Polycrystalline black phosphorus has been produced by the Bridgman method of combined temperature and pressure and found to be p type with room-temperature resistivity around one Ω-cm, in agreement with the observations of previous workers. From 300° to 700°K, the resistivity can be fitted by an expression ρav=4.6×10−3 exp(0.35/2kT) Ω-cm, and the Hall data by (ReT32)−1 = constant exp(−0.34/2kT). The product Rσ can be represented in this temperature range by (Rσ)av = 9×105T−1.4 cm2/V-sec. The deviation from this law toward lower mobility observed by Keyes from 250° to 350°K does not occur in these samples. The resistivity continues to rise even near liquid-helium temperature, and maxima in the Hall coefficient occur between 24° and 30°K. The low-temperature Rσ products also exhibit maxima, and the magnitude of this product is small near liquid-helium temperature. Although these characteristics are consistent with two-band conduction, the relative appearance of the two sets of maxima is not consistent with a prediction of the simplest two-band theory. An optical absorption edge is found in the 2- to 6-μ region and its temperature dependence yields dW/dT = 2.8×10−4 eV/deg. A sharp absorption peak observed near 21 μ moves to longer wavelengths with increasing temperature at a rate dW/dT = −2×10−6 eV/deg. At helium temperature the ac photoconductive response is substantially flat from one to 20 μ. At nitrogen temperature the response is peaked between 4 and 5 μ, while at room temperature a peak is observed in the same wave-length range but the sensitivity is considerably less.

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Polycrystalline black phosphorus has a hydrostatic piezoresistive coefficient πH=3.04×10−10 cm2/dyn independent of pressure and decreasing by 28% between 0° and 100°C, and a highly stress-sensitive uniaxial piezoresistive stress coefficient πl>3×10−9cm2/dyn under appropriate conditions. The thermoelectric power is in the range 330 to 413 μV/deg, and Young's modulus=6.89×1010±15% dyn/cm2, linear to at least 4×106 dyn/cm2 tension. The ultimate tensile strength is 3.4×107 dyn/cm2 and the ultimate compressive strength is 3.5×108 dyn/cm2. These properties of black phosphorus make it potentially useful for strain and hydrostatic gauges, and other transducer use. The advantages and disadvantages relative to other materials are discussed briefly.