DE2318201A1 - PROCESS FOR PRODUCING SINTERED ZIRCONIUM OXIDE BODIES AND MIXTURES TO BE TREATED ACCORDING TO THE PROCESS - Google Patents

Description

HWALD OPPERMANN MS OFFENBACH CMAlN). KAISBKSTKASSB 9 TELEPHONE (Mtl) St93IC. KABBL EWOPAT

April 10, 1973

Op / ef

37/14

Magnesium Electron Limited

Limn * s Lane

Clifton, Junction, Swinton,

Manchester,

England

Process for the production of sintered zirconium oxide bodies and mixtures to be treated according to the process

The invention relates to a method for the production of sintered zirconium oxide bodies and also extends to the provision of mixtures to be treated according to the method.

309843/1076

Because of its very high melting point of 2650 C and its chemically inert behavior at high temperatures, pure zirconium oxide appears at first glance to be very suitable as a material for the production of high-temperature-resistant ceramics. Compact bodies made of pure zirconium oxide, however, are subject to a destructive change in volume, which at around 1100 ^° C. is accompanied by a change in structure from monoclinic to tetragonal. To reduce this change in volume, it is known to use certain other oxides, e.g. Calcium oxide and magnesium oxide, to mix with zirconium oxide and to heat the mixture to convert the zirconium oxide into a cubic modification which is stable between room temperature and the melting point of the mixture. By limiting the additions of these other oxides to about 15 mol%, based on zirconium oxide, the melting point of the zirconium is only slightly affected. Ceramic bodies made from such "stabilized" zirconia have become well known articles of commerce. However, the manufacture of stabilized high density Zirkoniumoxidkörpern using calcium oxide 'as a stabilizer is difficult, while magnesium oxide-containing body destabilize at cyclic thermal treatments in the temperature range between 1000 and 1500 ⁰ C.

It is known that other oxides of the general formula R ₂ O- are able to bring about the desired cubic modification in the zirconium oxide if they are added in an amount of at least 6 minor in relation to the zirconium oxide. One of the first of those oxides where

2> 0 one has found this effect and what the subject of the largest number was followed by investigations, the Yttriumo.xid, Y ₂ ° 3 · ^In these studies it was found that cubic stabilized,

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6 Molt Yo ^ 3 ^ ^{ent a} l ^th d ^can be formed from the mixture of the oxides when heated to 2000 ⁰ C zirconia. These investigations indicated that oxides of other trivalent elements with an ionic radius similar to that of yttrium oxide, i.e. scandium and the rare earth metals from samarium (atomic number 62) to lutetium (atomic number 71), should also stabilize zirconium oxide in the same modification if they were added to zirconium oxide related quantities of 6 minor or larger can be added. Recent research has shown that adding smaller amounts of yttria to zirconia, e.g. B. from 4 up to 6 minor, tetragonally stabilized zirconium oxide results, which at elevated temperatures, ie temperatures up to about

2000 ⁰ C, undergoes no phase transformation.

Other developments have found that yttria stabilized zirconia based ceramics have additional useful properties. US Pat. No. 3,432,314 describes the preparation of mixtures of very pure zirconium oxide and yttrium oxide by controlled hydrolysis of their alkoxides. According to this publication, these mixtures can be ^{sintered at temperatures as low as 1450 °} C. and result in stabilized zirconium oxide ceramics which have an almost theoretical density and are light-permeable in thin sections. This document also describes ytterbium oxide and dysprosium oxide stabilized zirconium oxide obtained with the same production process.

It has been found that yttrium oxide in the production of at cyclic thermal treatment against destabilized · sation resistant zirconia ceramics the quay is ziumoxid similar, and the additional benefits to comprises that the ceramic thereby achieving a superior;

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Has corrosion resistance, ζ. Β. against molten Glass, molten metals and titanates, and a better electrical conductor at higher temperatures than with the help of calcium oxide stabilized zirconium oxide manufactured ceramic is.

It is evident that zirconia stabilized with yttria or one of the rare earth oxides ytterbia or dysprosia offers considerable possibilities for the production of strong, dense ceramic which ^{withstands temperatures above 2000 °} C. as well as extreme corrosion conditions. Pure yttrium oxide is a very expensive material because it is usually found in connection with oxides of rare earth metals in ores such as monazite or xenotime, from which it can only be separated by laborious and therefore time-consuming processes. Similarly, the rare earth metal oxides ytterbium oxide and dysprosium oxide are usually found in connection with other rare earth metal oxides, so that their pure preparation is expensive for the same reasons.

However, these separation processes produce intermediate products which contain between 35 and 70% yttrium oxide, the remainder of which consists essentially of oxides of rare earth metals with atomic numbers between 57 and 71.

The rare earth metals are usually divided into two subgroups: the cermetallic or "light" rare earth metal subgroup, consisting of elements with atomic numbers between 57 and 61, and the

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Yttrium "or" heavy "rare earth metal subgroup, consisting of rare earth metals with atomic numbers between §2 and 71. Yttrium itself, atomic number 39, is not a rare earth metal, although it occurs in nature in connection with rare earth metals. '

In the applicant's earlier patent application P 22 31 539.5, a stabilized zirconium oxide body was described, consisting of zirconium oxide and a yttrium oxide concentrate containing between 35 and 70 l of yttrium oxide contains and the rest of which, in essential oxides, is less common Earth metals are, the concentrate being present in such an amount that a total of at least 3 Moll Yttria and oxides of heavy rare earth metals are contained in the zirconia body.

By "stabilized zirconia body" is meant a stabilized zirconia in the form of grains or objects of a selected shape, such as bricks, Plates or other in the ceramic and refractory Materials manufacturing industry usual forum exercises, understood will.

According to the state of the art and the patent application P 22 31 539.5 of the applicant was the "stabilized Zirconium oxide body "produced using pure zirconium oxide, with in this context taking pure zirconia one with the help of chemical agents made from one of its ores like zircon, and zircon Saddeieylt obtained zirconium oxide is to be understood.

303043 / $ 107

The invention is based on the object of the known and to improve previously proposed methods for the production of zirconium oxide bodies, so that in particular Zirconia bodies of high density and little cracking can be obtained.

The inventive method for producing a sintered Zirkoniumoxidkörpers is characterized in that a compressed mixture consisting of 10 to 100 wt.% And 0 to 90% baddeleyite Gew.I pure zirconium oxide, wherein at least 10 wt. Of the mixture have an average grain size of not is greater than 5 microns, sinters. In pursuance of the inventive concept, the mixture is sintered for a period of 2 to 3 hours at a temperature between 1550 ^° C. and 1650 ^° C.

According to one embodiment of the invention, the device consists of 100 % baddeleyite, which is ground to an average grain size of no more than 2 microns. This results in high density bodies being formed during sintering.

It has been found that the products obtained with the aid of this preferred method are of high density and have only a small number of fine cracks. Pure zirconia, using the same process, gives products with very large cracks that cannot be used as a refractory material or that have completely disintegrated. The baddeleyite used can be in the form of used sand, containing 96 to 99% by weight of zirconium oxide (ZrO ₂ ). A typical example of baddeleyite has a chemical composition as follows:

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ZrO 98.5% wt.

it

TiO ₂ 0.5 wt

Fe ₂ O ₃ 0.3 wt

SiO ₂ '0.3 wt

The rest consists of very small percentages of undetermined impurities. It is composed of particles of the following sizes :

1 Λ of the whole larger than 72 stitches

20 I " ^lf ""100", 60 I """" 150 "

83 % """" 200 "

17 1 "" smaller "200"

It has been found to be advantageous for making high density products from such baddeleyite sand was to grind the sand to an average grain size of less than 2 microns (as measured by the Fisher instrument).

In a further preferred embodiment of the invention the composition consists of 10 is up to 90 wt. S baddeleyite sand and 10 to 90 wt. * Baddeleyite sand or pure zirconium oxide, the or the not previously more ground than 2 microns to an average grain size of assuming a corresponding mixture.

The mixture preferably contains 40 to 80 % baddeleyite and 60 to 20% zirconium oxide.

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The mixture is advantageously partially stabilized by including up to 2 Molls of yttrium oxide concentrate in it the mixture or up to 15 minor, preferably 2 to 5 minor, of either calcium or magnesium oxide or by including a mixture of yttria concentrate and calcium and / or magnesium oxide.

A conventional lubricant and binder is preferred, z. B. a polyethylene glycol wax in the mixture contained, which is then in a suitable steel mold in the required shape using a conventional brick press or flexible molds in an isostatic Press is compressed to reduce density fluctuations in the unfired state.

The sintering is preferably carried out at a peak temperature of over 1600 ^° C. over a period of time which is long enough to ensure that the mold is completely annealed at the peak temperature so that the end product receives the optimal mechanical properties.

A suitable yttria concentrate for stabilizing zirconia has an analysis as follows:

Y ₂ O ₃ 56 I.

Dy ₂ O ₃

18 I.

He ₂ O ₃ 4 I.

Ho ₂ O ₃ 3 %

Gd ₂ O ₃ 8 %

La ₂ O ₃ 0.2 %

CeO ₂ 0.5 I.

other oxides of rare earth metals 6 I.

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Such a yttria concentrate can be in the mixture of baddeleyite and zirconia as it was in the manufacture zirconia sintered body is used, and this mixture can be directly molded and be sintered.

However, it is preferred to anneal the mixture at a temperature of 1100 ° C. to 1200 ^° C. for two hours and then regrind the sintered product before it is pressed into the desired shape. It has been found that this additional step improves the compaction and flow properties of the mixture when pressed.

In a series of experiments the mixture during the Nächmahlens Preßgleit- as binders and 00:25 by weight was.% Polyethylene wax was added. From the post-ground

■■■■ '■■' ■ '■ "■' 2

Mixtures were molded using a pressure of 7.8 kg / mm and ^{sintered at a peak temperature of 1620 °} C. in a gas-fired furnace. These experiments are summarized in the table below.

20th

25th

Amount of yttria sintered density present Concentrate (MolIJ (g / cm ³ ) Phases +) 0.21 5.47 M. 0.43 5.60 M. 0.86 5.65 M + 51C 1.72 5.72 M + 20 I C 3.43 5.76 M + 40 IC +) X-ray determination M »apnoclinic C »cubic

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An examination of the compacted and sintered forms showed that those with a content of yttrium oxide concentrate of more than 0.2 minor were free of cracks and had densities of 5.6 to 5.7 g / cm. X-ray diffraction photographs of a pressed and sintered from a form derived powder consisting of 0.3 MoI ⁰ S yttria concentrate showed the presence of monoclinic zirconia and only a trace of the cubic phase.

Larger molds weighing between 1 to 2 kg were made using the same procedure as just described except that an isostatic pressing process was used to avoid density fluctuations in the fix unfired condition. The compacted and sintered forms produced were of high density and free from cracks and consisted essentially of monoclinic zirconia.

The invention is described below using examples:

example 1

Baddeleyite sand of the type described above wet milled to an average by the Fisher method using steel balls as milling aids certain particle size less than 2 microns. The resulting slurry was subjected to steel abrasion Addition of hydrochloric acid and freed by standing for several hours. The solid was then filtered out, washed with water and dried. Mixtures of the ground and unground baddeleyite sand were then put into

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the compositions according to the table below manufactured. 4 wt.! Polyethylene glycol wax were used in aqueous solution added to each of the mixtures as lubricants and binders »The mixtures were dried, granulated through a sieve of 20 mesh and at one

Pressure of 1400 kg / cm using an isostatic Press in rubber molds into right-angled blocks weighing approx. 2 kg each.

The blocks were then ^{sintered at 1650 °} C. for 3 hours and allowed to cool in the furnace.

Composition of the mixture Calculated density g / cm Uag-milled sand -Ground sand

33 t 67% 4.85

50 I 50 % 4.50

67 I 33 \ 4.20

It has been found that the sintered blocks are made of porous monoclinic zirconia, were crack-free, and had the fired densities shown in the table.

Example 2

Baddeleyite sand of the type described above wet ground to one through using yttria stabilized zirconia as a grinding aid The average particle size was determined by the Fisher method less than 2 microns. The milled mixture was added with 2% by weight of a yttria concentrate as is has been described above mixed. The mix became two

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Calcined at 1250 ^° C. for hours. The calcined product was wet-ground for two hours in a ball mill with a ball: charge ratio of 10: 1, with 0.5% by weight of polyethylene glycol wax being present as a lubricant and binder. The slurry was separated from the grinding balls and the remaining solid was filtered off, washed and dried. The dried material became a free flowing one

2 granulated powder which was pressed with a pressure of 7.8 kg / mm to give 19 mm long cylinders in steel molds. The raw cylinders were placed in a gas-fired oven at a peak temperature
Sintered for 3 hours.

a peak temperature of 1650 C for a duration of

The sintered cylinders had zero open pores and a burning density of 5.65 g / cm. A final examination the microstructure showed a range of grain sizes between 5 and 15 microns. The high density, hardness, low porosity and comparatively small grain size work together to make the cylinder usable for grinding purposes close.

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Claims (17)

Claims 1. A process for producing a sintered zirconium oxide body, characterized in that a compacted mixture consisting of 10 to 100 wt. % Baddeleyite and 0 to 90 wt.% Pure zirconium oxide, with at least 10 wt than 5 microns, sinters. 2. The method according to claim 1, characterized dad, sintering the mixture for a period of 2 to 3 hours at a temperature between 1550 ^0C and 1650 ^0C. 3. Process according to claims 1 or 2, characterized in that a mixture consisting of 100% baddeleyite, which is ground to a grain size of not more than 2 microns, is sintered. 4. The method according to claims 1 or 2, characterized in that one of 10 to 90 Gew.I baddeleyite sand and 10 to 90 Gew.I baddeleyite sand, which was previously ground to an average grain size of not more than 2 microns existing mixture sinters. 5. The method according to claims 1 or 2, characterized in that one of 10 to 90 Gew.t Baddeleyite sand and 10 to 90 Gew.I zirconium oxide, which was previously ground to an average grain size of not more than 2 microns, existing mixture sinters. 309843/1076 " ^U " 6. The method according to claim 5, characterized in that a sintering of 40 to 80 wt.% Baddeleyite sand and 20 to 60 wt.% Pure zirconia existing mixture. 7. The method according to any one of claims 1 to 6, characterized in that a mixture is used which contains no more than 2 minor yttrium oxide in the form of a yttrium oxide concentrate. 8. A method according to any one of claims 1 to 7, characterized in that a mixture is used which contains up to 15 minor calcium oxide and / or magnesium oxide. 9. The method according to claim 8, characterized in that a mixture is used which contains 2 to 5 Moll calcium and / or magnesium oxide. 10. The method according to any one of claims 1 to 9, characterized in that a mixture containing a lubricant is used. 11. The method according to claim 10, characterized in that the lubricant used is polyethylene glycol wax. 12. The method according to any one of claims 1 to 11, characterized in that the mixture is compressed by isostatic pressure. - 15 - 309843/1076 - is - 7318201 13. Mixture for the production of a sintered zirconium oxide body, characterized by one of 10 to 90 wt. I baddeleyite sand and 10 to 90 wt Micron was milled, existing composition. 14. Mixture according to claim 13, characterized by a composition containing 40 to 80 wt. I baddeleyite sand and 20 to 60 wt. I pure zirconium oxide. 15. Mixture according to claims 13 or 14, characterized by a content of not more than 2 Moll yttrium oxide, which is in the form of a yttrium oxide concentrate. 16. Mixture according to claims 13, 14 and 15, characterized by a content of up to 15 minor calcium oxide and / or magnesium oxide. 17. Mixture according to claim 16, characterized by a content of 2 to 5 minor calcium oxide and / or magnesium oxide. 309843/1076