Piezoresistivity is an electromechanical effect characterized by the reversible change in the electrical resistivity with strain. It is useful for electrical-resistance-based strain/stress sensing. ...The resistivity can be the volumetric, interfacial or surface resistivity, though the volumetric resistivity is most meaningful scientifically. Because the irreversible resistivity change (due to damage or an irreversible microstructural change) adds to the reversible change that occurs at lower strains, the inclusion of the irreversible effect makes the piezoresistivity appear stronger than the inherent effect. This paper focuses on the inherent piezoresistivity that occurs without irreversible resistivity changes. The effect is described by the gage factor (GF), which is defined as the fractional change in resistance per unit strain. The GF can be positive or negative. Strong piezoresistivity involves the magnitude of the fractional change in resistivity much exceeding the strain magnitude. The reversible effect of strain on the electrical connectivity is the primary piezoresistivity mechanism. Giant piezoresistivity is characterized by GF ≥ 500. This critical review with 209 references covers the theory, mechanisms, methodology and status of piezoresistivity, and provides the first review of the emerging field of giant piezoresistivity. Piezoresistivity is exhibited by electrically conductive materials, particularly metals, carbons and composite materials with conductive fillers and nonconductive matrices. They include functional and structural materials. Piezoresistivity enables structural materials to be self-sensing. Unfortunately, GF was incorrectly or unreliably reported in a substantial fraction of the publications, due to the pitfalls systematically presented here. The most common pitfall involves using the two-probe method for the resistance measurement.
review of exfoliated graphite Chung, D. D. L
Journal of materials science,
01/2016, Letnik:
51, Številka:
1
Journal Article
Recenzirano
Exfoliated graphite (EG) refers to graphite that has a degree of separation of a substantial portion of the carbon layers in the graphite. Graphite nanoplatelet (GNP) is commonly prepared by ...mechanical agitation of EG. The EG exhibits clinginess, due to its cellular structure, but GNP does not. The clinginess allows the formation of EG compacts and flexible graphite sheet without a binder. The exfoliation typically involves intercalation, followed by heating. Upon heating, the intercalate vaporizes and/or decomposes into smaller molecules, thus causing expansion and cell formation. The sliding of the carbon layers relative to one another enables the cell wall to stretch. The exfoliation process is accompanied by intercalate desorption, so that only a small portion of the intercalate remains after exfoliation. The most widely used intercalate is sulfuric acid. The higher concentration of residue in unwashed EG causes the relative dielectric constant (50 Hz) of the EG to be 360 (higher than 120 for KOH-activated GNP), compared to the value of 38 for the water-washed case. An EG compact is obtained by the compression of EG at a pressure lower than that used for the fabrication of flexible graphite. Compared to flexible graphite, EG compacts are mechanically weak, but they exhibit viscous character, out-of-plane electrical/thermal conductivity and liquid permeability. The viscous character (flexural loss tangent up to 35 for the solid part of the compact) stems from the sliding of the carbon layers relative to one another, with the ease of the sliding enhanced by the exfoliation process.
The thermal interface materials (TIMs) used for improving thermal contacts are considered in terms of the performance, performance consideration criteria, performance evaluation methods, and material ...development approaches. The performance is described mainly by the thermal contact conductance, which refers to the conductance across the thermal contact surfaces that sandwiches the TIM. This conductance depends on the conformability, thermal conductivity, and small‐thickness feasibility. However, the vast majority of published work does not consider this conductance, but only the thermal conductivity within the TIM. The highest TIM performance is exhibited by the thermal pastes and low‐melting alloys.
This perspective provides an overview of the performance of thermal interface materials (TIMs), with emphasis on the TIM performance criteria and the top‐performing TIMs. The performance is indicated by the thermal contact conductance rather than the thermal conductivity, due to the critical importance of the conformability in affecting TIM performance. The thermal resistance of the TIM‐contact interface is substantial.
This paper reviews carbon materials for significant emerging applications that relate to structural self-sensing (a structural material sensing its own condition), electromagnetic interference ...shielding (blocking radio wave) and thermal interfacing (improving thermal contacts by using thermal interface materials). These applications pertain to electronics, lighting (light emitting diodes), communication, security, aircraft, spacecraft and civil infrastructure. High-performance and cost-effective materials in various forms of carbon have been developed for these applications. The forms of carbon materials include carbon fiber, carbon nanofiber, exfoliated graphite, carbon black and composite materials. Short carbon fiber cement-matrix composites and continuous carbon fiber polymer-matrix composites are particularly effective for structural self-sensing, with the attributes sensed including strain, stress, damage and temperature. Flexible graphite as a monolithic material and nickel-coated carbon nanofiber as a filler are particularly effective for electromagnetic shielding. Carbon black paste, graphite nanoplatelet paste and flexible graphite (filled with carbon black paste) are particularly effective for thermal interfacing; carbon nanotube arrays are less effective than these pastes. The associated science pertains to the relationship among processing, structure and properties in relation to the abovementioned applications. The criteria behind the design of materials for these applications and the mechanisms of the associated phenomena are also addressed.
Absorption-dominant radio-wave (0.2–2.0 GHz) attenuation loss is comparatively reported for materials of high electrical conductivity, namely metals (aluminum and steel) and graphite. These materials ...exhibit similarly high absorption loss (≤ 91.5%) and similarly low reflection loss (≥ 8.5%), both as fractions of the total loss in dB. The absorption loss is high (< 55 dB) and the reflection loss is low (< 10 dB) for both graphite and the metals. The absorption-dominant attenuation loss of these high-conductivity materials is in contrast to the notion that high conductivity (due to the high impedance mismatch with air) generally causes reflection-dominant attenuation loss. The metals and graphite are in foil form, with the graphite being thicker than the metals. The linear absorption coefficient (directly related to the absorption loss per unit thickness) is lower for graphite (≤ 93 mm
−1
) than the metals (≤ 394 mm
−1
), due to the greater thickness of the graphite. The absorption loss and fractional absorption loss contribution increase with increasing frequency, whereas the reflection loss decreases, as consistent with electromagnetic theory. On the other hand, from the viewpoint of the fractional loss in power, reflection dominates over absorption for all three materials in the entire frequency range. For shielding, the metals are more effective than graphite if the absorption loss per unit thickness (< 3400 dB/mm) is considered. For stealth, graphite is advantageous to the metals in the low reflection loss, though it is disadvantageous in the low absorption loss per unit thickness.
This is a non-exhaustive but comprehensive review of materials for electromagnetic interference (EMI) shielding. It covers functional and multifunctional structural shielding materials. The materials ...include metals, carbons, ceramics, cement, polymers, hybrids and composites. Metals and carbons are the main functional materials. Ceramics, cement and polymers are typically not very effective, unless they are combined with a functional material. Due to the availability of numerous types of microcarbons and nanocarbons, shielding materials in the form of metal-carbon, ceramic-carbon, cement-carbon and polymer-carbon combinations have received much attention. Continuous carbon fiber composites and cement-based materials are dominant among structural shielding materials. The principles of shielding materials design are covered, with consideration of the science base and material structure. The common pitfalls in shielding materials research are also addressed.
•This is a review of materials for electromagnetic interference (EMI) shielding.•It covers functional and multifunctional structural shielding materials.•The materials include metals, carbons, ceramics, cement, polymers and hybrids.•Metals and carbons are the main functional materials.•The common pitfalls in shielding materials research are also addressed.
Through comparing carbons in the graphite allotrope family, namely isotropic graphite (IG), exfoliated-graphite-based flexible graphite (FG), graphite-flake thick film (not sintered), carbon fibers ...(PAN, pitch) and highly-oriented pyrolytic graphite (HOPG), all differing in the microstructure, this work addresses for the first time the factors (including the structure-property relationship) that govern the electric permittivity (kHz frequency) of carbons. The permittivity is more sensitive to the microstructural heterogeneities than the resistivity, which only slightly influences the permittivity. Smaller crystallite dimensions promote the permittivity. PAN-based carbon fibers exhibit higher permittivity than pitch-based carbon fibers, IG, FG or HOPG. The crystallographic preferred orientation is not influential to the permittivity, though it influences the resistivity. Interfaces (e.g., the grain boundaries in IG and the interface between the carbon layer ribbons in a fiber) promote the permittivity, being sites for the carrier-atom interaction that enables the polarization. A large number of sites is more important than a large site size for promoting the permittivity. The fiber diameter is not influential to both permittivity and resistivity. The thick film exhibits the highest relative permittivity (5 × 105), due to its low electrical connectivity. IG (grain size 12 μm) gives the lowest relative permittivity (5 × 102), due to its coarse microstructure.
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The absorption of radio wave is relevant to electromagnetic interference (EMI) shielding and low observability. The effect of the absorption distance much above the calculated skin depth on the ...absorption of radio wave (600–2000 MHz) is studied by determining the absorption loss per unit thickness for aluminum sheets of thickness ranging from 0.528 to 2.053 mm. Aluminum is dominantly used for EMI shielding. The absorption loss
SE
A
increases with increasing thickness, such that when the absorption distance
x
exceeds 1.3 mm, the increase is slight. The distance 1.3 mm corresponds to ~ 500 times the calculated skin depth for
x
= 0. Furthermore, when
x
exceeds 1.3 mm, the linear absorption coefficient
α
obtained from
SE
A
/thickness decreases linearly with increasing
x
. At
x
below 1.3 mm,
α
decreases with increasing
x
in a nonlinear non-exponential manner that is not far from linearity and corresponds to much less decrease for the same
x
than the exponential case. The non-exponential relationship indicates that the skin effect alone cannot explain the observed relationship. The observed high
α
at large
x
, at which the electric field is low, indicates nonlinear dielectric behavior. The factor
β
that relates
α
2
/
α
1
and
x
1
/
x
2
(where
α
1
is the
α
value at
x
1
, and
α
2
is the
α
value at
x
2
) increases with
x
and levels off at ~ 1.5 when
x
exceeds 1.3 mm. The effect of
x
on
α
is large compared to the effect of the frequency on
α
.
Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of ...their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr3 crystals using momentum-resolved neutron and X-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional sheets of correlated rotations in real space, and could represent precursors to proposed two-dimensional polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimization of their optical and thermal properties.Neutron and X-ray scattering measurements provide further insight into the anharmonic behaviour of lead halide perovskites, revealing that rotations of PbBr6 octahedra in CsPbBr3 crystals occur in a correlated fashion along two-dimensional planes.
Pyropermittivity refers to the effect of temperature on the electric permittivity of a material. It is an emerging thermoanalytical method that is relevant to materials characterization, ...capacitance-based temperature sensing and thermal energy harvesting. Pyropermittivity is well-known for electrically nonconductive materials, but not conductive materials, which include structural materials (e.g., carbon fibers). This work provides the first determination of the activation energy of permittivity and pyropermittivity-based energy density for any material, and that of the temperature coefficient of permittivity (associated carrier-atom interaction) for conductive materials. Pyropermittivity is discovered in carbon fibers. The permittivity temperature coefficient is positive for uncoated/nickel-coated carbon fibers and polycrystalline graphite of prior work, despite the difference in sign of the temperature coefficient of resistivity between the fibers and graphite. The pyropermittivity is stronger when the fiber is nickel-coated, but is yet stronger for graphite. The coefficient values are all higher than or comparable to those previously reported for nonconductive materials, indicating that nonconductivity does not necessarily enhance pyropermittivity. The activation energy of permittivity is ≤ 40 meV. The pyropermittivity-based volumetric energy density is higher when the carbon fiber is nickel-coated, and is lowest for the graphite. The greater energy density when the carbon fiber is nickel-coated is in line with the greater temperature coefficient of permittivity and the higher permittivity. The low energy density of graphite relates to the low permittivity. The highest energy density obtained is 1.73 × 10
–4
J m
−3
(nickel-coated carbon fiber for the temperature change of 50 °C); the temperature coefficient of permittivity is 1.37 × 10
–3
/K for this fiber.
