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• The medium and high-entropy transition mental disilicides present significantly reduced electrical conductivities. • The medium and high-entropy transition mental disilicides are promoting infrared emissivity materials for thermal protection applications. • The mechanism for the reduced thermal conductivity and high infrared emissivity are disclosed. Transition metal disilicides are widely used as heating elements and infrared emission coatings. However, the limited intrinsic infrared emissivity and high thermal conductivity are the main limitations to their applications as infrared emission coatings in the thermal protection system. To cope with these problems, four medium and high-entropy transition metal disilicides, i.e., (V 0.25 Ta 0.25 Mo 0.25 W 0.25)Si 2 (ME-1), (Nb 0.25 Ta 0.25 Mo 0.25 W 0.25)Si 2 (ME-2), (V 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2)Si 2 (HE-1), and (Cr 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2)Si 2 (HE-2), were designed and synthesized by spark plasma sintering method using transition metal binary disilicides as precursors. The introduction of multi-elements into transition metal disilicides not only improved the infrared emissivity but also reduced the electrical and thermal conductivity. Among them, (Cr 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2)Si 2 (HE-2) had the lowest electrical conductivity of 3789 S cm–1, which is over one order of magnitude lower than that of MoSi 2 (50000 S cm–1), and total infrared emissivity of 0.42 at room temperature, which is nearly double of that of TaSi 2. Benefiting from low electrical conductivity and phonon scattering due to lattice distortion, the medium and high-entropy transition metal disilicides also demonstrated a significant decline in thermal conductivity compared to their binary counterparts. Of all samples, HE-2 exhibited the lowest thermal conductivity of 6.4 W m − 1 K − 1 . The high-entropy transition metal disilicides also present excellent oxidation resistance at high temperatures. The improved infrared emissivity, reduced thermal conductivity, excellent oxidation resistance, and lower densities of these medium and high-entropy transition metal disilicides portend that they are promising as infrared emission coating materials for applications in thermal protection systems. [ABSTRACT FROM AUTHOR]

• High-entropy transition metal carbides with regulatable carbon stoichiometry (HE TMC x) achieve the promoting of formation ability from carbon stoichiometry deviations. • Bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25)C x (SPS-ZHTNC x) with carbon stoichiometry deviations show promoted sintering. • Bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25)C x (SPS-ZHTNC x) present tunable mechanical properties. Tremendous efforts have been dedicated to promote the formation ability of high-entropy transition metal carbides. However, the majority of methods for the synthesis of high-entropy transition metal carbides still face the challenges of high temperature, low efficiency, additional longtime post-treatment and uncontrollable properties. To cope with these challenges, high-entropy transition metal carbides with regulatable carbon stoichiometry (HE TMC x) were designed and synthesized, achieving improved ability for single phase solid solutions formation, promoting of sintering and controllable mechanical properties. Two typical composition series, i.e., easily synthesized (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25)C (ZHTNC) and difficultly synthesized (Zr 0.25 Hf 0.25 Ta 0.25 Ti 0.25)C (ZHTTC) are selected to demonstrate the promoting formation ability of single phase solid solutions from carbon stoichiometry deviations. Single phase high-entropy ZHTTC, which has been proven difficult in forming a single phase solid solution, can be prepared with the decrease of C/TM ratio under 2000 °C; while the high-entropy ZHTNC, which has been proven easy in forming a single phase solid solution, can be synthesized at lower temperatures with the decrease of C/TM ratio. The synergistic effect of entropy stabilization and reduced chemical bond strength gaining from carbon stoichiometry deviations is responsible for the formation of single phase solid solutions and the promoted sintering of HE TMC x. For example, the relative density of bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25)C x (SPS-ZHTNC x) increases from 90.98% to 94.25% with decreasing the C/TM atomic ratio from 0.9 to 0.74. More importantly, the room temperature flexural strength, fracture toughness and brittleness index of SPS-ZHTNC x can be tuned in the range of 384 MPa–419 MPa, 4.41 MPa⋅m1/2–4.73 MPa⋅m1/2 and 3.679 μm−1/2–4.083 μm−1/2, respectively. Thus, the HE TMC x prepared by adjusting the ratio of carbon to refractory transition metal oxides have great potential for achieving low temperature synthesis, promoted sintering and tunable properties. [ABSTRACT FROM AUTHOR]