RU2738880C9 - Method of obtaining and alumina ceramic material - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to inorganic chemistry and material science, specifically to composition and method of producing technical alumina ceramics, incl. structural purpose and armour ceramics. To produce dense ceramics, a charge is used, which is obtained by modifying Al₂O₃ powder with a nanosize coating of manganese and titanium oxides - precursors of manganese titanate. Coating is applied by liquid-phase method in organic solvents by xerogel formation with composition Mn-Ti-O-C-H and its annealing at temperature 450–700 °C. Free sintering of micropowder α-Al₂O₃ with median size of particles >1 mcm at 1300–1400 °C allows obtaining corundum ceramics with basic aluminium oxide content >95 wt. %, and with high mechanical properties (longitudinal velocity of sound ≥10.5 km/s, Young's modulus ≥360 GPa, microhardness ≥16 GPa).

EFFECT: technical result is expressed in uniform distribution in the ceramic of manganese titanate additive, which provides sintering at low temperature, and in corresponding reduction of content of structure defects.

5 cl, 7 ex

Description

The invention relates to the field of inorganic chemistry and materials science, namely to the composition and method of producing technical alumina ceramics, incl. structural purpose and armored ceramics.

It is known that low-melting additives to the main component (Al ₂ O ₃ ), which form a liquid phase during sintering, can be used to reduce the sintering temperature of alumina ceramics. One of the most effective such additives is manganese titanate MnTiO ₃ , which has a melting point and eutectic formation with aluminum oxide of about 1300-1350 ° C and can be used as the only additive ([Shirvinskaya A.K., Kachanova L.P. // Journal Applied chemistry. 1978. T. 51, No. 3. S. 506-510], [Ordanyan S.S., Samokhvalova T.I., Zaitsev G.P. // Refractories. 1992. No. 4. P. 10- 12]) or together with other additives that improve the structure and properties of alumina ceramics, such as zirconium dioxide ZrO ₂ , silicon dioxide SiO ₂ , magnesium oxide MgO, etc. (RU 2111935 Samokhvalova et al., RU 2501768 Zaitsev et al., US 4760038 Kinney et al., US 5147833 Manning et al., US 5658838 Trabelsi, WO 2013/146500 Mugishima et al.). In this case, to obtain high-quality thin technical ceramics (with high density and high mechanical characteristics), usually 1-5% MnTiO ₃ additives are introduced, and sintering of a powder mixture of α-Al ₂ O ₃ and MnTiO _{3 is} carried out at 1250-1400 ° C. The disadvantage of the methods for making ceramics according to the above sources is that the additive is introduced by grinding. Usually, the powder of basic α-Al ₂ O ₃ (corundum) and a mixture of manganese and titanium oxides (for example, rutile TiO ₂ and any of manganese oxides - MnO ₂ , Mn ₂ O ₃ , etc.) are subjected to grinding; instead of manganese oxide, manganese salts are also used, such as MnCO ₃ , which, when heated to 200-400 ° C, decompose to form manganese oxide. Moreover, during heating up to 900-1000 ° C, i.e. below the sintering temperature, manganese and titanium oxides enter into a chemical reaction with the formation of manganese titanate MnTiO ₃ . Alternatively, milling of MnTiO ₃ and Al ₂ O ₃ can be performed directly. However, the introduction of additives by grinding in mills is associated with significant energy consumption. In addition, even distribution of the additive often requires grinding the powder particles of the additive to a submicron size, i.e. long grinding (for example, about 10 hours). In addition, grinding is associated with some technological difficulties and inconveniences (dusting of fine dry powders when handling them; the need to clean the grinding chamber and balls of the mill after grinding, as well as sieves after sieving; the need to dry suspensions in case of wet grinding; grinding of impurities into the processed material from balls and chamber walls).

Avoiding grinding allows the method of introducing an additive in the form of a modifying coating on the particles of the base Al ₂ O ₃ . The simplest is the liquid-phase modification method, when the coating is applied from a solution. Known methods for modifying Al ₂ O ₃ powder with titanium and manganese oxides with precipitation of Mn and Ti compounds in aqueous solutions (see, for example, [Erkalfa N., Misirli Z., Baykara T. // Ceramics International. 1998. V. 24, No. 2. P. 81-90]). The disadvantage of these methods is that water-soluble titanium compounds - such as TiCl ₃ , TiOSO ₄ , potassium titanyl oxalate TiO (C ₂ O ₄ K) ₂ - are unstable, aggressive or deficient. In addition, the decomposition of inorganic chlorine- and sulfur-containing titanium compounds is accompanied by the release of harmful gases (sulfur oxides, etc.), which require disposal, and potassium is an undesirable impurity for fine alumina ceramics.

The technical task was to create a more efficient method for producing alumina ceramics, incl. armored ceramics.

The technical result was achieved by using a liquid-phase method for modifying the surface of A1 ₂ O ₃ particles with titanium and manganese oxides using organic solvents. In this case, an organic acid salt was used as the initial manganese-containing reagent, and an alcoholate served as the initial titanium-containing compound.

It is preferable to use readily available manganese acetate and titanium tetrabutylate (tetrabutoxytitanium) Ti (OC ₄ H ₉ ) ₄ as starting reagents. The latter, in contrast to titanium isopropoxide Ti (OC ₃ H ₇ ) ₄ , is more convenient to use - it is hydrolyzed more slowly by air moisture and evaporates more slowly. Low-toxic ethanol and acetylacetone are preferred as solvents.

Coating the Al ₂ O ₃ powder is preferably as follows. Manganese acetate in the form of tetra- or dihydrate is dissolved in ethyl alcohol. Titanium tetrabutylate is dissolved in acetylacetone. Both solutions are poured into a vessel with α-Al ₂ O ₃ powder. Alternatively, instead of α-Al ₂ O ₃ , metastable forms of aluminum oxide or aluminum hydroxides (γ-Al ₂ O ₃ , AlO (OH), etc.) can be used. While stirring the resulting suspension with a propeller stirrer, the solvents (ethanol, acetylacetone) and hydrolysis products of the reagents (butanol, acetic acid) are evaporated. In this case, a xerogel of the composition Mn-Ti-OCH is formed. By increasing the temperature, organic components and carbon are burned out of the xerogel, forming a coating of the Mn-Ti-O composition on the carrier particles (α-Al ₂ O ₃ ). In the simplest version, heat treatment is carried out in air, so that a sealed reactor with a specially created atmosphere is not required. Alternatively, a flow-through chemical reactor can be used. Evaporation occurs at 90-200 ° С, removal of organic residues and carbon - up to 450 ° С. One of the advantages of the proposed method for producing xerogel is its short duration in time. So, for example, for comparison, the classical sol-gel method requires a time of the order of a day or more for the maturation of gels.

The result is a fragile sinter that easily crumbles into powder. In contrast to the original Al ₂ O ₃ powder, the modified powder has a light brown color and better flowability.

When adding solutions of compounds of other elements, multicomponent additives can be added. For example, SiO ₂ can be introduced by adding an additional solution of tetraethoxysilane in acetylacetone. With a nonstoichiometric ratio of the starting Mn- and Ti-containing reagents, ceramics doped with manganese titanate and at the same time with manganese or titanium oxide can be obtained.

The resulting intermediate product - alumina powder coated with manganese and titanium oxides (possibly with additional oxides of other elements) - can be used for making ceramics by any known method, including by free sintering, slip casting, pressure sintering, making ceramics according to additive technologies. Semi-finished powder semi-finished products are formed by standard techniques (for example, by semi-dry uniaxial pressing).

The advantage of the proposed method is that in the course of the reactions of the formation of manganese and titanium oxides, inorganic compounds are not released, and the disposal of emissions can be reduced to the afterburning of volatile organic substances.

In the case when the raw material - the initial α-Al ₂ O ₃ powder - is a commercially available micropowder of low cost with an average particle size of more than 1 μm, a purity of ≈ 99.5-99.7%, the proposed method allows the production of technical ceramics with a sintering temperature ≈ 1300-1400 ° C with an MnTiO ₃ additive content _of about 2.5-4% of the mass. (in comparison with the sintering temperature ≈ 1700-1800 ° С without additives). Here, the sintering temperature means the temperature that provides compaction to a relative density of at least 97% with isothermal holding for not more than 3 hours.

At the same time, the simplest techniques of uniaxial semi-dry pressing with a temporary organic bond and free sintering in air provide sufficiently high mechanical properties of the resulting ceramics:

- longitudinal speed of sound ≥10.5 km / s;

- transverse speed of sound ≥6.0 km / s;

- Young's modulus ≥360 GPa;

- microhardness (with a load of 0.5 kg) ≥16 GPa;

- three-point bending strength ≥250 MPa.

The invention is illustrated by the following examples.

Example 1

a) Application of Mn-Ti-O coating on corundum powder

1.47 g (0.006 mol) of manganese acetate tetrahydrate Mn (CH ₃ COO) ₂ × 4H ₂ O was dissolved in 100 ml of ethanol (95%). 2.00 g (0.006 mol) of titanium tetrabutylate Ti (OC ₄ H ₉ ) _{4 was} dissolved in 12 ml of acetylacetone. The resulting solutions were poured into a glass with 30.0 g of corundum micropowder (median particle size 1.5 μm, BET specific surface 1.4 m ² / g). With stirring, the glass was heated to boiling of the liquid (100 ° C with a holding time of 45 min), transformation of the suspension into a paste (120 ° C, 10 min) and visual cessation of liquid evaporation (170 ° C, 10 min). The resulting mass was subjected to heat treatment in air in an oven heated to 450 ° C with a holding time of 15 min. The resulting brittle cake was pounded in a mortar.

The amount of starting reagents and support - Mn (CH ₃ COO) ₂ × 4H ₂ O, Ti (OC ₄ H ₉ ) ₄ and Al ₂ O ₃ - corresponds to the mass fraction of the modifier (calculated as MnTiO ₃ ) x _MnTiO3 = 3%.

The mass average thickness of the coating (based on MnTiO ₃ ) h according to equation (1) was 5 nm:

where Δm is the mass of the coating; m ₀ is the mass of the carrier powder (Al ₂ O ₃ ); ρ _c - coating density (4.55 g / cm ³ based on MnTiO ₃ ); s _m is the BET specific surface area of the starting Al ₂ O ₃ powder.

According to the data of scanning electron microscopy and electron microprobe analysis, the Mn-Ti-O coating is present on all Al ₂ O ₃ particles and has the character of a rough solid or island.

According to X-ray diffraction for separately synthesized xerogel (without powder Al ₂ O ₃₎ with the same proportions of reactants and heat treatment conditions, the modifier comprises a mixture of titania phases (rutile TiO _2), manganese sesquioxide (rhombohedral Mn ₂ O _3).

b) Sintering of modified corundum powder

25×5 мм и 6 стержней размером 3,6×5×32 мм. После сушки при комнатной температуре ≥3 суток, заготовки спекли на воздухе: нагрев 10°С/мин, выдержка 1 ч при 1400°С. Линейная усадка составила 17%. По данным рентгеновской дифракции спеченные образцы состояли из корунда с примесью MnTiO₃, т.е. в ходе нагрева при спекании прошла реакция взаимодействия оксидов марганца и титана покрытия с образованием титаната марганца:After adding a temporary binder to the powder (5% by weight, a 5% aqueous solution of polyvinyl alcohol), 3 tablets of the size

25 x 5 mm and 6 rods 3.6 x 5 x 32 mm. After drying at room temperature for ≥3 days, the workpieces were sintered in air: heating 10 ° С / min, holding for 1 h at 1400 ° С. Linear shrinkage was 17%. According to the X-ray diffraction data, the sintered samples consisted of corundum doped with MnTiO ₃ ; during heating during sintering, the reaction of interaction of manganese oxides and titanium of the coating took place with the formation of manganese titanate:

For the obtained ceramic samples measured: density ρ 3.87-3.88 g / cm ³ ; water absorption W 0.00%; longitudinal speed of sound V _L 10.6 ± 0.2 km / s; transverse speed of sound V _T 6.2 ± 0.1 km / s; Vickers microhardness (with a load of 0.5 kg) H _V 17.3 ± 1.1 GPa; three-point bending strength σ _f 263 ± 21 MPa. Strength was measured on rod specimens, other parameters were measured on pellets. Before measuring the mechanical properties (V _L , V _T , H _ν , σ _f ), the surface of the samples was ground.

The relative density of the samples ρ / ρ * was calculated based on the theoretical density ρ *:

where x _i is the mass fraction of the component, ρ _i is the density of the i component (3.99 g / cm ³ for Al ₂ O ₃ and 4.55 g / cm ³ for MnTiO3 according to the reference data). For x _MnTiO3 = 3%, the calculation by equation (3) gives ρ * = 4.00 g / cm ³ , respectively ρ / ρ * = 96.8%.

Young's modulus E was calculated using the equation (4):

The E value was 368 ± 6 GPa.

Example 2

Modes of coating and sintering are the same as in Example 1, but additionally before molding the preforms and sintering performed additional annealing modified powder Al ₂ O ₃ in air at 700 ° C for 1 hour. According to X-ray diffraction for separately synthesized xerogel (without powder Al ₂ O ₃ ) with the same proportions of reagents and heat treatment modes, the modifier consists of a mixture of titanium dioxide (rutile TiO ₂ ) and manganese sesquioxide (cubic Mn ₂ O ₃ ) phases. For the resulting ceramics ρ 3.89-3.90 g / cm ³ ; ρ / ρ * = 97.3%; W 0.00%; V _L 10.5 ± 0.3 km / s; V _T 6.11 ± 0.05 km / s; E 361 ± 8 GPa; H _V 16.3 ± 1.6 GPa; σ _f 257 ± 21 MPa. According to X-ray diffraction data, the sintered samples consisted of corundum doped with MnTiO ₃ .

Example 3

The modes of coating and sintering are the same as in example 1, but before the blanks are molded and sintered, the modified Al ₂ O ₃ powder is additionally annealed in air at 1000 ° C for 1 h. According to X-ray diffraction data for a separately synthesized xerogel (without Al ₂ O ₃ ) with the same proportions of reagents and heat treatment modes, the modifier consists of manganese titanate MnTiO ₃ .

According to the differential thermal analysis and thermogravimetry (DTA-TG) when heated separately synthesized xerogel air reaction (2) between Mn ₂ O ₃ + TiO ₂ to form MnTiO ₃ proceeds at 900 ° C, which is fixed by the endothermic effect and a reduction in weight. For the resulting ceramics ρ 3.85-3.88 g / cm ³ ; ρ / ρ * = 96.5%; W 0.1-0.2%; V _L 10.1 ± 0.4 km / s; V _T 5.8 ± 0.2 km / s; E 325 ± 13 GPa; H _V 16.0 ± 1.5 GPa; σ _f 259 ± 45 MPa. According to X-ray diffraction data, the sintered samples consisted of corundum doped with MnTiO ₃ .

As can be seen from examples 1-3, annealing the xerogel at temperatures of 450-700 ° C leads to the formation of coatings of manganese and titanium oxides Mn ₂ O ₃ and TiO ₂ , which are precursors of manganese titanate MnTiO ₃ . An increase in the annealing temperature to 1000 ° C with the formation of a coating directly from manganese titanate adversely affects the performance of ceramics sintered from a powder with a coating: open porosity (water absorption) appears, the speed of sound and Young's modulus decrease, and the spread of strength values increases.

Example 4

The modes of coating and sintering are the same as in example 1, but the sintering was carried out at 1350 ° C for 1.5 hours. For the resulting ceramics ρ is 3.88 g / cm ³ ; ρ / ρ * = 96.9%; W 0.00%; V _L 10.5 ± 0.2 km / s; V _T 6.17 ± 0.05 km / s; E 366 ± 7 GPa; σ _f 269 ± 36 MPa.

Example 5

The modes of coating and sintering are the same as in example 1, but the sintering was carried out at 1300 ° C for 3 hours. For the resulting ceramics ρ is 3.87 g / cm ³ ; ρ / ρ * = 96.7%; W 0.00%; V _L 10.5 ± 0.1 km / s; V _T 6.13 ± 0.13 km / s; H _V 17.7 ± 1.6 GPa; E 361 ± 12 GPa; σ _f 279 ± 30 MPa.

As can be seen from the comparison of examples 4.5 with example 1, by reducing the sintering temperature from 1400 to 1300 ° C with a simultaneous increase in the duration of isothermal holding from 1 to 3 hours, it is possible to obtain ceramics of the same density (ρ / ρ * = 97%) and without water absorption (without open porosity), while the strength increases from 260 to 280 MPa.

Example 6

The modes of coating and sintering are the same as in example 1, but the amount of initial reagents for the formation of xerogel and coating is reduced and corresponds to the mass, the proportion of MnTiO ₃ additive in the ceramic is 2%. For the resulting ceramics ρ 3.89-3.90 g / cm ³ ; ρ / ρ * = 97.5%; W 0.00%; V _L 10.51 ± 0.02 km / s; V _T 6.14 ± 0.08 km / s; E 365 ± 7 GPa; σ _f 217 ± 56 MPa. As can be seen, in terms of density and speed of sound, the obtained samples differ little from the samples presented in example 1. However, when measuring the strength, two out of six samples showed low strength less than 200 MPa (surges), so that the average value of σ _f dropped noticeably. For specimens with bursts of fracture strength, after testing, defects in the form of a light zone with an insufficient content of MnTiO ₃ additive were observed against a brown ceramic background. Obviously, these defects are caused by an insufficiently uniform distribution of the additive due to a decrease in the total content of the modifier in the powder.

Example 7 (comparative)

The composition and amount of the additive, the sintering mode are the same as in example 1, but the additive was introduced not as a coating on corundum powder, but according to the traditional technology by grinding. Separately, a Mn-Ti-OC-H xerogel was synthesized and annealed at 450 ° C to form a mixture of Mn ₂ O ₃ and TiO ₂ , similarly to example 1. Then 1.0 g of the synthesized Mn-Ti-O powder and 30.0 g of Al ₂ O ₃ powder was ground with balls in a planetary mill in stirring mode (70 rpm) for 45 min and then in grinding mode (250 rpm) for 45 min. In this case, the particles of the Mn-Ti-O powder, which were initially much larger and more fragile than the particles of basic Al ₂ O ₃ , having a microporous structure, were mainly subjected to grinding. The resulting mixture of Mn-Ti-O and Al ₂ O ₃ powders was molded and sintered as in example 1.

For the resulting ceramics ρ 3.90-3.92 g / cm ³ ; ρ / ρ * = 97.7%; W 0.00%; V _L 10.36 ± 0.03 km / s; V _T 6.21 ± 0.02 km / s; E 368 ± 2 GPa; H _V 17.1 ± 1.4 GPa; σ _f 252 ± 53 MPa. For the samples of this series, heterogeneity and defects that were not characteristic of the previous series were already visually detected (examples 1-6). So, during the initial grinding of the tablets on the diamond wheel, streaks appeared on the flat end surfaces, and a crack appeared on one of the tablets at the end surface. A large spread in strength values (± 53 MPa) also testifies to the greater heterogeneity of the samples made by the traditional technology.

It can be seen from the above examples that the optimum annealing temperature for coated corundum powder is 450-700 ° C. At higher annealing temperatures (from 900-1000 ° C), the coating material (a mixture of Mn ₂ O ₃ and TiO ₂ ) turns into manganese titanate, and a coating of ready-made MnTiO ₃ gives a deterioration in the properties of ceramics in comparison with a coating from its precursors. At annealing temperatures below 400 ° C, as shown by elemental analysis, incomplete removal of carbon from the Mn-Ti-OCH xerogel occurs, and the carbon impurity is undesirable for alumina ceramics.

The optimum temperature for sintering the coated corundum powder in air is 1300-1400 ° C. At lower temperatures, the required sintering time is excessively increased to achieve a relative density of at least 97%. At higher sintering temperatures, there is a strong growth of the ceramic grain and a decrease in its strength. The sintering temperature of 1300-1400 ° C is low enough for alumina ceramics, which gives savings in the cost of the used furnace equipment and energy consumption for sintering.

The optimum content of the modifier (coating on Al ₂ O ₃ powder) is 2.5-4% by weight, based on the addition of MnTiO ₃ in the finished ceramic. With a lower content of the modifier, a uniform distribution of the additive in the ceramic is not achieved and, as a result, internal defects are present in the sintered material that reduce the strength. At a higher content of the modifier, a partial outflow of the liquid phase of manganese titanate from the sintered workpiece is observed.

In addition to the disadvantages of the actual technological operation of grinding, mentioned earlier, from comparative example 7 it follows that the proposed method of introducing the additive makes it possible to achieve a significantly more uniform distribution of the sintering additive in the ceramic than the traditional method of grinding and thus improve its properties.

The achieved set of ceramic properties (absolute and relative density, sound speed, Young's modulus, hardness, strength) and the simplicity of the technology make it possible to use the proposed method for the manufacture of armored ceramics.

Claims (5)

1. A method of producing alumina ceramics, characterized in that as a raw material (charge) for sintering, Al ₂ O ₃ powder modified with manganese and titanium oxides is used in the form of a coating, which in the process of sintering or preliminary annealing at 900-1000 ° C turns into manganese titanate MnTiO ₃ , providing a liquid-phase sintering mechanism at a relatively low temperature of 1300-1400 ° C; and Al ₂ O ₃ powder particles are coated by forming a xerogel of the Mn-Ti-OCH composition from a solution of manganese and titanium compounds in organic solvents by evaporating the solvents followed by annealing in an oxidizing atmosphere at a temperature of 450-700 ° C. 2. The method according to claim 1, characterized in that the amount of the applied nano-sized coating corresponds to the mass fraction of the additive. 3. A material of alumina ceramics made according to claim 1, containing alumina doped with manganese titanate, with a concentration of manganese titanate in the ceramics of 2.5-4 wt.%. 4. Material according to claim 3, with the following characteristics: longitudinal speed of sound is not less than 10.5 km / s, Young's modulus is not less than 360 GPa, microhardness at a load of 0.5 kg is not less than 16 GPa, three-point bending strength is not less than 250 MPa. 5. Material according to claim 3, suitable for use as armored ceramics.