High-Temperature Stability and Phase Transformations of Titanium Carbide (Ti₃C₂Tₓ) MXene
High-Temperature Stability and Phase Transformations of Titanium Carbide (Ti₃C₂Tₓ) MXene
MXenes are a rapidly growing class of two-dimensional (2D) materials that have gained widespread attention due to their exceptional electrical conductivity, chemical activity, mechanical robustness, and versatility across technologies such as energy storage, catalysis, sensing, and composite reinforcement. Among all MXenes discovered to date, Ti₃C₂Tₓ stands out as the most widely studied and extensively applied member of the family.
However, as demand grows for materials that can operate in extreme conditions—particularly high temperatures—it has become increasingly important to understand how MXenes behave when exposed to elevated heat. Many emerging industrial applications require operation above 600–700 °C, such as metal and ceramic composites, high-temperature electronics, aerospace materials, and advanced catalytic systems. For these applications, it is essential to understand whether MXenes retain their structure, degrade, or transform into other compounds.
This study provides one of the most detailed analyses to date on how Ti₃C₂Tₓ MXene transforms between 600 and 1500 °C under inert environments, filling a major gap in the field.
Background: What Are MXenes and Why Do They Change at High Temperature?
MXenes are produced by selectively removing the “A layer” (often aluminum) from a 3D layered precursor material called a MAX phase. After etching, a 2D structure remains, composed of layers of transition metals (such as titanium) bonded with carbon or nitrogen. This structure is decorated with surface terminations (denoted as Tₓ), most commonly –O, –F, –Cl, and –OH groups.
Ti₃C₂Tₓ is derived from Ti₃AlC₂ MAX phase. It has:
high electrical conductivity (10,000–20,000 S/cm),
high mechanical stiffness,
good colloidal stability in water,
excellent electrochemical properties.
Yet the material’s high-temperature behavior is complex. Previous studies had shown:
Up to ~200 °C: interlayer water is released.
300–500 °C: –OH groups detach.
500–800 °C: –F groups desorb.
Above 800 °C: MXene begins to undergo structural decomposition.
Still, these earlier studies mainly focused on surface chemistry—not on how the MXene’s carbide core changes. The transformations of the core structure (Ti–C framework) at temperatures above 700 °C had not been fully mapped.
This study addresses that gap by using advanced in-situ XRD with a 2D detector (XRD²) and ex-situ annealing up to 1500 °C, directly observing how Ti₃C₂Tₓ evolves.
Materials Used in the Study
To understand whether MXene morphology affects its high-temperature transformation, researchers prepared two very different Ti₃C₂Tₓ films:
1. Single-Flake Films (Delaminated MXene)
Consist of individual or few-layered MXene flakes.
Highly ordered.
Smooth, uniform layer stacking.
Produced by delaminating the MXene using Li⁺ intercalation.
2. Clay Films (Multilayer MXene)
Thick, highly stacked, disordered MXene.
Contains many multilayer aggregates and irregular particle clusters.
Not delaminated; resembles a compact clay-like structure.
Both film types were annealed at temperatures from 600 to 1500 °C under inert gas (Ar or He) and analyzed by XRD, SEM, and EDS.
Key Finding #1: MXene Is Stable Up to 600 °C
The most fundamental conclusion is that:
Ti₃C₂Tₓ remains structurally intact up to ~600 °C.
Even though surface terminations begin to detach (especially –OH and –F), the main MXene structure stays two-dimensional.
This is important because it means that most practical applications—supercapacitors, batteries, EMI shielding, sensors—can safely operate below 600 °C without major structural degradation.
Key Finding #2: Between 700–1000 °C, MXene Transforms into 3D Carbide Phases
Above 700 °C, Ti₃C₂Tₓ begins a major transformation into three-dimensional (3D) titanium carbide phases:
The primary new phases formed are:
1. Ti₂C (ordered vacancy superstructure)
Appears around 17.8–18° (2θ).
Results from carbon vacancy ordering.
Typically forms in bulk Ti–C compounds after long annealing.
Indicates a partial breakdown of MXene’s layered nature.
2. TiCᵧ (non-stoichiometric cubic titanium carbide)
Major peaks appear around 36° and 77° (2θ).
The “y” value changes with carbon content.
This is the same well-known ceramic material used industrially for high-temperature applications.
The fact that both Ti₂C and TiCᵧ form simultaneously shows that MXene transforms through multiple parallel pathways depending on local carbon content.
Key Finding #3: Above 1000 °C, MXene Converts Almost Entirely into Cubic TiC
When the temperature exceeds 1000 °C, Ti₃C₂Tₓ MXene no longer retains any layered structure. Instead, it becomes:
Fully cubic TiCᵧ (3D crystalline titanium carbide).
This matches well-known phase diagrams of the Ti–C system.
At 1500 °C, only large, well-defined TiC grains remain, regardless of the starting MXene morphology.
This confirms that MXene does not survive extreme temperatures—but transforms into valuable high-temperature ceramic phases.
Morphology Matters: Single-Flake vs Clay MXene Behave Very Differently
One of the most novel contributions of this work is the discovery that the starting morphology strongly influences the final crystal structure.
A) Single-Flake (Delaminated) MXene → Lamellar 3D Carbides
Even after annealing at 900–1200 °C, single-flake MXene films retain a layered, lamellar grain morphology, even though their chemistry shifts to Ti₂C/TiC.
This means:
The 2D stacking sequence influences how new 3D crystals form.
The Ti₂C/TiC grains grow along the original MXene basal planes.
A partially layered structure survives even when the chemical composition changes.
This is a highly desirable trait for high-temperature composite reinforcement.
B) Clay MXene → Random Cubic TiC Grains
Clay films behave very differently due to their disordered structure.
After annealing:
They produce large, irregular cubic TiC grains.
There is no preferred orientation.
The layered structure is lost completely at much lower temperatures compared to single-flake films.
Thus, clay MXene is much less ideal for high-temperature structural applications.
Detailed Temperature-Based Summary
| Temperature | Single-Flake MXene | Clay MXene | Resulting Phases |
|---|---|---|---|
| ≤ 600 °C | Fully stable | Fully stable | Ti₃C₂Tₓ |
| 700 °C | Beginning of transformation | Beginning of transformation | Ti₂C + TiCᵧ |
| 800–900 °C | Strong lamellar Ti₂C/TiCᵧ formation | Formation of cubic TiC grains | Ti₂C + TiCᵧ |
| 1000 °C | Almost full conversion | Large cubic TiC grains | Ti₂C + TiCᵧ |
| 1200–1500 °C | Lamellar but 3D TiC | Fully cubic, random TiC | Pure TiCᵧ |
Electrical Conductivity Changes
The electrical conductivity of MXene is one of its most valuable properties. The study found:
At 150 °C, conductivity is very high (~17,000 S/cm).
At 600 °C, conductivity decreases.
At 700 °C, conductivity temporarily increases due to ordering after removal of surface groups.
Above 800 °C, conductivity drops sharply due to structural transformation.
At 1000–1500 °C, conductivity becomes much lower (similar to polycrystalline TiC).
This confirms:
MXene maintains its electrical properties only below about 700 °C.
Above this point, it stops behaving like a 2D conductor.
Why Do These Transformations Occur?
Several mechanisms explain the observed transformations:
1. Surface group removal destabilizes structure
–OH and –F groups detach → MXene layers become more reactive.
2. Carbon atoms migrate and vacancies form
Carbon movement increases drastically above 700 °C, enabling carbide phase rearrangement.
3. Ti–C bonds reorganize
The structure shifts to more thermodynamically stable cubic TiC at high temperatures.
4. Lamellar order slows transformation
In single-flake MXene, the inherent order delays and guides transformation.
Technological Significance of the Findings
This work is important for industries that want to integrate MXenes into real products.
1. MXene can safely operate up to 600 °C
This supports use in:
EMI shielding
Batteries
Supercapacitors
Flexible electronics
Sensors
2. Between 700–1000 °C, MXene becomes a beneficial carbide additive
Lamellar Ti₂C/TiC grains formed from single-flake MXene can reinforce:
Ceramics
Metal-matrix composites
High-temperature structural materials
3. Above 1000 °C, MXene fully converts into TiC
Since TiC is an important engineering ceramic—used in cutting tools, aerospace, and armor—MXene can act as a precursor for fine TiC grains with tunable morphology.
4. Single-flake MXene is superior for advanced composites
Because it retains lamellar structure even after transformation.
Final Conclusion
This study provides the most comprehensive look so far into how Ti₃C₂Tₓ MXene behaves at high temperatures. The major takeaways are:
MXene is stable up to 600 °C.
Between 700–1000 °C, it gradually converts to Ti₂C and TiCᵧ.
Above 1000 °C, MXene fully transforms into cubic TiC.
Single-flake MXene maintains lamellar morphology even after phase transformation.
Clay MXene becomes randomly oriented cubic grains.
Electrical conductivity decreases sharply after 800 °C.
These insights are crucial for applying MXenes in high-temperature composites, catalytic systems, and structural materials.
