R. Hauser

Applicability of LiNbO/sub 3/, langasite and GaPO/sub 4/ in high temperature SAW sensors operating at radio frequencies

The applicability of LiNbO/sub 3/, langasite and GaPO/sub 4/ for use as crystal substrates in high temperature surface acoustic wave (SAW) sensors operating at radio frequencies was investigated. Material properties were determined by the use of SAW test devices processed with conventional lithography. On GaPO/sub 4/, predominantly surface defects limit the accessible frequencies to values of 1 GHz. Langasite SAW devices could be operated up to 3 GHz; however, high acoustic losses of 20 dB//spl mu/S were observed. On LiNbO/sub 3/, the acoustic losses measured up to 3.5 GHz are one order of magnitude less. Hence, SAW sensors capable of wireless interrogation were designed and processed on YZ-cut LiNbO/sub 3/. The devices could be successfully operated in the industrial-scientific-medical (ISM) band from 2.40 to 2.4835 GHz up to 400/spl deg/C.

Wireless SAW based high-temperature measurement systems

The paper exemplifies the development of a surface acoustic wave (SAW) temperature measurement system, showing the advantages of wireless data transmission and passive sensor operation. It includes results of research on lithium niobate, langasite and gallium-orthophosphate, three piezoelectrica used as substrate crystals for the SAW devices. Critical parameters, limits and prospects of the materials as well as technological issues concerning the metallization and the lifetime of reflective SAW delay lines are discussed. The packaging and assembly of high temperature (HT) stable transponders is explained. A slot antenna, a patch antenna and a dipole antenna, thus providing maximum flexibility for the sensor geometry, are investigated. A phase matching algorithm, enhancing the measurement resolution by two orders of magnitude compared to a conventional peak detection algorithm, is described. A fully integrated SAW temperature sensor system is presented, basing on a frequency modulated radar operating in the 2.45 GHz ISM band and designed for a world wide legal usage

Properties of radio frequency Rayleigh waves on langasite at elevated temperatures

The properties of Rayleigh waves operating both at radio frequency (RF) and at high temperatures (HT) on langasite crystal cuts with Euler angles of (0/spl deg/, 138.5/spl deg/, 26.6/spl deg/) and (0/spl deg/, 30.1/spl deg/, 26.6/spl deg/) are investigated. By evaluating the frequency response of surface acoustic wave (SAW) test devices the SAW velocity, the coupling factor, and the propagation attenuation have been determined experimentally in a temperature range from 20/spl deg/C to 500/spl deg/C. The SAW devices could be operated up to frequencies of 3 GHz. However, considering a future sensor application a limiting factor is a strong increase of the acoustic losses in conjunction with frequency and temperature.

Gd substitutions in the TmCo2 Laves phase: the onset of long-range magnetic order in the itinerant subsystem

We found that in TmCo2 the molecular field does not exceed the critical value necessary to induce long-range magnetic order in the Co d-electron subsystem, which is in contrast to the other heavy RCo2 compounds. Below Tc=3.8 K a first-order magnetic phase transition at 3.4 K exists, which is due to a rearrangement of the Tm 4f magnetic moments. When substituting Tm by Gd long-range magnetic order appears in the Co d subsystem. The onset of the magnetic order in the d-electron subsystem can most clearly be seen from thermal expansion measurements. There is an increasing positive volume anomaly with increasing Gd content below Tc. The estimated magnetostriction constant lambda 111 of TmCo2 is -4.1*10-3, in agreement with the single-ion model. Pronounced discontinuities at Tc are characteristic for the transport properties (resistivity and thermopower) for TmCo2 as well as for the pseudobinary Tm1-xGdxCo2 compounds. At elevated temperatures the physical properties of all these compounds are dominated by spin fluctuations.

CePd2Ga3: A new ferromagnetic Kondo lattice

Various temperature-, pressure- and field dependent investigations on CePd2Ga3 indicate this ternary compound as belonging to the group of ferromagnetically ordered Kondo lattices, with the Curie temperatureTC=6K and the Kondo temperatureTK=4K. The first excited crystal field level of this hexagonal compound is about 40 K above the crystal field ground state, while the overall splitting is much larger.

A wireless SAW-based temperature sensor for harsh environment

A SAW-based temperature sensor system is presented. Much effort was placed in the development of a high temperature resistive transponder compromising of both a SAW device and an antenna stable up to 400 /spl deg/C for at least a few days. The system solution also includes a low-cost reader unit employing the FSCW (frequency stepped continuous wave) principle operating in the ISM band from 2.4 GHz to 2.4835 GHz. Spare space on the SAW device has been employed also to implement an ID for identification purpose. Hence, this solution also allows for tagging of objects exposed to harsh environments.

SAW strain sensitivity of selected rayleigh wave crystal cuts

ST-quartz is commonly used for pressure sensor applications. Drawbacks are its frequency limitations and poor coupling coefficient. In particular for medical applications, where the temperature range is limited and the antenna size is critical, good signal coupling and higher frequencies may trump the temperature effects. This work compares experimental data on the sensitivity to mechanical loads of langasite 138.5°, lithiumtantalate 112.2° and ST-quartz. The measurements were performed on a dedicated test stand, using differential delay lines. The impulse responses versus varying mechanical loads were measured. The strain and stress distributions in the SAW elements were calculated from the measured mechanical loads using advanced FEM methods considering anisotropic elasticity. Sensitivity values to stress and strain have been deduced for the investigated substrates.

High temperature stable SAW based tagging system for identifying a pressure sensor

Driven by the need for identifying an AVL pressure sensor inside of a combustion engine, various methods for this purpose have been examined. The solution found has to be compatible with the constraints placed by both the harsh environmental conditions acting on a sensor mounted close to the combustion chamber and the reading technique employed to monitor the data. Thus the final solution, preferably mounted inside of the pressure sensor, has to withstand shock, vibrations, high temperatures and temperature gradients, to provide a high isolation resistance and to fit into small size bore. Furthermore, the solution has to cope with additional demands such as low-costs or minor modifications of the sensor only.

Ceramic patch antenna for high temperature applications

For a high-temperature stable SAW device, serving as both an RFID-tag and a temperature sensor, a hermetic case has to be provided to protect the SAW device from moisture, contaminations and harsh environments. Furthermore, for wireless readout a high-temperature stable antenna has to be connected to the SAW device. In order to fulfil both purposes adequately an alumina based ceramic package was designed accordingly and successfully tested up to 400degC