Direct Integration of High-Quality 2D Carbide MXene Flakes for Electromagnetic Interference Shielding
2.3. Formation of a High-Quality Parent MAX Phase 1. Optimization of the sintering process
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measuring ≈41 × 41 mm2 for measurement purposes. The total EMI shielding effectiveness (SET) of the vacuum-filtered Ti3C2Tx MXene/PP membrane and painted MXene ink/filter paper is defined as the logarithm of the proportion of incident and (Pi) transmitted (Pt) powers of radiation, which is the sum of absorption (SEA), reflection (SER), and multiple reflections (SEM):92
SET[dB] = 10 log (𝑃𝑖
𝑃𝑡) = SEA+ SER+ SEM (2.1) The SEA and SER could be achieved with following calculation:
SEA[dB] = −10 log ( 𝑇
1−𝑅) = −10log ( |𝑆21|2
1−|𝑆11|2) (2.2) SER[dB] = −10 log(1 − R) = −10 log(1 − |𝑆11|2) (2.3)
where the reflection (R), transmittance (T), and absorbed (A) power coefficients are derived by scattering factors (S11 and S21).
2.3. Formation of a High-Quality Parent MAX Phase
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Figure 2.3. Structural characterization of sintered Ti3AlC2. (a) XRD results of Ti3AlC2 MAX phase pellet heated at different temperatures, (b) XRD results of specimens heated at 1500 °C at varying periods, and (c) temperature-dependent chemical composition calculation. Photos of a cold-pressed specimen (d), before and (e) after the reactions at ≈1,480 °C for 2 h, and (f) grinded specimens. SEM micrographs of (g) surface of heated pellets and (h) the grinded MAX precursor powder with layered morphology. (i) EDX result of a specimen exhibiting the elemental components of the Ti3AlC2 MAX phase, and (j) XRD results of commercially available and produced Ti3AlC2 MAX phase specimen in comparison with hexagonal patterns of Ti3AlC2 (JCPDS 10-074-8806).
As the heating temperature climbed to 1,450 °C, the Ti3AlC2 phase grew to 86.6 vol% while TiC decreased to 13.4 vol%, indicating that a larger amount of Ti–Al reacted with TiC to build interleaved A layers resulting in Ti3AlC2. According to a reported study that examined the mechanism for the synthesis of Ti3AlC2 from raw materials, chemically incorrect Ti–Al intermediate phases were generated after Al melted at ≈660 °C.93, 94 Consequently, a liquid state generated by intermediate phases that melted at 1,350 °C enhanced the sintering with TiC at a higher temperature (1,420 °C) to yield Ti3AlC2.
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Consequently, it is feasible to ascribe the construction of ideal Ti3AlC2 via solid-liquid processes to the high annealing temperature chosen above the melting temperature. Nonetheless, as the heating temperature increased to 1,500 °C, the proportion of Ti3AlC2 phase fell marginally to 84.2 vol%. Around 1,550 °C, TiC rose more and a C intensity formed, exhibiting the same trend as the previously described Ti3AlC2 phase which thermally degrades at 1,580 °C.20 Increasing the heating time from 2 to 4 h had no effect on the peak shift or the full width at half maximum (FWHM), with the exception of a proportional increase in the amount of Ti3AlC2 due to the prolonged process time (Figure 2.3b and Figure 2.4e–h). PVA does not exist in any of the measurement, indicating that the polymer agent was eliminated fully above its boiling temperature (≈228 °C).
Through EDX analysis, the elemental composition of Ti, Al, and C in the heated sample was determined (Figure 2.3c). Chemical composition ranges of x = 2.80–3.04 and y = 1.75–2.38 were investigated for TixAlCy, with Al content held constant; this suggests that sintering circumstances, such as temperature, influenced the final chemical compositions. Using optimized process conditions, an optimum Ti3AlC2 MAX phase pellet was synthesized based on these findings. Following 2 h of sintering at 1,480 °C, the pellet's overall color modified from black to brilliant grey (Figure 2.3d and 2.3e).The layer structure of the precursor phase was visible in SEM micrographs of the sample surface and as- milled sample, indicating that a precursor phase formulated across the whole pellet when over 0.5 g of the raw powder was introduced (Figure 2.3f–h). As reported by other investigations, XPS results revealed crystal nanosheets with strong ceramic Ti and C bonds and interleaved A layers with comparatively weak metal bonds (Figure 2.5).25, 61, 77, 78, 95Ti-C (relevant to Ti-carbide) and Ti(IV) oxide (TiO2) might be attributed to Ti3AlC2 Ti 2p peaks located at ≈455 eV and ≈459 eV, respectively.96C 1s peaks located at ≈281.9 and ≈285 eV corresponded to Ti–C and C–C bonds, respectively, while Al species locating planar sites were found at ≈72.3 eV for the Al 2p peak (with oxidized species at ≈74.6 eV).97, 98 The EDX measurement revealed the precise amounts of Ti, Al, and C in pellets, as well as the composition of Ti3AlC2 (Figure 2.3i).
2.3.2. Comparison with a commercial MAX phase
The synthesis of the hexagonal configuration of Ti3AlC2, comparable to JCPDS file card number 10-074-8806, was confirmed by XRD examination (Figure 2.3j). To determine the necessity of pelletizing on the number of impurities and producing a ideal Ti3AlC2 phase, a comparison was made with other samples, commercially prepared precursor and manufactured MAX powder in the absence of pelletizing (Table 2.1). Commercially prepared precursor phase exhibited irregular morphologies, erroneous compositions, impurity phases including Ti2AlC and AlTi3, and a high C intensity in the XRD analysis, resulting in a poor volumetric proportion of Ti3AlC2 (Figure 2.6). Sintering without
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pelletization also resulted in significant impurity phases due to the large inter-particle distance in decompressed and bulky materials, which made atomic reaction problematic (Figure 2.7). Contrastively, when a little quantity of raw materials (0.6 g) was pelletized and annealed, the major processes during the annealing procedure, namely compaction and surface reduction, happened among the compressed particles, as seen by sample shrinkage. The compaction could encourage the previously indicated Ti3AlC2 growth mechanism, leading to totally synthesized Ti3AlC2 that corresponds to a reported studies for heating with a variety of powder materials by compressing and microwave heating.99 With the exception of some unreacted TiC, the produced precursor phase with compressing had significantly less impurities, demonstrating its greater quality and purity.
Figure 2.4. Analysis of the phases of as-milled pellets sintered at varying temperatures and times.
(a, e) position of the main peaks, (b, f) FWHM, (c, g) integrated intensities, and (d, h) volumetric fraction of Ti3AlC2 and TiC.
Figure 2.5. XPS study of powdered Ti3AlC2 MAX and Ti3C2Tx MXene phases as produced. (a) Ti 2p (b) C 1s, and (c) Al 2p spectra.
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Table 2.1. Comparative assessment of the volume fraction, atomic ratio, and exfoliation yield of MXenes in Ti3AlC2 MAX phases produced by three distinct techniques.
Methods
Volume Fraction [vol%] Atomic Ratio
Exfoliation Yielda) Ti3AlC2
(MAX Phase)
TiC (Impurity
Phase)
Ti (3) Al (1) C (2)
Commercial MAX 32.0 68.0 3.40 1 2.32 0.56 ± 0.14%
Sintered MAX (this work)
Bulk
(5.84 g) 28.6 71.4 3.04 1 2.77 2.35 ± 1.86 %
Pellet
(0.6 g) 91.2 8.8 3.13 1 2.06 55.5 ± 12.8%
a) Exfoliation yield was estimated by the calculation of (weight of delaminated MXenes)/(weight of precursor powder).
Figure 2.6. Structural characterization of commercially manufactured MAX. (a-c) Low proportion of layered structure in SEM micrographs of commercially available Ti3AlC2 MAX powder. (d) EDX result of commercially available precursor phase reveals chemically incorrect composition for Ti3AlC2. (e) Commercial MAX powder with a complex XRD pattern comprising intermediates. (f) Polycarbonate (PC) membrane after vacuum-filtration of an MXene colloid delaminated from commercially available precursor phase reveals inadequate amount of exfoliated MXene flakes.
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Figure 2.7. Sintering without pelletization. Photos of (a) prepared raw material sample and (b) morphology being sintered at ≈1,480 °C for 2 h. (c) XRD result of as-sintered Ti3AlC2 MAX phase powder with a large proportion of TiC as an impurity phase, without pelletizing.
