claimed in claim 1, comprising sintering crystalline powders which contain the crystal phase.

8. Oven windows comprising the material as claimed in claim 1.

9. Cookware comprising the material as claimed in claim 1.

10. Telescope mirrors or other passive elements for optical technologies comprising the material as claimed in claim 1.

11. Joining material having low coefficients of thermal expansion comprising the material as claimed in claim 1.

12. Glazing material having low coefficients of thermal expansion comprising the material as claimed in claim 1.

13. The material as claimed in claim 2, wherein a sum of concentration(s) of the at least one further constituent does not exceed the concentration of Zn.

14. The material as claimed in claim 1, wherein the composition is selected from the group consisting of Ba.sub.0.9Sr.sub.0.1Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.8Sr.sub.0.2Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.1Sr.sub.0.9Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.01Sr.sub.0.99Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Mg.sub.0.5Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.3Mg.sub.0.7Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Co.sub.0.5Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5ZnCoSi.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.9Mn.sub.0.1Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Mn.sub.0.5Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.8Ni.sub.0.2Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.7Ni.sub.0.3Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.6Ni.sub.0.4Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.9Cu.sub.0.1Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.7Cu.sub.0.3Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Cu.sub.0.5Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.2Si.sub.1.9Ge.sub.0.1O.sub.7, and Ba.sub.0.5Sr.sub.0.5Zn.sub.1.7Fe.sub.0.3Si.sub.2O.sub.7.

15. The material as claimed in claim 1, wherein the composition is selected from the group consisting of Ba.sub.0.8Sr.sub.0.2Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.1Sr.sub.0.9Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.01Sr.sub.0.99Zn.sub.2Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Mg.sub.0.5Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Co.sub.0.5Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Mn.sub.0.5Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.8Ni.sub.0.2Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.7Ni.sub.0.3Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.1.9Cu.sub.0.1Si.sub.2O.sub.7, Ba.sub.0.5Sr.sub.0.5Zn.sub.2Si.sub.1.9Ge.sub.0.1O.sub.7, and Ba.sub.0.5Sr.sub.0.5Zn.sub.1.7Fe.sub.0.3Si.sub.2O.sub.7.

16. A material based on a crystal phase of composition Ba.sub.1-xSr.sub.xM.sub.2Si.sub.2-yGe.sub.yO.sub.7 with 0.1<x<1 and 0y2, where M comprises Zn as a constituent such that the material has a negative coefficient of thermal expansion or a coefficient of linear thermal expansion of <1.Math.10.sup.6 K.sup.1 and M further contains at least one further constituent selected from the group consisting of Mg, Mn, Co, Ni, Fe and Cu, wherein the crystal phase is present in a volume concentration of >50% and other phases present being other crystal phases or one or more glass phases of a different chemical composition, and a sum of concentration(s) of the at least one further constituent does not exceed the concentration of Zn.

Description

DETAILED DESCRIPTION

(1) A very low coefficient of expansion means 1.Math.10.sup.6K.sup.1 to 1.Math.10.sup.6K.sup.1.

(2) We provide crystalline phases of the formula AM.sub.2Si.sub.2O.sub.7 (A=Sr, Ba and M=Zn, Mg, Fe, Ni, Co, Cu, Mn) that can be produced by conventional ceramic processes or crystallization from glasses. The compositions indicated form solid solutions, with the elements indicated as component M being able to be replaced by one another in virtually any concentration. The stoichiometry of these silicates and their structure can differ to a greater or lesser extent. SiO.sub.2 can also be replaced by GeO.sub.2.

(3) The compound BaZn.sub.2Si.sub.2O.sub.7 is present as monoclinic low-temperature phase at room temperature and this is transformed at 280 C. into an orthorhombic high-temperature phase (J. H. Lin, G. X. Lu, J. Du, M. Z. Su, C.-K Loong, J. W. Richardson Jr., J. Phys. Chem. Solids 60, 1999, 975-983). The coefficient of thermal expansion of the low-temperature phase is very high, at 13-16.Math.10.sup.6 K.sup.1, the phase transformation is associated with a volume increase of 2.8% and the high-temperature phase then has, depending on composition, a very small or even negative thermal expansion at high temperature (M. Kerstan, M. Mller, C. Rssel, J. Solid State Chem. 188, 2012, 84-91).

(4) Incorporation of SrO into crystals of the formula AM.sub.2Si.sub.2O.sub.7 (A=Ba, Sr and M=predominantly Zn, in addition also Mg, Fe, Ni, Co, Cu, Mn) effects stabilization of the high-temperature phase having a low and/or negative expansion so that this is also stable at room temperature. The component M should always have Zn as a main constituent, i.e., the proportion of Zn in the component M should be >50%. If, in addition to SrO, relatively high concentrations of Co, Mg, Fe, Ni, Cu and Mn are incorporated instead of Zn, this leads to stabilization of high-expansion phases having the structure of low-temperature BaZn.sub.2Si.sub.2O.sub.7. However, relatively small concentrations of Co, Mg, Fe, Ni, Cu and Mn can be incorporated into the crystal without the low-temperature phase being stabilized. This means that the high-temperature phase is retained at room temperature even on incorporation of Co, Mg, Fe, Ni, Cu and Mn.

(5) The material can be sintered from powders, with the above-described Ba.sub.1-xSr.sub.xM.sub.2Si.sub.2O.sub.7 phase being the main crystal phase. The powder used for sintering can, however, contain one or more other crystalline or amorphous phases. The powders described can be produced by solid-state reactions from, for example, oxides and carbonates or else by other methods, for example, by sol-gel or coprecipitation processes or else by gas-phase reactions.

(6) The material can also be obtained by controlled crystallization of glass. This is possible because the components of the crystalline phases are also constituents of many relatively crystallization-stable glasses. If an only relatively small amount of SiO.sub.2 is added to a Ba.sub.1-xSr.sub.xM.sub.2Si.sub.2O.sub.7 composition, a relatively stable glass can be obtained. The stability of the glass can be further improved by addition of further components such as, for example, B.sub.2O.sub.3, La.sub.2O.sub.3 or ZrO.sub.2, since these components in low concentrations suppress nucleation in our chemical compositions. However, in higher concentrations, La.sub.2O.sub.3 or ZrO.sub.2 can have the opposite effect, i.e., cause nucleation and thus crystallization, and consequently act as nucleating agents.

(7) Since most glasses and glass-ceramics having high proportions of BaO and/or SrO have a very high coefficient of thermal expansion, the coefficient of expansion of glass-ceramics can be varied within a wide range by variation of the BaO/SrO ratio without important glass properties, e.g., the glass transition temperature or the coefficient of expansion of the pure glass, being significantly altered.

(8) Furthermore, the component M is also of considerable importance in the composition Ba.sub.1-xSr.sub.xM.sub.2Si.sub.2O.sub.7. In M=Zn, the crystalline phase can be stabilized as soon as approximately 10 mol % of the BaO is replaced by SrO. Incorporation of components other than Zn also leads to stabilization of the high-expansion low-temperature phase. As component M, it is possible to use ions of the following divalent metals: Mg, Ni, Co, Fe, Cu, Mn.

(9) If too much Zn is replaced by other components, the low-temperature phase which has a high coefficient of expansion is stabilized. This effect makes it possible to produce glasses having approximately the same properties, but whose expansion behavior after crystallization can be varied considerably. These glass-ceramics therefore represent a possible way of controlling the coefficient of expansion in a targeted manner within a wide range.

(10) Our ceramics and glass ceramic are illustrated below with the aid of working examples.

Working Example 1

(11) The compositions indicated in the following table are produced by conventional ceramic processes (solid-state reaction). This means that the starting raw materials are heated below the liquidus temperature for 30-50 hours. During this time, the powders are repeatedly milled and homogenized (4 to 10 times). Phase purity is checked by x-ray powder diffraction. BaCO.sub.3, SrCO.sub.3, ZnO, SiO.sub.2, GeO.sub.2, MgO, MnCO.sub.3, NiO, Fe(COO).sub.2.2H.sub.2O, Co.sub.3O.sub.4 and CuO are used as raw materials. Two crystal structures can be stabilized. X-ray powder diffraction is used to check whether the monoclinic or orthorhombic modification is stable at room temperature. The latter has the desired negative thermal expansion. Which of the two phases is stable at room temperature can be seen in the table. In some compositions, the two phases are present side-by-side. The iron-containing samples were produced under an argon atmosphere.

(12) TABLE-US-00001 Production Ortho- Mono- temp. Foreign Composition rhombic clinic [ C.] phases BaZn.sub.2Si.sub.2O.sub.7 x 1200-1250 Ba.sub.0.96Sr.sub.0.04Zn.sub.2Si.sub.2O.sub.7 x 1200-1250 Ba.sub.0.94Sr.sub.0.06Zn.sub.2Si.sub.2O.sub.7 x 1200-1250 Ba.sub.0.9Sr.sub.0.1Zn.sub.2Si.sub.2O.sub.7 x x 1150-1200 Ba.sub.0.8Sr.sub.0.2Zn.sub.2Si.sub.2O.sub.7 x 1150-1200 Ba.sub.0.5Sr.sub.0.5Zn.sub.2Si.sub.2O.sub.7 x 1200-1250 Ba.sub.0.1Sr.sub.0.9Zn.sub.2Si.sub.2O.sub.7 x 1150-1200 Ba.sub.0.01Sr.sub.0.99Zn.sub.2Si.sub.2O.sub.7 x 1150-1200 Sr.sub.2ZnSi.sub.2O.sub.7, Zn.sub.2SiO.sub.4 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Mg.sub.0.5Si.sub.2O.sub.7 x 1250-1300 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.3Mg.sub.0.7Si.sub.2O.sub.7 x x 1250-1320 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Co.sub.0.5Si.sub.2O.sub.7 x 1250-1300 Ba.sub.0.5Sr.sub.0.5ZnCoSi.sub.2O.sub.7 x 1200-1250 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.9Mn.sub.0.1Si.sub.2O.sub.7 x 1250-1300 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Mn.sub.0.5Si.sub.2O.sub.7 x x 1150-1200 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.8Ni.sub.0.2Si.sub.2O.sub.7 x 1150-1200 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.7Ni.sub.0.3Si.sub.2O.sub.7 x x 1100-1150 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.6Ni.sub.0.4Si.sub.2O.sub.7 x 1150-1200 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.9Cu.sub.0.1Si.sub.2O.sub.7 x 1150-1200 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.7Cu.sub.0.3Si.sub.2O.sub.7 x x 1100-1150 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.5Cu.sub.0.5Si.sub.2O.sub.7 x 1050-1100 Ba.sub.0.5Sr.sub.0.5Zn.sub.2Si.sub.1.9Ge.sub.0.1O.sub.7 x 1250-1350 Ba.sub.0.5Sr.sub.0.5Zn.sub.1.7Fe.sub.0.3Si.sub.2O.sub.7 x 1030-1060

(13) Powders that predominantly contain the orthorhombic modification display, after prior isostatic pressing and subsequent sintering, negative or very low coefficients of thermal expansion.

Working Example 2

(14) Production of a ceramic having the composition Ba.sub.0.2Sr.sub.0.8Zn.sub.2Si.sub.2O.sub.7 by the acetates and also colloidal SiO.sub.2.

(15) For this purpose, barium acetate, strontium acetate and zinc acetate are dissolved in the correct stoichiometry in deionized water. The appropriate amount of colloidal SiO.sub.2 having a particle size of 50 nm is added to the dissolved acetates. While stirring continually, the water is evaporated over a period of one day until a viscous mass is formed. This is dried, milled by a ball mill, mixed with a 1% strength solution of polyvinyl alcohol in water and dried again. The powder obtained is subsequently uniaxially pressed and sintered at 1130 C. The ceramic obtained is examined by X-ray powder diffraction. The crystal structure corresponds to that of high-temperature BaZn.sub.2Si.sub.2O.sub.7, as was also found in Working Example 1. The ceramic is phase-pure and displays a coefficient of linear thermal expansion of 12.Math.10.sup.6 K.sup.1.

Working Example 3

(16) A glass having the composition 8 BaO.8SrO.34ZnO.46SiO.sub.2.1ZrO.sub.2.3La.sub.2O.sub.3 is melted at a temperature of 1450 C. in a platinum crucible. The glass displays a very low viscosity (<10 Pas) at this temperature. Crystallization of a cylindrical specimen having a length of about 20 mm and a diameter of about 8 mm at 900 C. for five hours gives a coefficient of linear thermal expansion of 0.5.Math.10.sup.6K.sup.1 (measured in the temperature range of 25 to 300 C.).

Working Example 4

(17) A glass having the composition 8 BaO.8SrO.30ZnO.5MgO.45SiO.sub.2.2ZrO.sub.2 2La.sub.2O.sub.3 is melted at 1400-1450 C. in a platinum crucible. After crystallization at 900 C. for five hours, the glass has a coefficient of linear thermal expansion of 2.Math.10.sup.6 K.sup.1 (measured in the temperature range of 25 to 300 C.).

Working Example 5

(18) A glass having the composition 8 BaO.8SrO.30ZnO.5CoO.45SiO.sub.2.1ZrO.sub.2.1La.sub.2O.sub.3.2B.sub.2O.sub.3 is melted at 1350-1450 C. in an Al.sub.2O.sub.3 crucible. After crystallization at 900 C. for five hours, the glass-ceramic has a coefficient of linear thermal expansion of 2.4.Math.10.sup.6 K.sup.1 (measured in the temperature range of 25 to 300 C.).

Working Example 6

(19) A glass having the composition 8BaO.8SrO.34ZnO.44SiO.sub.2.1ZrO.sub.2.1La.sub.2O.sub.3.4B.sub.2O.sub.3 is comminuted to an average particle size of 10 m. The glass powder obtained in this way is mixed with a 1% strength solution of polyvinyl butyral in water and dried. The powder is subsequently cold isostatically pressed with the aid of a silicone mold. The compact is finally sintered at 900 C. for five hours. During heating, the powder is densified by viscous flow, and the glass subsequently crystallizes. A coefficient of linear thermal expansion of 4.1.Math.10.sup.6 K.sup.1 is obtained in the temperature range of 100 to 500 C.

Working Example 7

(20) A glass having the composition 7.5BaO.7.5SrO.32ZnO.47SiO.sub.2.5ZrO.sub.2.1La.sub.2O.sub.3 is melted at 1350-1450 C. The glass transition temperature is 695 C. Crystallization at 815 C. for 20 hours leads to formation of crystals having the composition Ba.sub.0.5Sr.sub.0.5Zn.sub.2Si.sub.2O.sub.7. Small amounts of ZrO.sub.2 additionally crystallize. The latter acts as nucleating agent. The glasses crystallized at 815 C. have a coefficient of linear thermal expansion of 2.1.Math.10.sup.6 K.sup.1.

Working Example 8

(21) A ceramic having the composition Sr.sub.0.5Ba.sub.0.5Zn.sub.1.9Ni.sub.0.1Si.sub.2O.sub.7 is produced by a solid-state reaction. For this purpose, the appropriate amounts of BaCO.sub.3, SrCO.sub.3, ZnO, NiO and SiO.sub.2 are slurried in ethanol and subsequently milled in a planetary mill to an average particle size of <3 m. The mixture of starting materials and ethanol is subsequently dried at 110 C. for three hours. The powder formed in this way is converted into the corresponding virtually phase-pure ceramic by a heat treatment at 1200 C. for ten hours. The ceramic is then once again milled to an average particle size of <3 m and uniaxially pressed. The compact is subsequently sintered at 1200 C. for ten hours. A dilatometric measurement indicates a coefficient of expansion of 19.3.Math.10.sup.6K.sup.1 in the temperature range of 100 to 300 C.

Working Example 9

(22) A glass having the composition 8BaO8SrO30ZnO5MgO45SiO.sub.23B.sub.2O.sub.3 is melted at 1350 C. in a platinum crucible. The raw materials BaCO.sub.3, SrCO.sub.3, ZnO, MgO, SiO.sub.2 and H.sub.3BO.sub.3 are used for this purpose. The glass is poured into a steel mold which has been preheated to 700 C. and subsequently transferred into a cooling furnace that has likewise been preheated to 700 C., and subsequently cooled at 2 K/min. Crystallization of the glass block in the temperature range of 700 to 780 C. produces a glass-ceramic which has a thermal expansion close to zero, measured in the temperature range from room temperature to 650 C. This can thus be used as support material for optical components.

(23) Materials made from the compositions can thus be utilized in oven windows, cookware, telescope mirrors and other passive elements for optical technologies. Glasses having SiO.sub.2 concentrations of >98%, borosilicate glasses or glass-ceramics based on lithium aluminosilicate having a thermal expansion of virtually zero can comprise such materials. We also provide glazing products having low coefficients of thermal expansion and SiO.sub.2 concentrations of >98%, borosilicate glasses or glass-ceramics based on lithium aluminosilicate having a thermal expansion of virtually zero comprised of such materials.