background:

The MAX phase is a group of ternary compounds whose general formula is Mn+1AXn (where M is a transition element, a is usually from the IIIAo IVA group in the periodic table, and X can be C or N, N=1-3). They have a nano-amination structure, which gives them unusual properties. Maximum combination of metal and ceramic properties, such as high rigidity, good mechanical properties at high temperatures, high corrosion and oxidation resistance, and good thermal and electrical conductivity.

In addition, they show the mechanical damping properties (damping) of laminates: they deform to form kinks, which allows them to absorb a lot of energy. These characteristics make them ideal candidates for specific high-temperature applications. Good oxidation performance is a necessary condition for the material to be used at high temperatures; in order to maintain its performance, an oxidation protective layer needs to be formed.

In this context, the team headed by SATsipas launched a research and published it in the Journal of Alloys and Compounds on November 29, 2020 Beijing time with the title "Thermophysical properties of porous Ti2AlC and Ti3SiC2 produced by powder metallurgy" It aims to analyze the thermophysical properties of porous Ti2AlC and Ti3SiC2 prepared by powder metallurgy.

Summary:

The researchers conducted in-depth studies on the physical and chemical properties of Ti2AlC and Ti3SiC2 MAX phase compounds with controlled porosity and grain size obtained by powder metallurgy technology to determine their use in vehicle catalytic devices, heat exchangers or impact-resistant structures. Applicability.

The study was carried out in different amounts (20-60 vol.%) and different space holder sizes (250-1000xh m) of balanced consolidated specimens and specimens without space holders. According to the maximum use temperature of each material, the oxidation test is carried out at different temperatures.

In order to understand the oxidation mechanism, the oxidation kinetics was analyzed to determine the effect of porosity in each case. In addition, the structure and composition of the oxide layer formed after the test were analyzed by scanning electron microscope (SEM). Study electrical and thermal conductivity at room temperature and temperatures up to 1000°C.

The researchers discussed the effects of pore size and cell wall thickness, and measured the permeability of the foam. The influence of microscopic porosity and macroscopic porosity on permeability is discussed, and the thermal compensation coefficient of the prepared foam is measured at the same time.

These porous largest phases have suitable properties for catalytic substrates, heat exchange, high temperature filters or volumetric solar receivers.

process:

Ti2AlC usually forms a dense, thin and well-adhesive Al2O3 layer on the surface of the material. On the other hand, Ti3SiC2 forms a double-layer structure: the outside is a TiO2 layer, and the inside is a combined layer of SiO2 and TiO2. The oxidation mechanisms of the largest phases of Ti2AlC and Ti3SiC2 both show parabolic volume dynamics.

In addition, it has been determined that the oxide layer grows through diffused oxygen to the inner part of the material and diffused titanium to the outside, while silicon atoms are oxidized in situ. Table 1 summarizes the relevant oxidation studies performed on the MAX phase.

Although most of the largest phase studies are based on dense materials, due to the combination of their unique properties, people are more and more interested in the high performance application characteristics of porous largest phase materials, such as catalytic devices, heat exchangers or impact-resistant structures on vehicles .

There are few studies on the preparation of porous maximum phase materials. L. Hu et al. prepared Ti2AlC foam with controllable porosity using NaCl as pore formation. T. Fey et al. prepared Ti2AlC foam material by gel casting method and simulated its elastic behavior using femi modeling. Although people are very interested in the largest porous phase, so far, there are few detailed studies on the oxidation behavior of the largest porous phase.

Gonzalez-Julian studied the oxidation resistance of the largest phase of porous Cr2AlC. The purpose of this work is to study the thermophysical properties of the porous Max stage Ti2AlC Ti3SiC2 and control the size and amount of porosity, determine the suitability of these foams for solar volume receivers, heat exchangers in contact with active heat transfer fluids, high temperature filters or The substrates are high temperature electrodes of catalytic combustion equipment, solid oxide fuel cells, components and high temperature corrosion resistance, friction and wear in nuclear fuels and structural materials.

The average equivalent diameter and average cell wall thickness of the macroporosity, and their respective standard deviations.

For a foam material with a volume% of 60, the size distribution of the three foam materials:

250-400 runners m, 400-800 runners m and 800-1000 runners m,

a) Ti2AlC and b) Ti3SiC2.

a) Ti2AlC Ti3SiC2 at 1000ºC and b) Ti2AlC Ti3SiC2 at 900ºC.

The oxidized Ti2AlC sample is at 1000ºC for a 24-hour period of 10,

a) Dense surface, b) 20 vol.% holds 250-400µm space frame size and

C) 60 vol.% holds the size of the internal clip with a space of 800-1000 µm.

Oxidize Ti3SiC2 sample at 900ºC for 10 24 hours cycle,

a) Dense surface, b) 20-250 and C) 60-800 inside.

a) Ti2AlC and b) Ti3SiC2 porous sample composition diagram.

The effect of the average pore size on the thermal conductivity of the fixed porosity in the material:

24vol.% (Ti2AlC) and 66vol.% (Ti3SiC2). The result is 300ºC.

analyze:

This study investigated the thermophysical properties of the largest porous phases Ti2AlC and Ti3SiC2 with different pore sizes and different porosities. In all cases, the oxidation kinetics of the porous samples of Ti2AlC and Ti3SiC2 were much faster at the beginning of the first cycle (24 hours) than during the rest of the test period.

This indicates that a protective oxide layer is formed in the first 24 hours. The oxidation kinetics of Ti2AlC follows a cubic law. The quality obtained samples have no space holder and logarithmic method with the lowest sample porosity (20-250), while the large-scale observation samples are rarely obtained with higher porosity.

For Ti3SiC2, the kinetics are logarithmic for all sparse densities and pore sizes were studied. The cyclic test was carried out for 24 hours at 1000°C (Ti2AlC) and 900°C (Ti3SiC2) for a total of 240 hours. All tested Ti2AlC and Ti3SiC2 samples showed resistance to oxidation and thermal shock.

The protective layer formed in the porous samples of the two materials is dense, adheres well, has no cracks, and resists thermal cycling, although the surface area is increased due to the presence of pores.

Porosity and size affect mass gain: the larger the exposed area, the greater the mass gain. For porous Ti2AlC, the change from closed porosity to open porosity greatly increases its mass gain. The porosity changes from closed to open. As the porosity increases, the mechanism of oxygen diffusion through the sample changes, and the microstructure of the oxide layer inside the pores also changes.

On the other hand, all Ti3SiC2 porous samples have oxide layers with similar characteristics on the inner surface of the pores. There was no mass loss in all samples during the formation of the oxide layer, indicating that there was no spalling phenomenon, which proved that the porous material prepared by the powder metallurgy method in this study has good strength and quality.

Therefore, these porous maximum phases can be considered for exhaust gas catalytic devices, high temperature electrodes, burner devices or applications under combustion conditions.

The increase in permeability is related to the increase in porosity and pore size: both are conducive to the flow of gas through the sample. The difference in permeability between Ti2AlC and Ti3SiC2 pore structures with similar porosity is related to the macroscopic porosity. The greater the macroscopic porosity, the greater the permeability.

Under similar porosity, the linear CTE increases slightly with the increase of the average pore size, and the increase of the porosity reduces the linear CTE. The electrical conductivity and permeability exhibited by these largest phase foams make them good candidates for catalytic combustion devices, high temperature electrodes, or solid oxide fuel cell substrates, or heat exchangers in contact with aggressive heat transfer fluids.

Link to this article： What are the thermophysical properties of porous Ti2AlC and Ti3SiC2 produced by powder metallurgy?

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