Vibration in electromagnetically heated steel

When a ferromagnetic steel billet was heated by induction a large increase in the amplitude of longitudinal vibration frequently occurred as a result of resonance. This happened when a natural frequency of the bar coincided with twice the heating frequency or multiples thereof. The temperature at which resonance occurred depended on a number of factors including billet length and heating power. Resonance was most often observed when the surface temperature of the billet reached the Curie point. It is well established that magnetostrictive vibrations occur in a ferromagnetic material subjected to an alternating electromagnetic field, but existing data suggests that linear magnetostriction decreases towards the Curie point. Linear magnetostriction was measured in a sample of mild steel up to 800ºC using a high temperature strain gauge. The magnetostriction constant 100 was calculated assuming an average grain orientation in mild steel. The data was found to be comparable to that published for single crystals of iron. It was discovered that linear magnetostriction was responsible for resonance below 600ºC but not for temperatures near the Curie point. Other possible causes of resonance such as forces produced by the interaction between eddy currents and the alternating electromagnetic field, the alpha to gamma phase transformation and the existence of a thin ferromagnetic layer were investigated. None were found to account for resonance in bars of mild steel heated by induction. Experimental work relating to the induction heating of steel is compared to previous work on the subject of electromagnetic generation of ultrasound where a similar increase of the amplitude of longitudinal waves in steel is reported at the Curie point. It is concluded that the two phenomena are related as they show strong similarities.

The effect of barium substitution on the ferroelectric properties of Sr2 Nb2 O7 Ceramics

This work revealed that the solid solution compounds of Sr 2-xBaxNb2O7 are promising lead-free materials for high-temperature piezoelectric sensor application. These compounds were confirmed as ferroelectric materials with high Curie points (> 900°C) by their piezoelectric activity after poling, ferroelectric domain switching in their P-E hysteresis loops and thermal depoling behavior. The effect of Ba substitution on the structure and properties of Sr 2-xBaxNb2O7 (x < 1.0) was investigated. The solid solution limit of Sr2-xBaxNb 2O7 was determined by XRD as x < 0.6. The a-, b-, c- axes, and cell volume increase with Ba addition. The textured ceramics of Sr2-xBaxNb2O7 were prepared for the first time. The highest d33 was measured as 3.6 ± 0.1 pC/N for Sr1.8Ba0.2Nb2O7. © 2012 The American Ceramic Society.

Piezoelectric and dielectric properties of Ce substituted La2Ti2O7 ceramics

The ferroelectric and dielectric properties of cerium (Ce) substituted La2Ti2O7 (LTO) have been investigated. Single phase, dense La2-xCexTi2O7 (x=0.15, 0.25, 0.35) ceramics were prepared by spark plasma sintering. The solubility limit of Ce in La2-xCexTi2O7 was found to be between 0.35 and 0.5. The a-, b- and c-axes of the unit cell decrease with increasing Ce substitution. The Curie point (Tc) of La2-xCexTi2O7 (x=0, 0.15, 0.25, 0.35) decreases and dielectric constant and loss increase with increasing Ce substitution. Cerium can increase the d33 of La2Ti2O7. The highest d33 was 3.9±0.1pC/N for La1.85Ce0.15Ti2O7 textured ceramic. © 2012 Elsevier Ltd.