JPH0333671B2 - - Google Patents

Description

Claim 1: A metal oxide powder is prepared by a hydrolysis technique consisting of hydrolyzing a suitable metal alkoxide, washing, drying, granulating and calcining; Wet ball milling the powder, draining the supernatant resulting from the wet ball milling, forming the powder from the supernatant into at least one powder condensate, and sintering the at least one powder condensate into a ceramic body. In the method for producing a ceramic material body selected from the materials consisting of Al ₂ O ₃ , ZrO ₂ and TiO ₂ , the wet ball milling process
A method characterized in that it is carried out with an aqueous slurry having a pH of over 2.4.

2. The ceramic material contains TiO ₂ ,
2. The method of claim 1, and wherein said alkoxide is selected from titanium ethoxide, titanium isopropoxide, and titanium butoxide.

3. The method according to claim 1 or 2, wherein the PH is about 2.1.

4. Claim 1, wherein said sintering is carried out at a temperature between 650°C and 1200°C, preferably 850°C,
The method according to item 2 or 3.

5 The ceramic material further contains Al, Nb and Ta.
5. A method according to any one of claims 1 to 4, comprising an element selected from:

6. The method according to any one of claims 1 to 5, wherein alcohol is added to the alkoxide.

TECHNICAL FIELD The present invention relates to a process for manufacturing ceramics having a sintering step at reduced temperatures.

BACKGROUND OF THE INVENTION Devices manufactured from ceramic materials have many important technical applications. For example, they are magnets,
Useful in capacitors, thyristors, thermistors and many other applications. Many techniques have been developed for the production of these materials, many if not all requiring high temperature sintering processes.

Many types of devices that use ceramic materials are electrical devices that require one or more electrical contacts. However, high sintering temperatures, typically 1400°C, greatly limit the materials that can be used to form the electrodes. Many of the electrode materials are precious metals,
Since the prices of these metals have increased significantly in recent years, a method of manufacturing ceramic materials that allows the use of lower sintering temperatures and therefore cheaper electrode materials is desired. Of course, lower sintering temperatures can also be used in the production of other types of devices, for example due to the energy costs and lower compositional control enabled by lower sintering temperatures in the presence of volatile components. Ease is also desirable because it is desirable.

Multilayer ceramic capacitors are an example of electrical equipment where lower sintering temperatures are desirable. The manufacture of multilayer ceramic capacitors typically requires placing electrode materials between layers of ceramic material. Both the ceramic layer and the electrode material layer are co-sintered at elevated temperature in an oxidizing atmosphere. Barium titanate ceramic powders commonly used in multilayer ceramic capacitors typically require sintering temperatures of approximately 1400°C. At such relatively high temperatures, noble metals such as Ag-Pd alloys are typically required as electrodes. Ag
Alternatives such as electrodes consisting of Ag cannot be used unless ceramic materials can be sintered at temperatures lower than the melting point of Ag, approximately 962°C.

Various solutions have been taken to reduce sintering temperatures. One solution is to reduce the sintering temperature by chemical means, such as adding dopants to the ceramic composition prior to sintering. However, this solution is often undesirable because the dopants have a deleterious effect on the dielectric properties of the ceramic composition. The second solution is to produce ceramic powders with optimal physical properties for sintering at low temperatures. 1 of this method
An example is the Journal of the American Ceramic Society.
64, pp. 19-22, 1981. This paper reports the use of a non-agglomerated micron-sized powder of ZrO ₂ stabilized Y ₂ O ₃ that can be sintered at 1100°C. This temperature is approximately 300°C lower than normal sintering temperatures.

SUMMARY OF THE INVENTION According to the present invention, ceramic materials can be sintered to a high density at relatively low temperatures by ball milling and sintering in a PH range that maximizes the dispersion of metal oxide powder in a slurry. can. The metal oxide powder is prepared, in a preferred embodiment, by a method that includes hydrolyzing an alkoxide to form a slurry, drying, granulating, and calcining the metal oxide powder in the slurry. Manufactured by. In a further preferred embodiment, alcohol is added to the alkoxide. In yet another preferred embodiment, the ceramic material produced by the method includes _TiO2 . In a more preferred embodiment, the ceramic material produced by this method includes TiO ₂ produced by hydrolysis of Ti-alkoxides such as isopropoxide, and has a theoretical density of
Sintered to 99%. This temperature is approximately 500°C lower than the normal sintering temperature.

DETAILED DESCRIPTION Ceramic oxide materials can be sintered at relatively low temperatures by using unagglomerated ceramic powders with very small particle sizes. The metal oxide powder is ball milled in a slurry having a PH value in a region that maximizes dispersion of the powder in the slurry.
This PH controls the surface charge of the particles and reduces their tendency to agglomerate. Small particle size is desirable because it leads to smaller voids in the dense material that can be easily removed during the sintering process, thereby allowing higher densities to be achieved. However, it is not just the particle size that is small. The particles also have to be dispersed in the slurry to prevent agglomeration, which again creates large voids,
As a result, the density after sintering becomes low, which is not sufficient.

In a preferred embodiment, the powder is produced by a process that begins with hydrolysis of an alkoxide.
If the viscosity of the alkoxide is undesirably high at the desired process temperature, the viscosity can be reduced by dissolving the alkoxide in a suitable alcohol. Adding alcohol also reduces the size of the particles. If several metals are desired to be present in the ceramic material, more than one alkoxide is used. The use of alkoxides provides a convenient means of introducing the desired metal atoms while producing reaction products, metal oxides and alcohols, which do not react after hydrolysis.

After hydrolysis, the metal oxide powder typically undergoes an intermediate treatment step before ball milling. Suitable intermediate steps typically include drying of the powder in the slurry produced during hydrolysis, followed by granulation and calcination of the powder. After ball milling is complete, the slurry is allowed to stand for a period of time to allow larger particles to settle out. After settling for a while, the supernatant slurry is drained to densify the particles. In this method, smaller particles are selected to increase the resulting density.

The degree of powder dispersion that should be maximized during ball milling depends on the PH of the solution. Studies on slurry viscosity and sedimentation can be used to determine the amount of powder dispersed and therefore the critical PH value and PH region in the slurry. Generally, slurries with more highly dispersed powders have relatively lower viscosities than slurries with less dispersed powders. There are papers in the literature, such as Journal of the Ceramic Society of America, 12th, No.
394-398, 1949, shows the relationship between the apparent viscosity of a slurry and its pH. Oxide surfaces generally result in a true surface charge and a potential difference between the oxide surface and the liquid medium remote from the oxide surface.
The hypothesis is that it has ionization sites that can release H ⁺ or OH ^- . This potential difference is commonly referred to as the zeta potential, and analysis has shown that it reaches its maximum value at a critical PH value or region. These values and regions indicate that the ionization of the oxide surface depends on the pH of the liquid medium in which the oxide is in equilibrium.
determined by the fact that it depends on Since all particles carry the same charge and have the same zeta potential, electrostatic repulsion between the particles can prevent their close proximity or flocculation. If the zeta potential is large enough, the electrostatic repulsion becomes significant and overcomes the Van der Waals attraction, and the particles remain dispersed in the liquid. A relatively low viscosity is desirable as it results in well-dispersed particles.

The settling velocity of solid particles can be calculated from the Stokes equation, which states that velocity increases with the square of the particle size. As described above, sedimentation typically proceeds slowly in a slurry containing fine particles. As an example, TiO ₂ powder produced as described below has a particle radius of approximately 0.02 μm and, when well dispersed, has a settling rate of less than 0.1 mm/day.

However, the microparticles tend to agglomerate, with each agglomerate consisting of 10 ³ to 10 ⁵ loosely bound particles with a rather low solids content. Since the size of the aggregates is much larger than the particle size, the settling velocity of the aggregates is much greater than that expected for individual particles. If the aggregates consist primarily of particles of approximately the same size, their settling rate is inversely proportional to the solid fraction in the aggregate. Thus, the settling rate is relatively high even when the fine particles form bulky, low-density aggregates. As a result, the degree of dispersion of the powder is directly related to the settling rate.

Although the method is well suited for use with Al ₂ O ₃ , ZrO ₂ , and TiO ₂ , it will be described with particular reference to TiO ₂ . Micron-scale TiO ₂ powder is produced by hydrolysis of titanium alkoxides such as titanium ethoxide, titanium isopropoxide and titanium butoxide. The hydrolysis reaction is represented by the following chemical equation: 2Ti(OC _o H _2o+1 ) ₄ +2H ₂ O→2TiO(OC _o H _2o+1 )
₂ +4C _o H _2o+1 OH(1) 2TiO(OC _o H _2o+1 ) ₂ →TiO ₂ +Ti(OC _o
H _2o+1 ) ₄ (2) The net reaction is: Ti(OC _o H _2o+1 ) ₄ +2H ₂ O→TiO ₂ +4C _o
H _2o+1 OH(3) (where n is for titanium ethoxide, titanium isopropoxide and titanium butoxide,
n=2, 3 and 4, respectively). Each mole of titanium alkoxide thus reacts with 2 moles of water to produce 1 mole of TiO ₂ and 4 moles of alcohol.

Certain cationic dopants, e.g. Al, Nb
and Ta to control the electrostatic properties TiO ₂
often preferably added to. This dopant can be introduced in the form of a metal alkoxide,
This dissolves in the titanium alkoxide. for example
If Al is the desired dopant, for example, a suitable amount of aluminum butoxide is dissolved in titanium butoxide. If Ta or Nb is the desired dopant, for example tantalum ethoxide or niobium ethoxide is dissolved in the titanium ethoxide. Since both the cationic dopant and the precursor of TiO ₂ are present in a miscible solution, a homogeneous distribution of the dopant into the TiO ₂ is achieved.

The melting point of titanium alkoxide is close to typical processing temperatures, eg, 25°C, so liquid alkoxides are very sticky at these temperatures. The viscosity of the alkoxide can be reduced by dissolving the alkoxide in a suitable alcohol, such as titanium ethoxide in ethyl alcohol, titanium isopropoxide in isopropyl alcohol, and titanium butoxide in butyl alcohol.

The alcohol content of the diluted alkoxide solution is desirably less than 95 percent. Dilute solutions also yield _TiO2 hydrolysis reaction products with smaller particle sizes. For example, the size of _TiO2 particles produced using an alkoxide solution consisting of 10 percent titanium isopropoxide and 90% isopropyl alcohol is approximately 25 percent of the particle size produced using undiluted titanium isopropoxide. It is. Particle size is _TiO2
The hypothesis is that particle size is reduced by dilution with alcohol, as it is determined by the frequency of Brownian collisions between molecules. In dilute conditions _TiO2 molecules are less concentrated and their collision frequency is reduced.

The processing method proceeds as follows. The reaction preferably takes place at a temperature at which the titanium alkoxide, such as isopropoxide, remains liquid, such as 25°C. When a titanium isopropoxide solution comes into contact with water, it immediately reacts to form a cloudy precipitate. Titanium isopropoxide can be sprayed into stirring water, for example. To minimize contamination with _CO2 , preferably an inert gas such as nitrogen is blanketed over the moving water.

The liquid is removed from the slurry by filtration through a suitable paper having pores of small diameter, typically smaller than 2-3 μm. The resulting liquid is a clear liquid, indicating that the size of the precipitate agglomerates formed during hydrolysis is larger than the paper pores. Transfer the residue and mix well with distilled water, preferably 3
Repeat distillation to remove any unreacted titanium isopropoxide. The slurry is then passed through to remove the liquid. The residue is then mixed with isopropyl alcohol and filtered again. mixing with alcohol,
The washing and subsequent filtration is preferably repeated several times, for example three times, to remove residual water.

The filtered powder is then dried at about 100°C for about 1 hour. The dry powder is granulated through a sieve with holes of e.g.
Calcinate for 16 hours at a temperature of 550°C. The calcined powder is then placed in a polypropylene container to a pH of approximately
Grind for about 3 hours using ZrO ₂ milling media in HCl-H ₂ O solution with 2.1.

The slurry has a solids content of approximately 14 weight percent.

A pH of 2.1 appears to be the most preferred as it maximizes the dispersion of the TiO ₂ powder in the aqueous solution. about
Any pH within the range of 1.8 to about 2.4 can be advantageously used. Outside this range, aggregates are likely to form.

After ball milling, leave the powder slurry for 72 hours. The supernatant slurry, which consists only of the smallest particles in suspension, is removed with a siphon and densely condensed, for example, by centrifugation. The powder condensate obtained is dried at about 105° C. for about 16 hours. The resulting aggregates were then heated to 650°C and 1200°C.
sintering at temperatures between Flowing oxygen can be used as the sintering atmosphere.

The density of the obtained material was measured by Archimedes' principle using CCl ₄ as the sinking liquid. The relative density determined was based on a theoretical density of 4.26 g/cm ³ . Produced by the method described above and heated at 850 °C in oxygen.
Both doped and undoped TiO ₂ sintered at 0.degree. C. for 2 hours had a density greater than 99.5 percent of theoretical and a grain size less than 0.5 .mu.m.

that there are relatively simple laws governing the time required for sintering at a given temperature to produce geometrically similar changes in two or more strands of otherwise identical solid particles with different particle sizes; is generally recognized by those skilled in the art. As an example, if it takes time t ₁ to sinter a spherical particle of radius r ₁ at a given temperature, then
The time required to sinter a particle with radius r ₂ t ₂
is given by the formula: t ₂ = (r ₂ /r ₁ =) ⁿ t ₁ (the value of n in the formula depends on the mass transport mechanism during sintering). The value of n is 3 for lattice diffusion and 4 for grain boundary diffusion. It is further assumed by those skilled in the art that, in general, the required sintering time increases exponentially with the inverse of temperature. For samples with low temperatures and small grain sizes, grain boundary diffusion is generally the more important mass transport mechanism leading to densification. Although grain boundary diffusion in TiO ₂ has not been reported in the literature, the activation energy of grain boundary diffusion is generally smaller than that of lattice diffusion. Therefore, it is expected that the smaller the particle size, the lower the sintering temperature.

Prior to low temperature sintering, it is important that the TiO ₂ powder is well dispersed before agglomeration. In the above method, large aggregates formed during hydrolysis and calcination were removed by settling. Therefore, only small, unagglomerated particles are condensed into a dense body by centrifugation. After removing the aggregates, the density distribution in the produced samples became more uniform. The pore size distribution becomes more uniform and densification proceeds more easily without being limited and delayed by residual pores that are present when the sample has a non-uniform density.

Powder agglomerates can also be produced by methods other than centrifugation techniques. For example, an agglomerate having the desired uniform density distribution can also be obtained by isostatic pressing.

After sintering has been completed, the production of devices, for example the production of electrical contacts to the ceramic material, can continue.