Abstract
A polycrystalline ceramic solid and a method for producing a polycrystalline ceramic solid are disclosed. In an embodiment a polycrystalline ceramic solid includes a main phase with a composition of the general formula: (1-y)Pb.sub.a(Mg.sub.bNb.sub.c)O.sub.3-e+yPb.sub.aTi.sub.dO.sub.3 with 0.055≤y≤0.065, 0.95≤a≤1.02, 0.29≤b≤0.36, 0.63≤c≤0.69, 0.9≤d≤1.1, and 0≤e≤0.1, and optionally one or more secondary phases, wherein, in each section through the solid, a proportion of the secondary phases relative to any given cross-sectional area through the solid is less than or equal to 0.5 percent, or wherein the solid is free of the secondary phases.
Claims
1. A polycrystalline, ceramic solid comprising: a main phase with a composition of the general formula:
(1−y)Pb.sub.a(Mg.sub.bNb.sub.c)O.sub.3-e+yPb.sub.aTi.sub.dO.sub.3 with 0.055≤y≤0.065, 0.95≤a≤1.02, 0.29≤b≤0.36, 0.63≤c≤0.66, 0.9≤d≤1.1, and 0≤e≤0.1; and optionally one or more secondary phases, wherein, in each section through the solid, a proportion of all secondary phases added together relative to any given cross-sectional area through the solid is less than or equal to 0.5 percent, or wherein the solid is free of the secondary phases.
2. The solid according to claim 1, wherein, in each section through the solid, the proportion of all secondary phases added together relative to any given cross-sectional area through the solid is less than or equal to 0.3 percent.
3. The solid according to claim 1, wherein the main phase is formed of grains with an average grain size, and wherein the average grain size, measured as a number-related median value using static image analysis, amounts to between 4 μm and 9 μm, inclusive.
4. An electrode comprising: the solid according to claim 1; and an electrical contact arranged at the solid.
5. A method for producing a polycrystalline, ceramic solid, wherein the solid comprises a main phase with a composition of the general formula:
(1−y)Pb.sub.a(Mg.sub.bNb.sub.c)O.sub.3-e+yPb.sub.aTi.sub.dO.sub.3, with 0.055≤y≤0.065, 0.95≤a≤1.02, 0.29≤b≤0.36, 0.63≤c≤0.66, 0.9≤d≤1.1, and 0≤e≤0.1; and optionally one or more secondary phases, wherein, in each section through the solid, a proportion of all secondary phases added together relative to any given cross-sectional area through the solid is less than or equal to 0.5 percent, or wherein the solid is free of the secondary phases, the method comprising: providing starting materials comprising elements Mg, Nb, Ti and Pb; producing a mixture comprising the starting materials; calcining the mixture to produce a calcined mixture; processing the calcined mixture into a green body; and sintering the green body, wherein, to control a lead content, sintering the green body proceeds in a closed system.
6. The method according to claim 5, further comprising adding in excess a Pb-containing starting material prior to sintering the green body.
7. The method according to claim 5, wherein a first starting material of the starting materials is Mg.sub.1/3Nb.sub.2/3O.sub.2.
8. The method according to claim 5, wherein a second starting material of the starting materials is TiO.sub.2.
9. The method according to claim 5, wherein in a third starting material of the starting materials is PbO or Pb.sub.3O.sub.4.
10. The method according to claim 5, wherein producing the mixture comprises producing the mixture by wet grinding.
11. The method according to claim 5, wherein the calcining the mixture comprises calcining the mixture at a temperature of between 800° C. and 860° C. inclusive.
12. The method according to claim 5, further comprising adding TiO.sub.2 and/or Nb.sub.2O.sub.5 to the calcined mixture before processing the calcined mixture, wherein a proportion of added TiO.sub.2 and/or Nb.sub.2O.sub.5 amounts to 0.01 to 0.4 weight percent relative to a weight of the calcined mixture.
13. The method according to claim 5, wherein processing the calcined mixture comprises: grinding the calcined mixture; adding a binder to the calcined mixture; spray drying the calcined mixture with the binder to produce ceramic pellets; and compression-molding the ceramic pellets to produce the green body.
14. The method according to claim 5, wherein sintering the green body comprises sintering at a temperature of 1150° C. to 1280° C. inclusive.
15. The method according to claim 5, wherein sintering the green body proceeds in the closed system, wherein the closed system is a closed container, and wherein the container contains at least one of the materials selected from the group consisting of Al.sub.2O.sub.3, ZrO.sub.2 and MgO.
16. The method according to claim 15, wherein the closed container has an interior in which one or more green bodies are arranged such that the degree of filling by volume of all the green bodies relative to the volume of the interior amounts to at least 30 vol. %.
17. A method for producing an electrode, the method comprising: performing the method for producing the solid according to claim 5; and applying an electrical contact to the solid.
18. The method according to claim 17, wherein the electrical contact is applied by applying and stoving a paste, and wherein the stoving is performed at a temperature of 680° C. to 760° C. inclusive.
19. A polycrystalline, ceramic solid comprising: a main phase with a composition of the general formula:
(1−y)Pb.sub.a(Mg.sub.bNb.sub.c)O.sub.3-e+yPb.sub.aTi.sub.dO.sub.3 with 0.055≤y≤0.065, 0.95≤a≤1.02, 0.29≤b≤0.36, 0.63≤c≤0.69, 0.9≤d≤1.1, and 0≤e≤0.1; and optionally one or more secondary phases, wherein the main phase is formed of grains with an average grain size, wherein the average size, measured as a number-related median value using static image analysis, amounts to between 4 μm and 9 μm, inclusive, and wherein, in each section through the solid, a proportion of all secondary phases added together relative to any given cross-sectional area through the solid is less than or equal to 0.5 per cent, or wherein the solid is free of the secondary phases.
20. The solid according to claim 19, wherein, in each section through the solid, the proportion of all secondary phases added together relative to any given cross-sectional area through the solid is less than or equal to 0.3 per cent.
21. A method for producing a polycrystalline, ceramic solid, wherein the solid comprises a main phase with a composition of the general formula:
(1−y)Pb.sub.a(Mg.sub.bNb.sub.c)O.sub.3-e+yPb.sub.aTi.sub.dO.sub.3 with 0.055≤y≤0.065, 0.95≤a≤1.02, 0.29≤b≤0.36, 0.63≤c≤0.69, 0.9≤d≤1.1, and 0≤e≤0.1; and optionally one or more secondary phases, wherein, in each section through the solid, a proportion of secondary phases relative to any given cross-sectional area through the solid is less than or equal to 0.5 per cent, or wherein the solid is free of the secondary phases, the method further comprises: providing starting materials comprising elements Mg, Nb, Ti and Pb; producing a mixture comprising the starting materials; calcining the mixture to produce a calcined mixture; adding TiO.sub.2 and/or Nb.sub.2O.sub.5 to the calcined mixture before processing the calcined mixture, wherein a proportion of added TiO.sub.2 and/or Nb.sub.2O.sub.5 amounts to 0.01 to 0.4 weight per cent relative to a weight of the calcined mixture processing the calcined mixture into a green body; and sintering the green body, wherein, to control a lead content, sintering the green body proceeds in a closed system.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention is explained further below with reference to figures. In this case, a conventional polycrystalline, ceramic solid (reference) is compared with a polycrystalline, ceramic solid (specimen) according to embodiments:
(2) FIGS. 1A and 1B show scanning electron micrographs (BSE micrographs) of a conventional solid (FIG. 1A) and one according to embodiments (FIG. 1B).
(3) FIGS. 2A and 2B show scanning electron micrographs (SE micrographs) of a conventional solid (FIG. 2A) and one according to embodiments (FIG. 2B).
(4) FIGS. 3A and 3B show element distribution images for the element magnesium of a conventional ceramic solid (FIG. 3A) and one according to embodiments (FIG. 3B).
(5) FIGS. 4A and 4B show tables with EDX results for the elemental composition of the main phase (FIG. 4A) and the secondary phase (FIG. 4B) of a conventional ceramic solid.
(6) FIG. 5 shows results for elemental composition (EDX results) of the main phase of the ceramic solid according to embodiments.
(7) FIGS. 6A and 6B show EDX/EBSD overlay images (EBSD=electron backscatter diffraction) for a conventional solid (FIG. 6A) and the solid according to embodiments (FIG. 6B).
(8) FIGS. 7A and 7B show the results of the evaluation of the grain sizes for a conventional solid (FIG. 7A) and the solid according to embodiments (FIG. 7B).
(9) FIG. 8 shows electrical capacitance versus temperature for a polycrystalline ceramic solid according to embodiments and a conventional polycrystalline ceramic solid with secondary phases.
(10) FIG. 9 shows the relationship between loss factor and temperature for a polycrystalline, ceramic solid according to embodiments and a conventional polycrystalline, ceramic solid with secondary phases.
(11) FIGS. 10A and 10B show a container with gap (FIG. 10A) and a completely closed container (FIG. 10B) as may be used to produce the specimen and to produce solids corresponding to the reference.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
(12) The figures and results are described in detail below:
(13) FIGS. 1A to 2B are in each case scanning electron micrographs. These and the further electron micrographs and measurement results described hereinafter were obtained using a Zeiss Merlin Compact VP scanning electron microscope. All four micrographs were captured at 1000 times magnification with an acceleration voltage of 20 kV in a vacuum of in each case around 2.2*10.sup.−6 mbar. The specimen and the reference are in each case polycrystalline, ceramic solids. Both specimen and reference were sawn up, embedded, sanded and polished for the scanning electron micrographs. To prevent charges, the polished section was vapor-coated with a thin carbon layer.
(14) FIGS. 1A and 1B show backscattered electron (BSE) contrast images (BSE micrographs) for the reference (FIG. 1A) and the specimen (FIG. 1B). FIGS. 2A and 2B, on the other hand, show secondary electron (SE) contrast images (SE micrographs) for the reference (FIG. 2A) and the specimen (FIG. 2B). The BSE and SE micrographs of FIGS. 1A and 2A were each captured at the same point of the reference. Likewise, the BSE and SE micrographs of FIGS. 1B and 2B were captured at the same point of the specimen. BSE micrographs provide a good material contrast (phase contrast), while more topographical information can be obtained using SE micrographs. Dark spots are visible on the BSE micrographs of reference and specimen. These dark spots are predominantly attributable to pores, since both the specimen and the reference similarly have a degree of porosity, albeit slight overall. The light background, on the other hand, is in each case attributable to the main phase.
(15) As already mentioned, SE micrographs allow conclusions to be drawn about the surface topography of the solid under investigation. The SE micrographs of FIGS. 2A and 2B also show the dark spots which are visible on the BSE micrographs, but the SE micrographs of the reference of FIG. 2A allow differentiation between two different types of dark spot, while FIG. 2B does not show different types of dark spot. FIG. 2A contains dark spots with light borders and dark spots without light borders. The dark spots with light borders are attributable to pores. The light borders are caused by the change in topography in the region of a pore. However, FIG. 2A also has dark areas without light borders, which are not attributable to pores but rather to secondary phases, as will be further explained below. Dark spots belonging to the secondary phase are each marked in FIG. 2A and also in FIG. 1A with the aid of circles. In contrast thereto, FIG. 2B only shows pores, but no secondary phases. The reference is distinguished by a considerable proportion of secondary phases, while the specimen according to embodiments does not have any secondary phases. The secondary phases marked in FIGS. 1A and 2A are distinguished by a partly acicular or angular structure. They have a different chemical composition from the otherwise light main phase, which forms the background to the micrographs. This is clear in particular with the aid of an investigation into the chemical composition of the main and secondary phases of the reference and the sole main phase of the specimen (FIGS. 3-5).
(16) FIGS. 3A and 3B show element distribution images for the element magnesium for the reference (FIG. 3A) and the specimen (FIG. 3B), which were obtained using SEM-EDX measurements (EDX denotes energy-dispersive X-ray spectroscopy). For EDX measurements an Oxford SDD 80 mm.sup.2 detector was used (Aztec). The images show the distribution of magnesium for the reference and the specimen manufactured by the method according to embodiments. The element distribution images in turn show the same spots which have already been depicted in FIGS. 1 and 2. Light spots indicate a high magnesium content. It is clear from a comparison of FIGS. 3A and 3B that the reference has Mg-rich spots. The secondary phase, which is present in the reference, is thus an Mg-rich secondary phase. Element distribution images also make it possible to quantify the proportion of the secondary phase in the polycrystalline, ceramic solid of the reference. An evaluation of Mg element distribution images of conventional, ceramic solids shows a high proportion of undesired secondary phases. For instance, the reference has a secondary phase which, for a section through the solid, on average shows a proportion of the secondary phase relative to a cross-sectional area through the solid of 0.7 percent. In contrast, the ceramic solid according to embodiments of FIG. 3B is free of Mg-rich spots. It does not have a secondary phase.
(17) Further element distribution images were furthermore captured for the elements C, O, Ti, Nb and Pb for the reference and the specimen. What is noteworthy here is that the element distribution images for lead (Pb Mal micrographs) for the spots which belong to the Mg-rich secondary phase of the reference indicate lead depletion relative to the main phase. From the different element distribution images it is clear that the specimen is free of undesired secondary phases, while the reference has an Mg-rich and simultaneously low-Pb secondary phase. The most important results of the investigation of the elemental composition of the main phase of the specimen and reference and the secondary phase of the reference are brought together in the tables of FIGS. 4A, 4B and 5.
(18) FIGS. 4A and 4B show the EDX results from comparative scanning electron microscopy for the reference, wherein FIG. 4A reproduces the EDX results of the main phase and FIG. 4B the EDX results of the secondary phase of the ceramic solid. FIG. 5 shows EDX results from comparative scanning electron microscopy for the specimen. Four EDX spectra are shown in each case. The measured proportions of the elements O, Mg, Ti, Nb and Pb are plotted in atom percent for each spectrum. The average value was formed in each case from the 4 spectra for the proportions of Mg, Ti, Nb and Pb. Experience shows that the proportion of light elements, such as oxygen, is underestimated in EDX measurements. Normalization for determining the empirical formula was therefore appropriately undertaken such that the total content of Mg+Ti+Nb corresponds in total to 1. The resultant coefficients of the associated chemical formula may likewise each be inferred from the tables. The coefficients respectively expected on the basis of weighed-out quantities have additionally been indicated for the main phase. A comparison of FIGS. 4A and 5 indicates that the lead content deviates less from the ideal composition in the case of the specimen according to embodiments. The lead content of the main phase of the reference is, at 0.941, markedly lower than the ideal value 1.0. In contrast, the main phase of the specimen comes markedly closer to the ideal value. Finally, conclusions may be drawn from FIG. 4B about the chemical composition of the secondary phase. As has already been mentioned above, the secondary phase is rich in Mg and low in Pb. The Nb content is slightly higher than in the main phase. Without being limited to the theory, the secondary phase appears to be most readily described by the formula Mg.sub.2/3Nb.sub.1/3O.sub.3 phase.
(19) FIGS. 6A and 6B show EDX/EBSD overlay images (EBSD=electron backscatter diffraction) for the reference (FIG. 6A) and the specimen (FIG. 6B). A forward scatter detector (FSD) was used for the micrographs. The EBSD measurements were carried out on etched specimens. In this respect, the following settings were selected for the reference and specimen respectively: acceleration voltage 20.00 kV; specimen tilt (degrees) 69.99°; hit ratio 94.25% to 94.99%; capture speed 66.25 to 66.35 Hz. The phases for the micrographs were, based on the phase Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3: a=4.05 Å; b=4.05 Å; c=4.05 Å; α=90.00°; β=90.00°; γ=90.00°; space group 221; ICSD database. From the figures it is particularly easy to compare the grain sizes of the crystallites of the reference and of the specimen. The specimen is distinguished by distinctly larger grain sizes.
(20) A quantitative evaluation of the differences in grain size is shown in FIGS. 7A and 7B. Determination of the equivalent circular diameter (ECD) has already been explained above. It is clear in particular from the figures that the specimen according to embodiments (FIG. 7B), at 5.32 μm, has a markedly larger d.sub.50, measured as a number-related median value using static image analysis, than the reference, at 3.79 μm (FIG. 7A). The overall grain size distribution is thus shifted in the specimen to larger grain sizes compared to the reference specimen. This shows that the method according to embodiments, by means of which the specimen was manufactured, leads not only to the avoidance of secondary phases, but at the same time also to larger crystallites, whereby improved electrical capacitances (FIG. 8) and lower power losses (FIG. 9) are achieved.
(21) FIG. 8 compares the electrical capacitances of the specimen and the reference. The graph shows the dependency of the electrical capacitance, stated in nanofarads [nF], on the temperature in degree centigrade [° C.]. The measurements were each carried out at 200 kHz and 1V. Both solids have an electrical capacitance maximum in the temperature range of between 30 and 42° C. This is attributable to the comparable chemical composition of the main phase. From a comparison of the measurement curves obtained, it becomes clear that the electrical capacitance of the specimen is constantly markedly higher over the entire measured temperature range than the electrical capacitance of the reference. The capacitance is on average around 5% higher for the specimen according to embodiments.
(22) FIG. 9 shows the dependency of the loss factor on the temperature in degree centigrade [° C.]. The measurements were each carried out at 200 kHz and 1V. For specimen and reference the loss factor drops as the temperature increases. In contrast to the reference, the loss factor for the important temperature range between 20 and 45° C. is markedly lower, however, which means that if the solid according to embodiments is used in electrodes, low power losses are obtained. This leads to greater efficiency and above all to lower self-heating.
(23) FIGS. 10A and 10B illustrate how the reference and the specimen, as described in FIGS. 1-9, may be obtained. The specimen is an inventive polycrystalline, ceramic solid according to the first aspect, obtained using the inventive method of the fourth aspect. In this case, the sintering step F) was carried out in a container according to FIG. 10B with a container body (1) and a container plate (2). Together these form a closed container, in the interior (3) of which the sintering step F) of the method according to embodiments is carried out. To this end, one or more green bodies are arranged in the interior (3) of the closed container. The closed container forms a closed system which prevents outgassing of PbO. The shape of the container may be varied. The material of the container is selected such that it is not suitable for absorbing PbO and is impermeable to PbO, so enabling particularly effective control of the lead balance during sintering. In contrast, conventional polycrystalline ceramic solids required for high capacitance electrodes for treating patients are not sintered with sufficient lead balance control. This leads to outgassing of PbO during sintering and thus to inhomogeneities in the solid. The inventors found in particular that this is responsible for the formation of undesired secondary phases, as may be found in conventional ceramic solids. The secondary phases lead to a reduction in capacitance and lend conventional ceramic solids a yellowish color shade. The consequences of lack of lead balance control are shown taking the reference as an example. The reference may be obtained by sintering in an arrangement according to FIG. 10A. FIG. 10A shows a container body (1) and the container plate (2) and means for providing a gap (4) between container body and container plate. The container of FIG. 10A thus has a gap. The size of the gap amounts to 5 mm. A degree of gas exchange is thus possible between the interior (3) of the container and the surrounding environment. This leads to some of the Pb of the solid being released during sintering in the form of PbO. In contrast to the specimen, the reference obtained in this way has a yellow color shade.
(24) The specimen and reference were obtained as follows:
(25) In both cases first of all green bodies of the same composition were produced. To this end, in each case 34.9494 kg Mg.sub.1/3Nb.sub.2/3O.sub.2, 83.8043 kg Pb.sub.3O.sub.4 and 1.7488 kg TiO.sub.2 were weighed out. The starting materials were preground to a target d50 grain size of around 1.0 μm in 100 liters of deionized water. The resultant mixture was subjected to spray drying. The mixture was then calcined for 6 hours at 820° C., ground to a d50 grain size of around 0.8 μm and spray-granulated with 3 weight percent PVA binder.
(26) The green bodies were made from the ceramic pellets by compression-molding. The compressed density was 4.8 g/ml. The green bodies were decarbonized at 450° C.
(27) The specimen was obtained by sintering in a closed MgO container according to FIG. 10B. The reference was obtained by sintering in an MgO container according to FIG. 10A. The gap was 5 mm in this case. 1250° C. was selected in each case as the sintering temperature. The retention time at 1250° C. was 4 hours. The degree of filling by volume in the case of the specimen was around 45 vol. %.
(28) The invention is not restricted by the description given with reference to the exemplary embodiments. Rather, the invention encompasses any novel feature and any combination of features, including in particular any combination of features in the claims, even if this feature or this combination is not itself explicitly indicated in the claims or exemplary embodiments.
