Turbostratic boron nitride (t-BN) powder prepared by mechanical grinding of hexagonal boron nitride (h-BN) under the nitrogen atmosphere with the oxygen concentration less than 1 ppm was consolidated by spark plasma sintering (SPS). The turbostratic structure was maintained after the consolidation, which was characterized by XRD, Raman and IR spectra. The t-BN consolidated without any additives at 1900°C for 10 min under applied pressure of 100 MPa showed a bulk density of 95%. The bending strength, Young's modulus and Shore hardness of the consolidated t-BN were 72 MPa, 259 GPa and 117 Hs, respectively. They were much higher than those of conventionally consolidated h-BN. On the other hand, the thermal conductivity was 3 W/mK and much lower than the compact of h-BN.

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Although oxygen therapy rapidly improves arterial oxygen saturation in idiopathic pulmonary arterial hypertension, the effects of chronic administration of oxygen are unknown. The purpose of the present study was to investigate the effects of chronic oxygen therapy on the histological changes and survival rate in rats with idiopathic pulmonary arterial hypertension. Idiopathic pulmonary arterial hypertension was induced by monocrotaline injection. The rats were then randomly assigned to receive or not receive oxygen therapy (O2 group and non-O2 group, respectively). The rats in the O2 group were exposed to a high (90%) oxygen environment from day 17 following injection of monocrotaline, when hypoxemia was first observed. The pulmonary arteriole walls were significantly thicker in monocrotaline-injected rats than in saline-injected rats as vehicle on day 19 and were significantly thicker in the rats that received oxygen therapy than in the rats that did not. Right ventricular inflammations were significantly higher in monocrotaline-injected rats than in saline-injected rats on day 19 and were significantly higher in the rats that received oxygen therapy than in the rats that did not. By day 20 after injection of monocrotaline, the survival rate was significantly lower in the rats that received oxygen therapy than in those that did not. Superoxide dismutase activity in the lungs was higher in monocrotaline-injected rats than in saline-injected rats on day 19 after monocrotaline injection and was also higher in the saline-injected rats that received oxygen therapy than in the saline-injected rats that did not. No interaction was detected between monocrotaline injection and oxygen therapy. These results suggest that chronic oxygen therapy worsens the histological changes and survival rate in idiopathic pulmonary arterial hypertension. The fact that degradation of the histological changes and survival rate was accompanied by increase in superoxide dismutase activity suggests that antioxidant capacity may contribute to the degradation.

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