DE102017124404A1 - Yttrium-coated aluminum nitride powder, its preparation and use - Google Patents

Abstract

The invention relates to a yttrium-coated aluminum nitride powder (AlN powder) usable for producing an aluminum nitride sintered body, to the production and use thereof. Among other things, the method provides that yttrium stearate from the solution is deposited on the AlN powder to be coated.

Description

The invention relates to a yttrium-coated aluminum nitride powder (AlN powder) usable for producing an aluminum nitride sintered body, its production and use.

Technological background

Aluminum nitride ceramic has an expanding field of application due to its good thermal conductivity, corrosion resistance and mechanical strength.

Aluminum nitride ceramics are produced in the ceramic process by processing a finely divided AlN powder. The treatment is generally carried out in the steps: admixing a sintering aid, adding an organic pressing aid, optionally increasing the flowability by granulation, press molding, thermal removal of the pressing aid (presintering) and sintering. It should be noted that AlN is sensitive to air (oxygen) above 650 ° C and humidity.

Like all compounds with high covalent bond content, AlN has a low sinterability. In addition, because of the thermal instability of the material at temperatures above 2000 ° C, a sintering improvement by using higher sintering temperatures is not possible, sintering aids must be added. A significant increase in sintering speed occurs when, at sintering temperature, a molten phase forms between the particles to be sintered. The liquid phase facilitates effective mass transport through dissolution and elimination processes.

One component of the molten phase during sintering is Al ₂ O ₃ , which is always present as an impurity in an amount of a few tenths to a few percent in AlN. For the other components of the melt phase, a number of proposals have been made, mainly for admixing solid oxides, halides or compounds that can be thermally converted into oxides and halides based.

• • der Oxide der Erdalkalimetalle, der Seltenen Erden, der Übergangselemente der 4., 5. und 6. Gruppe des Periodensystems, von Al₂O₃ und/oder SiO₂ [ DE 33 37 630 A1 ];
• • der Erdalkalimetalloxide, von Y₂O₃ und/oder CaF₂ [ DE 35 34 886 A1 ];
• • mindestens einer Halogenverbindung von einem Metall der Gruppe der Erdalkalimetalle, der Seltenen Erden sowie von Yttrium und einer halogenfreien Verbindung aus der Gruppe derselben Metalle. Die halogenfreie Verbindung kann auch ein Aluminat sein [ DE 36 27 317 A1 ; EP 0207465 A1 ];
• • von Erdalkalialuminat und einem Oxid oder einer oxidbildenden Verbindung (Carbonat, Oxalat, Nitrat, Nitrit, Bicarbonat, Sulfat, Sulfit, Chlorat, Acetat) eines Elementes der Seltenen Erden [ DE 37 23 774 A1 ];
• • mindestens einer Verbindung der Erdalkalimetalle oder Seltenen Erden und mindestens ein Element oder dieses enthaltende Verbindung der Gruppen 14, 15, 16 und 17 des Periodensystems [ EP 0207465 A1 ];
• • der Boride und Carbide von Ca, Sr, Ba oder der Seltenen Erden [ US 4711861 A ];
• • der Acetylide von Ca, Sr, Ba, Na, K, Rb, Cs, Cu, Ag, Mg, Cd, Hg, Zn, Al, Ce [ US 4650777 A ];
• • einer Mischung von CaO mit W- und/oder Mo-Oxid [ EP 0267623 A2 ];
• • der Silicide, Carbide, Boride, Nitride (außer BN, AlN, Be₃N₂); und
• • von CaCN₂ [ EP 0260865 A1 ].

So it is suggested the mixing

• • the oxides of the alkaline earth metals, the rare earths, the transition elements of the 4th, 5th and 6th group of the periodic table, of Al ₂ O ₃ and / or SiO ₂ [ DE 33 37 630 A1 ];
• The alkaline earth metal oxides of Y ₂ O ₃ and / or CaF ₂ [ DE 35 34 886 A1 ];
• • at least one halogen compound of a metal of the group of the alkaline earth metals, the rare earths and of yttrium and a halogen-free compound from the group of the same metals. The halogen-free compound may also be an aluminate [ DE 36 27 317 A1 ; EP 0207465 A1 ];
• • alkaline earth aluminate and an oxide or an oxide-forming compound (carbonate, oxalate, nitrate, nitrite, bicarbonate, sulphate, sulphite, chlorate, acetate) of a rare earth element DE 37 23 774 A1 ];
• At least one compound of the alkaline earth metals or rare earths and at least one element or compound of the groups 14, 15, 16 and 17 of the periodic table containing the same [ EP 0207465 A1 ];
• The borides and carbides of Ca, Sr, Ba or the rare earths US 4711861 A ];
• The acetylides of Ca, Sr, Ba, Na, K, Rb, Cs, Cu, Ag, Mg, Cd, Hg, Zn, Al, Ce US 4650777 A ];
• A mixture of CaO with W and / or Mo oxide EP 0267623 A2 ];
• The silicides, carbides, borides, nitrides (except BN, AlN, Be ₃ N ₂ ); and
• • of CaCN ₂ [ EP 0260865 A1 ].

In addition to the mechanical admixing of the sintering aids, it has also been proposed to mix the sintering aids in dissolved form with the AlN and then to evaporate the solvent ( DE 40 39 183 A1 ).

Yttria (Y ₂ O ₃ ) has become established in the art as a sintering aid for AlN. It is usually added to the AlN as a ground powder. Particularly fine crystalline powders are obtained by the thermal decomposition of yttrium stearate (Synthesis of dispersed Y ₂ O ₃ Nanopowder from Yttrium Stearate, Jinsheng Li, Xudong Sun et al., Key Engineering Materials Vol., 544 (2013), 3-75).

Although good ceramic properties can be achieved with some of these sintering-promoting agents, all of the mentioned proposals are associated with disadvantages resulting from the solid state of the sintering-promoting agents and the associated necessity of intensive mechanical mixing with the AlN powder. During the mixing process, the particle size distribution of the sintering aid powder is not significantly changed. As a result, larger sintering aid agglomerates remain, which can form second phases in the AlN ceramic with grain sizes of AlN and optionally also larger. Furthermore, the addition of temporary granulating and pressing aids is necessary for shaping the fine powders.

In addition to the problems associated with the poor sinterability of AlN, the susceptibility to hydrolysis of the starting powder must be taken into account in the production of AlN ceramics. The finer and thus more sinter-active an AlN powder, the greater the risk of a reaction with moisture traces, which leads to the formation of unwanted aluminum hydroxide.

This susceptibility to hydrolysis is to be considered at all stages of the manufacturing process. This is especially true for process stages in which auxiliaries (pressing aids, granulation aids, sintering aids) are added. Occasionally, the AlN is passivated with organic matter and thus made less sensitive to moisture z. B. by additives such as stearic acid ( EP 0588171 A1 ). Nevertheless, all common methods used in ceramic technology that use aqueous solutions or slurries have to be considered critically.

This also applies to the use for suspensions of organic solvents and sintering aids. Again, the sintering aid agglomerates remain substantially preserved. The addition of the sintering aid via an anhydrous solution has also been proposed ( DE 40 39 183 A1 ). However, in this case the range of possible sintering aids is limited and further steps for the temporary introduction of pressing aids are required.

Summary of the invention

One or more of the disadvantages of the prior art described can be eliminated or at least reduced with the aid of the invention.

1. a) Ansetzen einer ersten Lösung von Stearinsäure und einer Yttrium(III)-Verbindung, bevorzugt Y(NO₃)₃ × 6 H₂O, in einem nicht-wässrigen Lösungsmittel, insbesondere Isopropanol;
2. b) Herstellen einer Dispersion aus der ersten Lösung und einem AlN-Ausgangspulver;
3. c) Abscheiden von Yttriumstearat auf dem AlN-Ausgangspulver durch Zusatz einer Base, bevorzugt Ammoniak, zur Dispersion; und
4. d) Entzug des Lösungsmittels.

According to a first variant of the method according to the invention for producing an yttrium-coated aluminum nitride powder (AlN powder) which can be used to produce an aluminum nitride sintered body, the method comprises the steps:

1. a) preparing a first solution of stearic acid and an yttrium (III) compound, preferably Y (NO ₃ ) ₃ × 6 H ₂ O, in a non-aqueous solvent, in particular isopropanol;
2. b) preparing a dispersion of the first solution and an AlN starting powder;
3. c) depositing yttrium stearate on the starting AlN powder by adding a base, preferably ammonia, to the dispersion; and
4. d) removal of the solvent.

The use of stearic acid has several advantages. For example, stearic acid passivates the AlN powder against the attack of water. Only in this way can a surface reaction of the water of crystallization with the use of Y (NO ₃ ) ₃ .6H ₂ O and the ever-present water content of an added ammonia solution with the AlN powder be prevented in particular. Furthermore, stearic acid reacts with the dissolved yttrium (III) to yield yttrium stearate. The yttrium stearate also passivates the AlN surface. The stearic acid is completely non-toxic and, if still present, can be eliminated thermally without residue.

1. a) Ansetzen einer zweiten Lösung von Yttriumstearat in einem nicht-wässrigen Lösungsmittel;
2. b) Herstellen einer Dispersion aus der zweiten Lösung und einem AlN-Ausgangspulver; und
3. c) Abscheiden von Yttriumstearat auf dem AlN-Ausgangspulver durch Entzug des Lösungsmittels.

According to a second variant of the method according to the invention for producing an yttrium-coated aluminum nitride powder (AlN powder) which can be used to produce an aluminum nitride sintered body, the method comprises the steps:

1. a) preparing a second solution of yttrium stearate in a nonaqueous solvent;
2. b) preparing a dispersion of the second solution and an AlN starting powder; and
3. c) deposition of yttrium stearate on the AlN starting powder by removal of the solvent.

The solvent of the second solution is preferably an organic liquid having a boiling point of 70 ° C or higher. More preferably, the solvent of the second solution is selected from the group consisting of benzene, gasoline, cyclohexane, cyclopentane, ethylbenzene, isooctane, xylene, polymethylbenzene, turpentine oil, and mixtures thereof. A mass ratio of yttrium stearate to the solvent of the second solution may range from 1: 5 to 1:15.

In both process variants, the deposited yttrium stearate is preferably thermally decomposed. The yttrium stearate formed decays to yttrium oxide, which forms a liquid aluminate phase above all with the aluminum oxide adhering to the AlN at the sintering temperature.

Another aspect of the invention relates to yttrium-coated aluminum nitride powder prepared according to the previously described methods.

The invention further relates to the use of the yttrium-coated aluminum nitride powder of the present invention for producing an aluminum nitride sintered body (or aluminum nitride ceramic) and aluminum nitride sintered bodies prepared by pressing and sintering this yttrium-coated aluminum nitride powder.

The invention is therefore based on the finding that on the AlN powder according to a first variant of the method of liquid phase a Yttriumverbindung is deposited, wherein the deposited solid compound passivates the AlN powder, acts as a molding aid in the molding and then serves as a sintering aid after their thermal decomposition. In the ceramic remains in this way a very evenly distributed, extremely finely divided oxide, which converts at sintering temperature with the alumina contained in AlN to yttrium aluminate phases, which have a much smaller grain size after formation of the ceramic structure than the dominant AlN matrix. In other words, the aluminum nitride sintered body made with the AlN powder coated according to the invention differs morphologically from aluminum nitride sintered bodies made on the basis of a conventional AlN powder.

In the second variant of the process, the AlN powder is mixed with a passivating yttrium-containing solution, from which a likewise passivating pressing assistant is deposited in fine distribution on the AlN powder. This pressing aid is also thermally converted into the sintering aid after the press molding, wherein the sintering aid phase is present in the finished AlN ceramic with very fine distribution.

The solution protects the AlN powder from attack by water. The yttrium stearate contained in the solution according to the second process variant or the likewise passivating yttrium stearate separated off from the solution by raising the pH according to the first process variant thus serves first as a passivating agent and as a pressing assistant which, after shaping and thermal decomposition, passes into the associated oxide and forms liquid sinter-promoting yttrium aluminates at sintering temperature with the alumina always accompanying the AlN powder. Due to the self-adjusting uniformly fine distribution of the yttrium oxide, the sintering compaction is promoted at the same time, so that lower sintering temperatures are necessary.

A particular advantage of the method is that the uniform distribution of the sinter-promoting yttria component on the surface of the AlN particles makes direct contact with the Al ₂ O ₃ layer which also covers the surface of the AlN particles. This accelerates the formation of the liquid phase and thus the entire sintering process.

The thermal decomposition of the stearate releases larger amounts of gas. This can lead to defects in the ceramic body during debindering with thick ceramic shards. It is advantageous in this case to accommodate part of the yttrium oxide already in the AlN starting powder. Thus, the AlN can be soaked in a solution of Y-stearate in xylene. At about 620 ° C, AlN remains with finely divided yttria. Yttrium stearate can now be deposited again on the liquid phase AlN powder already mixed with yttrium oxide. In other words, after the thermal treatment, the process steps of the two process variants following the process step a) are preferably repeated.

Optionally, the AlN powders to be coated may contain alloying portions of other nitrides or oxides of alkali metals, metals of Group 3 of the Periodic Table, rare earths or silicon in amounts up to 10% by mass.

Further preferred embodiments of the invention will become apparent from the following description.

Detailed description of the invention

The invention will be explained in more detail with reference to embodiments.

Embodiment 1

42.7 g of Y (NO ₃ ) ₃ .6H ₂ O (yttrium nitrate) and 144 g of C ₁₇ H ₃₅ COOH (stearic acid) are dissolved in 1.6 l of isopropanol at 60 ° C. 2.5 kg AlN are added to the solution. The slurry is heated to 60 ° C with gentle stirring. After reaching the temperature are diluted with 56 g of 25% ammonia solution in 185 g of isopropanol, the pH of the solution to pH = 7 raised. Here, Y-stearate precipitates on the surface of the AlN powder. Y (NO ₃ ) ₃ × 6 H ₂ O + 3 NH ₄ OH + 3 C ₁₇ H ₃₅ COOH → (C ₁₇ H ₃₅ COO) ₃ Y + 3 NH ₄ NO ₃ + 9 H ₂ O

The slurry is further stirred gently for 1 h and then dried in 2 stages (8 h at 70 ° C, isopropanol evaporated) and 8 h at 180 ° C (ammonium nitrate is thermally decomposed). The resulting powder is soft and has a bulk density of 0.6 g / cm ³ .

To improve the flowability and pressability, the powder is isostatically compacted at 50 MPa, comminuted and sieved. The granules have a bulk density of 1.1 g / cm ³ . Now, the granules are compacted with 100 MPa and the compact in air by heating to 620 ° C entbindert (decomposition of yttrium stearate from about 400 ° C and crystallization of yttrium oxide from about 500 ° C) and at 1810 ° C for 2 h in one Nitrogen-hydrogen atmosphere (volume ratio 1: 1) sintered. The AlN ceramic achieves a density of 3.26 g / cm ^3, a mean grain size of 4 μm and a uniform distribution of the yttrium aluminate phase in the gussets of the AlN grains. The yttria content of the ceramic is 1 mass%.

Embodiment 2

2.5 kg AlN are stirred at 100 ° C with a solution of 850 g of Y-stearate in 3 kg of xylene to a pulp. The slurry is dried and tempered at 620 ° C. The cooled mass is sieved (0.5 mm) and subjected to the procedure of Embodiment 1.

The AlN ceramic achieves a density of 3.27 g / cm ^3, a mean grain size of 5 μm and a uniform distribution of the yttrium aluminate phase in the gussets of the AlN grains. The yttria content of the ceramic is 6% by mass.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 3337630 A1 [0006]
• DE 3534886 A1 [0006]
• DE 3627317 A1 [0006]
• EP 0207465 A1 [0006]
• DE 3723774 A1 [0006]
• US 4711861 A [0006]
• US 4650777 A [0006]
• EP 0267623 A2 [0006]
• EP 0260865 A1 [0006]
• DE 4039183 A1 [0007, 0012]
• EP 0588171 A1 [0011]

Claims (10)

A process for producing an yttrium-coated aluminum nitride powder (AlN powder) usable for producing an aluminum nitride sintered body, the process comprising the steps of: a) preparing a first solution of stearic acid and an yttrium (III) compound in a non-aqueous solvent; b) preparing a dispersion of the first solution and an AlN starting powder; c) depositing yttrium stearate on the starting AlN powder by adding a base to the dispersion; and d) removal of the solvent. Method according to Claim 1 in which ammonia is used as base and ₃ × 6 H ₂ O as yttrium (III) compound Y (NO ₃ ). Method according to Claim 1 or 2 in which the solvent of the first solution is isopropanol. A process for producing an yttrium-coated aluminum nitride powder (AlN powder) usable for producing an aluminum nitride sintered body, the process comprising the steps of: a) preparing a second solution of yttrium stearate in a nonaqueous solvent; b) preparing a dispersion of the second solution and an AlN starting powder; and c) deposition of yttrium stearate on the AlN starting powder by removal of the solvent. Method according to Claim 4 in which the solvent of the second solution is an organic liquid having a boiling point of 70 ° C or higher. Method according to Claim 5 in which the solvent of the second solution is selected from the group consisting of benzene, gasoline, cyclohexane, cyclopentane, ethylbenzene, isooctane, xylene, polymethylbenzene, turpentine oil and mixtures thereof. A method according to any one of the preceding claims, wherein the deposited yttrium stearate is thermally decomposed. Yttrium-coated aluminum nitride powder prepared by a process according to any one of the preceding claims. Use of the yttrium-coated aluminum nitride powder according to Claim 8 for producing an aluminum nitride sintered body. Aluminum nitride sintered body prepared by pressing and sintering the yttrium-coated aluminum nitride powder according to Claim 9 ,