Electret, piezoelectret, piezoresistivity and stress-dependent electric permittivity are reported in unmodified steels. Structural stress/strain self-sensing based on piezoelectret/piezoresistivity is demonstrated under tension. Structural self-powering is shown by the electret's inherent electric field, (2.224 × 10−5) V m−1 and (1.051 × 10−5) V m−1, and power density, 29.2 and 41.8 W m−3, for low carbon steel and stainless steel, respectively, being enabled by the electret's electrical conductivity. The free-electron-movement-enabled electrets are supported by the asymmetry in the polarization-induced apparent resistance relative to the true resistance upon polarity reversal. The electric field increases linearly with the inter-electrode distance l. An l increase causes the amount of participating free electrons to increase and the fraction of free electrons that participate to decrease; when l is tripled, the amount is increased by a factor of 1.011 and 1.021 for low carbon steel and stainless steel, respectively, while the fraction of free electrons that participate is decreased by a corresponding factor of 0.337 and 0.340. The higher values for stainless steel are consistent with the higher relative permittivity (2 kHz), 1.23 × 106 and 2.89 × 106 for low carbon steel and stainless steel, respectively. The capacitance (2 kHz) and electric field (DC) of the piezoelectret decrease nonlinearly with increasing stress, due to electret weakening; the decrease is reversible at stress ≤210 MPa, but is irreversible at stress ≤340 MPa (elastic regime). This effect is stronger for low carbon steel than stainless steel. The piezoelectret coupling coefficient d33 is −(6.6 ± 0.1) × 10−7 and −(3.6 ± 0.2) × 10−7 pC N−1 for low carbon steel and stainless steel, respectively. The relative permittivity (2 kHz) decreases nonlinearly by ≤14% with stress ≤340 MPa. The piezoresistivity involves the DC resistivity decreasing nonlinearly and reversibly by ≤10% with stress ≤340 MPa; the gage factor is −1030 and −800 for low carbon steel and stainless steel, respectively. The reversibility upon unloading is superior for piezoresistivity than piezoelectret.
Pyropermittivity, pyroresistivity, and pyroelectret refer to the effect of temperature on permittivity, resistivity, and inherent electric field. They are emerging thermoanalytical methods that are relevant to capacitance-based, resistance-based, voltage-based temperature self-sensing, and thermal energy harvesting. This work provides the first determination of temperature coefficients of permittivity (associated with AC polarization), resistivity (associated with DC conduction), and inherent electric field (associated with DC polarization) for carbon-carbon composites (C/C). The coefficients are -(5.1±0.2)×10 -3 K -1 , (2.0±0.1)×10 -3 K -1 , and (0.43±0.02) K -1 in terms of resistivity, permittivity, and inherent electric field, respectively. The highest coefficient for the inherent electric field means DC polarization is more sensitive to temperature. The activation energy required for the inherent electric field ((0.25±0.01) eV) is significantly higher than for resistivity ((56.6±1.6) meV) and permittivity ((21.4±0.6) meV), suggesting that the formation of electrets through carrier-atom interaction is more challenging during heating than the interaction under AC conditions and carrier drift under DC conditions. Mild heating increases the volumetric power density (by 600 times), gravimetric power density (by 600 times), and current density (by 28 times) significantly. The pyropermittivity-based energy density is (4.96±0.15) × 10 -6 J/m 3 when the temperature increases from 20 °C to 70 °C. The temperature self-sensing and self-powering behavior make C/C a highly promising smart material.
To address the issues of low strength, poor water stability, and hazardous substance leaching associated with using phosphogypsum (PG) as a direct road-based material, the traditional approach involves employing inorganic cementing materials to stabilize PG, effectively addressing the problems. This study innovatively utilizes the xanthan gum (XG) and sodium methylsiliconate (SM) as curing agents for PG to solve the above problems. An organic curing agent stabilized PG was prepared by dry mixing XG and PG. The unconfined compressive strength, water stability, and leaching behavior of stabilized PG were investigated, the leaching behavior was characterized by ion leaching concentration, and the mechanisms behind the strength development of stabilized PG were explored by SEM and FTIR. The experimental results indicate that the single incorporation of XG reduced the strength and water stability of stabilized PG, while the single incorporation of SM had a limited effect on strength and water stability. In addition, the dual incorporation of XG and SM significantly improved the strength and water stability of stabilized PG. At the same time, the dual incorporation of XG and SM greatly reduced the leaching of hazardous substances from stabilized PG. These results demonstrate the feasibility of using stabilized PG for road base materials.
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