| Alpha-Fe2O3 Ceramics as Negative Temperature Coefficient Thermistors |
93 |
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| Authors:
J. Certo; C. S. Furtado; A. R. Ferreira; J. M. Perdigao; COIMBRA UNIV (PORTUGAL) CENTRO DE ELECTROTECNIA
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 | The electrical behavior of negative temperature coefficient (NTC) thermistors can be expressed by the relationship pi = pi(sub 25)*expBETA(1/T-1/ 298). Good thermistors are thus obtained through the adequate control of the parameters beta and pi(sub 25), which has been achieved in alpha-Fe2O3 ceramics by a convenient processing. Reducing in nitrogen and then reoxidizing in air has lead to thermistors with good values for beta and reasonable ones for pi(sub 25). ... |
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| Development of an Acoustical Visualization System for Non-Destructive Testing of Composites |
92 |
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| Authors:
J. B. Santos; J. M. Perdigao; A. R. Ferreira
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| In order to characterize composite materials, we have developed an array of ultrasonic transducers and its electronic control such as the excitation system that incorporate the delays implementation and pulse generation, and the reception system where we have basically a analogic-to- digital (ADC) converter in each line, an amplification and filtering module and memories to save the signals coming from A/D converters for later processing. (Author). |
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| A Nomogram for Ultrasonic Wave Velocity Determination |
DEC 91 |
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| Authors:
J. M. Perdigao; J. B. Santos; A. R. Ferreira
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| For some kind of scientists, namely mechanical ones, it is difficult to find the ultrasonic wave velocity in different media. However, the mechanical properties of these are often well known. In this paper we represent a nomogram, based on two families of circles representing mechanical properties, for determination of the ultrasonic wave velocity in different media. It is easy to use and has general application. (Author). |
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| gaussian model for evaluating the coherent and incoherent scattering of ultrasonic waves in non-homogeneous media |
DEC 89 |
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| Authors:
J. M. Perdigao; A. R. Ferreira; J. B. Santos
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| the phenomena occurring in ultrasonic wave propagation in nonhomogeneous media may be described using simple models derived from theoretical and experimental studies. more precisely, two kinds of waves can be evidenced and used in the establishment of theoretical models: the coherent wave and the incoherent one. the latter presenting a phase and amplitude strongly dependent on random distribution of heterogeneities. the relative levels of the coherent and the incoherent waves ... |
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