SI21615A - LOW SINTERABLE FERROELECTRIC CERAMICS BASED ON Li2O DOPED (Ba, Sr) TiO3 - Google Patents

Abstract

The subject of invention is a ferroelectric ceramics based on (Ba, Sr)TiO3, whose sintering temperature is lowered to less than 920 degrees Celsius with addition of Li2O, which allows it to be sintered simultaneously with metallic silver. The low sinterable ferroelectric ceramics according to the invention is manufactured according to standard production procedures for ceramic elements, i.e. by casting or piling up thick layers or creation of thin layer films. The ceramics is characterised in that to a solid solution of (Ba1-xSrx)TiO3, where x is greater than 0 and less than 1, up to 20 w. % Li2O are added in the form of oxide or in any other form, which decomposes during thermal treatment, enabling the reaction of Li+ ions with BST.

Description

Low -interactive ferroelectric ceramics based on (Ba, Sr) TiO ₃ doped with Li ₂ O

The subject of the invention is ferroelectric ceramics based on (Ba, Sr) TiO ₃ , which with the addition of L12O lowers the sintering temperature to <920 ° C, which allows simultaneous sintering with metallic silver.

A new development trend emerging in microwave technology enables the integration of passive components (filters, capacitors, resonators) and significant miniaturization compared to conventional dielectric components. This is made possible by the low temperature co-sintered ceramics (LTCC) technology. The essence of LTCC technology is the integration of passive components in multifunctional, three-dimensional modules. These modules consist of individual ceramic layers with a microwave circuit that is connected into a three-dimensional structure. In order to remove the organic binder and thicken the ceramic layers and the electrode metal, such a three-dimensional composite structure must be thermally treated. The sintering takes place at a relatively low temperature, about 900 ° C, which is conditioned by the characteristics of the electrode metal in the module. Co-sintering of metal and ceramic layers is one of the most demanding processes in the production of LTCC modules. Large product imperfections can occur due to uneven metal shrinkage rates and ceramics during sintering. Due to unequal shrinkage of different layers of the sample, various defects can occur during the sintering process, such as cracks, wrinkling, twisting, cracking, delamination, etc., which cause a significant deterioration of the module's characteristics or even loss of functionality.

Due to the specific construction of the LTCC, it is necessary to select very compatible materials which, in addition to the corresponding dielectric properties, must satisfy a number of other requirements related to simultaneous sintering. One of these requirements is the low sintering temperature. Metal melting points are a limiting factor when using high sintering temperatures. Ceramics must therefore be sintered at a temperature below 50 ° C to 100 ° C below the melting point of the electrode. Melting the electrodes would damage the internal structure of the module and thus cause a loss of functionality. The selection of metals is very limited because low microwave dielectric losses are associated with high conductivity of the electrode metal. Due to its good conductivity and resistance to oxidation, silver is most commonly used in LTCC technology, which has a melting point at 961.9 ° C.

Due to the complexity of thermal processing of LTCC modules, the functionality of the ceramic layers in today's LTCC systems is limited to low-dielectric substrate materials, while high-dielectric capacitor, ferrite, semiconductor materials, etc. are being intensively developed, all of these materials being considered from the point of view of thermal treatment and chemical compatibility. compatible with existing LTCC technology.

The inventive material is classified in the new materials category for LTCC technology and for all other technologies requiring the simultaneous sintering of ceramics with a silver electrode, such as technologies for making some versions of adaptive dielectric resonators [PK Petrov et al., Elect. Lett., 37, 2001, p. 17], multilayer capacitors [SM Rhim et al., J. Am. Ceram. Soc., 83, 2000, p. 3009] and elements with a positive temperature coefficient of resistance [JK Lee et al., J. Am. Ceram. Soc., 85, 2002, p. 1173]. The material is based on a solid solution of BaTiCb and SrTiCF with the general formula (Bai_ _x Sr _x ) TiO3 (hereinafter BST). It is a ferroelectric and is characterized by the fact that the dielectric constant depends on the prestress, or that the dielectric constant can be controlled by the voltage. Such materials are called electrically flexible materials. BST is a very well researched electrically adaptable material that can be used in the form of a monolithic ceramic element or in the form of thick or thin films. The electrical properties of the solid solution with the composition (Bao 6Sro.4) Ti03 are: dielectric constant -3400, dielectric losses tan5 = 10 'm electrical adaptability Δε = 16.4%. Properties can be significantly altered by low concentrations of dopants (eg Fe, Y, Ga, Mn). In particular, dielectric losses can be reduced even below tanb = 5TO ' ³ and increase the electrical adaptability to Δε = 23.3% [SB Hemer et al., Mater. Lett., 15, 1993, p. 317],

The basic problem that prevents the use of BST in LTCC technology and all other devices requiring the simultaneous sintering of ceramics and silver is the sintering temperature being too high. This amounts to dust with a composition (Bao.6Sro.4) Ti03 with an average particle size below 1 pm of about 1350 ° C. In the past, some studies have already been conducted to try to lower the BST sintering temperature accordingly. With the addition of B2O3, the sintering temperature of BST can be lowered to 1100 to 1150 ° C, leaving the dielectric and ferroelectric properties of such a material similar to the properties of undoped BST [S. M. Rhim, J. Amer. Ceram. Soc., 83, 2000, p. 1145], a significant decrease in sintering temperature could not be achieved by using alumosilicate fluxes [H.F. Cheng et al., J. Am. Ceram. Soc., 76, 1993, p. 827],

It is an object of the invention to produce a BST powder having a sintering temperature <920 ° C and being chemically compatible with metallic silver under these conditions. This would allow for the simultaneous sintering of BST dust and silver electronic elements.

According to the invention, the problem is solved according to an independent patent claim.

The invention will be described on the basis of embodiments and illustrations showing:

Figure 1: Dynamic dilatometer recording of the thickening (Bao.6Sro.4) Ti03 powder with an average particle size of 0.8 pm and the addition of 1.5% L12O (heating rate is 10 ° C / min),

Figure 2: Isothermal dilatometer thickener (Bao.6Sro.4) Ti03 powder with an average particle size of 0.8 pm and the addition of 1.5% L12O,

Figure 3: Microstructure of ceramics (Bao 6Sro.4) Ti03 with the addition of 1.5% L12O, sintered for 2h at 900 ° C,

Figure 4: Frequency and temperature dependence of the dielectricity of ceramics with the composition (Bao.6Sro.4) Ti03 with the addition of 0.5 wt% L12O sintered for 2h at 900 ° C.

According to the invention, up to 20% by weight of L12O in the form of oxide or any other form which, during thermal treatment, decomposes and enables the reaction of Li ⁺ ions with BST (eg L12CO3 or CH3COOL1 - Li-acetate). Part of the BST thus reacts with Li ⁺ to form the Li-titanate phase, and the excess BaO and SrO is compensated by the formation of the (Ba, Sr) -ortotitanate phase. The reaction described results in the surface activation of the remaining BST phase resulting from the generation of surface structural defects and the formation of a liquid phase during the sintering process. This accelerates compaction even at very low temperatures (<920 ° C). Despite the presence of the liquid phase, due to the very low temperatures of thermal treatment, BST grains do not grow, but their surface structure regenerates. After sintering, BST contains ceramics with low concentrations of Li-titanate and (Ba, Sr) -ortotitanate phases, as well as a matrix BST phase, which provides adequate dielectric properties as well as electrical adaptability.

Example 1:

To the pre-reacted powder with the composition (Bao.6Sro.4) Ti03 was added 0.5 wt% Li ₂ O in the form of a 5% solution of acetic acid Li-acetate. The mixture was homogenized and calcined at 10OH at 850 ° C. The calcinate is ground to an average particle size of less than 1 pm and the powder thus prepared is compressed into sintered wheels for 2 hours at 900 ° C. The dielectricity of the ceramics thus prepared at 20 ° C and a frequency of 1MHz is 3470, the dielectric loss of tanb <10 ^-4 , and the temperature and frequency dependence of the dielectricity is shown in the graph in Figure 4. Electrical adaptability at room temperature and bias of 2kV / mm is 15.7%.

Example 2:

Pre-reacted powder with the composition (Bao.6Sro.4) Ti03 was added 0.5 wt% Li ₂ O in the form of polycrystalline L12CO3. The mixture was homogenized and calcined at 10OH at 850 ° C. The calcinate is ground to an average particle size of less than 1 pm and the powder thus prepared is compressed into sintered wheels for 2 hours at 900 ° C. The dielectricity of the thus prepared ceramics at 20 ° C and the frequency 1MHz is 3456, the dielectric loss tand <10 ^-4 , and the temperature and frequency dependence of the dielectricity is the same as in Execution Case 1. Electrical flexibility at room temperature and bias 2kV / mm is 15.7%

Example 3:

To the pre-reacted powder with the composition (Bao.6Sro.4) Ti03 was added 1.5 wt% Li ₂ O in the form of a 5% solution of acetic acid Li-acetate. The mixture was homogenized and calcined at 10OH at 850 ° C. The calcinate is ground to an average particle size of less than 1 pm and the powder thus prepared is compressed into sintered wheels for 2 hours at 900 ° C. The dielectricity of the thus prepared ceramic at 20 ° C and the frequency 1MHz is 3260, the dielectric losses tan5 = 7-10 “ ⁴ , and the temperature and frequency dependence of the dielectricity is not significantly different from the one shown in embodiment 1. Electrical adaptability at room temperature and bias of 2kV / mm is 15.4%

Embodiment 4: Pre-reacted powder with composition (Bao gSroUTiCb was added 2.5 wt% Li ₂ O in the form of a 5% solution of acetic acid Li-acetate. The mixture was homogenized and calcined lOh at a temperature of 850 ° C. The calcinate was ground to an average particle size of less than 1 pm and the powder thus prepared is compressed into sintered coils for 2h at 900 ° C. The dielectricity of the ceramics thus prepared at 20 ° C and 1MHz is 2730, and the dielectric losses tand = 1-10 ³ , and the temperature and frequency dependence of the dielectricity in none does not differ significantly from the dependence shown in embodiment 1. Electrical flexibility at room temperature and bias of 2kV / mm is 15.3%.

Experiments were tested for compositions containing up to 20 wt% Li ₂ O and found that they all function as described in the patent. Depending on the dielectric properties achieved, ceramics containing up to 10% by weight are better. Li ₂ O. Ceramics with a maximum of 5 wt. % Li ₂ O, which is also described in embodiments.

Low-interactivity ferroelectric ceramics based on (Ba, Sr) TiO3 doped with Li ₂ O according to the invention, which is made according to standard methods of making ceramic elements, by pouring or depositing thick layers or forming thin films, characterized in that it is a solid solution ( Bai. _X Sr _x ) TiO3 with 0>x> l added up to 20 wt.% Li ₂ O in the form of oxide or any other form that decomposes during thermal treatment and enables the reaction of Li ⁺ ions with BST.

Claims (1)

PATENT APPLICATION: 1. Low-interactivity ferroelectric ceramics based on (Ba, Sr) TiC> 3 doped with Li ₂ O, manufactured using standard methods of ceramic element fabrication, by pouring or depositing thick layers or forming thin films, characterized in that it is a solid solution (Bai_ _x Sr _x ) TiO3 with 0>x> l added up to 20% by weight, preferably up to 10% by weight. Li ₂ O and more preferably up to 5 wt. % Li ₂ O in the form of oxide or any other form which decomposes during thermal treatment and enables the reaction of Li ⁺ ions with BST.