Investigations on the effects of microwaves on hard rock

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This study investigates the response of various types of hard rock to microwave irradiation. Two basic laboratory setups have been implemented and the consequences of microwave irradiation on the behaviour of rock samples have been analysed. Small-scale low-power microwave irradiation was performed with a 3.2 kW standard household microwave. Large-scale experiments were performed with a self-designed high-power microwave apparatus operating at a maximum output power of 30 kW. Irradiation times vary from a few seconds to minutes. With the low-power apparatus only rocks with relatively good microwave absorption properties (basalt, gabbro) could be sufficiently heated and significantly damaged, represented by a reduction in sound wave velocity from 5000 to 3500 m/s after 120 s of irradiation. The influence of the sample geometry is obvious from the crack pattern. Evidence for consequences of differential heating, as proposed in various literature sources, could not be found. This can presumably only be achieved by short irradiation with higher power than applied in the present thesis. The occurrence of small amounts of water (1-2 wt.%) in porous rocks has a positive influence on the destruction of rock by building up vapour pressure leading to stresses in the rock exceeding its tensile strength. High-power microwave irradiation with up to 30 kW leads to heating and cracking of all investigated rock types. The different rock types (fine grained strong absorbers, coarse grained strong and weak absorbers, water bearing) show completely different effects of damage. Rocks with strong microwave absorption (greenstones) heat rapidly (ΔT = 270 °C in 6 s with 25 kW) with coinciding fast generation of temperature gradients and spallation of craters at the surface. Here cracks are not aligned to grain boundaries. In weakly absorbing rocks (sandstone, granite) temperature rises are less distinctive (ΔT = 240 °C in 30 s with 25 kW for granite) radial cracks develop originating in the centre of irradiation. Cracks are largely bound to grain boundaries or cleavage planes of single minerals but are assumed not to originate in intergranular temperature stresses but in the global stresses induced by the stronger heating of the centre of the irradiated volume. Small amounts of water in the porous volume of sandstone play a major role in high power irradiation as well. Excellent microwave absorption by water leads to fast generation of vapour pressure and explosive generation of craters with up to 6 cm of depth. An attempt was made to quantify damage induced by microwave irradiation by a linear cutting test-rig. Despite the observation of a trend towards a reduction of cutting forces after microwave irradiation the large scattering of results does not allow for a reliable conclusion.
Translated title of the contributionUntersuchungen zu den Auswirkungen von Mikrowellenbestrahlung auf Hartgesteine
Original languageEnglish
  • Moser, Peter, Assessor A (internal)
  • Kuchar, Friedemar, Assessor B (internal)
Publication statusPublished - 2013

Bibliographical note

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  • microwave
  • hard rock
  • alternative rock cutting
  • microwave disintegration

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