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Co-firing of low temperature co-fired ceramics (LTCC) and functional ceramics like ferrites is a promising approach to increase the level of integration in future microsystems, and to create new applications for LTCC technology. Besides the development of compatible material combinations, the configuration of the sintering process is an important issue for successful co-firing. A method is presented to derive the linear shrinkage mismatch of a material combination based on density data calculated from the master sintering curves (MSCs) of the individual materials. The influence of the firing profile on the constraint in the combined multilayer can be anticipated using this method. To investigate and improve the co-firing of ferrite and dielectric tape, the shrinkage mismatch with respect to heating rate was studied. A significant reduction of shrinkage mismatch was found for increased heating rates. The calculated results are verified by lateral shrinkage measurements on combined laminates.
Recent advances in the development of high gauge factor thin films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion (9.4 ppm/K) and a low modulus of elasticity (82 GPa) for the application as support material for thin-film sensors. In the first part, constantan foil strain gauges were fabricated from this material by tape casting, pressure-assisted sintering, and subsequent lamination of the metal foil on the planar ceramic substrates. The accuracy of the assembled load cells corresponds to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the low-temperature co-fired ceramic (LTCC) substrates for fabrication of accurate strain gauges. In the second part, to facilitate the deposition of thin-film sensor structures to the LTCC substrates, pressure-assisted sintering step is modified using smooth setters instead of release tapes, which resulted in fabrication of substrates with low average surface roughness of 50 nm. Titanium thin films deposited on these substrates as test coatings exhibited low surface resistances of 850 Ω comparable to thin films on commercial alumina thin-film substrates with 920 Ω. The presented material design and advances in manufacturing technology are important to promote the development of high-performance thin-film strain gauges.
Recent advances in the development of high gauge factor thin-films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion and a low modulus of elasticity for the application as support material for thin-film sensors. Constantan foil strain gauges were fabricated from this material by tape casting, pressure-assisted sintering and subsequent lamination of the metal foil on the planar ceramic substrates. The sensors were mounted on a strain gauge beam arrangement and load curves and creep behavior were evaluated. The accuracy of the assembled load cells correspond to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the LTCC substrates for fabrication of accurate strain gauges. To facilitate the deposition of thin film sensor structures onto the LTCC substrates, the pressure-assisted sintering technology has been refined. By the use of smooth setters instead of release tapes substrates with minimal surface roughness were fabricated. Metallic thin films deposited on these substrates exhibit low surface resistances comparable to thin films on commercial alumina thin-film substrates. The presented advances in material design and manufacturing technology are important to promote the development of high performance thin-film strain gauges.
Roughening of zirconia dental implants is a common clinical practice to improve ingrowth behavior. It depends on the manufacturer of the implant at which stage of the manufacturing process and by which method the surface is roughened. Systematic studies on this topic are rarely found in the literature. Therefore, the influence of surface treatment on the strength of a dental zirconia was investigated as part of a research project on the development of ceramic implants. The material under test was a commercial zirconia consisting of a Y-TZP matrix and Ce-TZP inclusions in the sintered state. This material is characterized by a slightly higher fracture toughness and slightly reduced strength compared to typical 3Y-TZP. Sets of samples were sandblasted in the white-fired or sintered condition. The ball-on-three-ball-strength of these samples was measured and compared to the strength of as-fired samples and polished samples. The complete study was performed two times for validation of the results. It is found that the average strength of TZP ceramics differs by almost 500 MPa depending on the surface treatment. Conventionally sintered specimens with as-fired surface exhibit a strength of 880 MPa. Sandblasting in the white fired state reduces the strength to 690 MPa. Both polishing and sandblasting in the sintered condition result in an increase in strength to about 1180 MPa. Comparative microstructural investigations, roughness measurements and X-ray phase analyses were carried out to determine the causes of these huge differences in strength. These findings may challenge the practice of white body surface treatment and give reason for further investigations on other commercial dental TZP materials.
Glass-ceramic composites as insulation material for thermoelectric oxide multilayer generators
(2021)
Thermoelectric generators can be used as energy harvesters for sensor applications.
Adapting the ceramic multilayer technology, their production can be
highly automated. In such multilayer thermoelectric generators, the electrical insulation material, which separates the thermoelectric legs, is crucial for the performance of the device. The insulationmaterial should be adapted to the thermoelectric regarding its averaged coefficient of thermal expansion α and its sintering temperature while maintaining a high resistivity.
In this study, starting from theoretical calculations, a glass-ceramic Composite material adapted for multilayer generators fromcalciummanganate and Calcium cobaltite is developed. The material is optimized towards an α of 11 × 10−6 K−1 (20–500◦C), a sintering temperature of 900◦C, and a high resistivity up to 800◦C.
Calculated and measured α are in good agreement. The chosen glass-ceramic composite with 45 vol.% quartz has a resistivity of 1 × 107 Ωcm and an open porosity of <3%. Sintered multilayer samples from tape-cast thermoelectric oxides and screen-printed insulation show only small reaction layers. It can be concluded that glass-ceramic composites are a well-suited material class for insulation layers as their physical properties can be tuned by varying glass composition or dispersion phases.
The relationship between fracture toughness and Yttria content in modern zirconia ceramics was revised. For that purpose, we evaluated here 10 modern Y2O3-stabilized zirconia (YSZ) materials currently used in biomedical applications, namely prosthetic and implant dentistry. The most relevant range between 2-5 mol% Y2O3 was addressed by selecting from conventional opaque 3 mol% YSZ up to more translucent compositions (4-5 mol% YSZs). A technical 2YSZ was used to extend the range of our evaluation. The bulk mol% Y2O3 concentration was measured by X-Ray Fluorescence Spectroscopy. Phase quantification by Rietveld refinement considered two tetragonal phases or an additional cubic phase. A first-account of the fracture toughness (KIc) of the pre-sintered blocks is given, which amounted to 0.4 – 0.7 MPa√m. In the fully-densified state, an inverse power-law behavior was obtained between KIc and bulk mol% Y2O3 content, whether using only our measurements or including literature data, challenging some established relationships. A linear relationship between KIc and the fraction of the transformable t-phase was established within the range of 30–70 vol%.
This study combines three different approaches to lower the sintering temperature of Sm-doped CaMnO3 to save energy in production and facilitate co-firing with other low-firing oxides or metallization. The surface energy of the powder was increased by fine milling, sintering kinetics were enhanced by additives, and uniaxial pressure during sintering was applied. The shrinkage, density, microstructure, and thermoelectric properties were evaluated. Compared to micro-sized powder, the use of finely ground powder allows us to lower the sintering temperature by 150 K without reduction of the power factor. By screening the effect of various common additives on linear shrinkage of CaMnO3 after sintering at 1100 ○C for 2 h, CuO is identified as the most effective additive. Densification at sintering temperatures below 1000 ○C can be significantly increased by pressure-assisted sintering. The power factor at room temperature of CaMnO3 nano-powder sintered at 1250 ○C was 445 μW/(m K2). Sintering at 1100 ○C reduced the power factor to 130 μW/(m K2) for CaMnO3 nano-powder, while addition of 4 wt.% CuO to the same powder led to ∼290 μW/(m K2). The combination of fine milling, CuO addition, and pressureassisted sintering at 950 ○C resulted in a power factor of ∼130 μW/(m K2). These results show that nano-sized powder and CuO addition are successful and recommendable strategies to produce CaMnO3 with competitive properties at significantly reduced temperatures and dwell times.
Dielectric breakdown of insulators is a combined electrical, thermal, and mechanical failure. The exact breakdown mechanism in ceramics and the formulation of useful models are still subject of investigation. Recent studies highlighted that several experimental aspects of dielectric breakdown strength testing affect the test results, and thus impede the recognition of fundamental principles.
Excess field strength near the electrode can lead to premature breakdown in the insulating liquid. This would cause superficial damage to the test specimen and thus falsify the measurement results.
The field strength distribution is influenced by the ratio of permittivity of the sample and the surrounding insulating liquid. Premature breakdown depends on the breakdown strength of the liquid and the actual test voltage. The test voltage again depends on the specimen thickness.
To systematically investigate these relations, a numerical simulation study (FEM) of the electric field distribution in a typical testing rig with cylindrical electrodes was performed. The permittivity of the sample and the insulating liquid was parameterized, as well as the sample thickness. The electric field distribution was calculated for increasing test voltage. Field strength maxima are compared to experimental breakdown strength of typical insulating liquids and experimental breakdown locations on alumina. Strategies are discussed to adjust the insulation liquid and the sample thickness to reduce the influence of the testing setup on the dielectric breakdown strength results.
For the first time, an ab initio calibration for absolute Mg isotope ratios was carried out, without making any a priori assumptions. All quantities influencing the calibration such as the purity of the enriched isotopes or liquid and solid densities were carefully analysed and their associated uncertainties were considered. A second unique aspect was the preparation of three sets of calibration solutions, which were applied to calibrate three multicollector ICPMS instruments by quantifying the correction factors for instrumental mass discrimination. Those fully calibrated mass spectrometers were then used to determine the absolute Mg isotope ratios in three candidate European Reference Materials (ERM)-AE143, -AE144 and -AE145, with ERM-AE143 becoming the new primary isotopic reference material for absolute isotope ratio and delta measurements. The isotope amount ratios of ERM-AE143 are n(25Mg)/n(24Mg) = 0.126590(20) mol/mol and n(26Mg)/n(24Mg) = 0.139362(43) mol/mol, with the resulting isotope amount fractions of x(24Mg) = 0.789920(46) mol/mol, x(25Mg) = 0.099996(14) mol/ mol and x(26Mg) = 0.110085(28) mol/mol and an atomic weight of Ar(Mg) = 24.305017(73); all uncertainties were stated for k = 2. This isotopic composition is identical within uncertainties to those stated on the NIST SRM 980 certificate. The candidate materials ERM-AE144 and -AE145 are isotopically lighter than ERM-AE143 by 1.6 ‰ and 1.3 ‰, respectively, concerning their n(26Mg)/n(24Mg) ratio. The relative combined standard uncertainties are ≤0.1 ‰ for the isotope ratio n(25Mg)/n(24Mg) and ≤0.15 ‰ for the isotope ratio n(26Mg)/ n(24Mg). In addition to characterizing the new isotopic reference materials, it was demonstrated that commonly used fractionation laws are invalid for correcting Mg isotope ratios in multicollector ICPMS as they result in a bias which is not covered by its associated uncertainty. Depending on their type, fractionation laws create a bias up to several per mil, with the exponential law showing the smallest bias between 0.1 ‰ and 0.7 ‰.
Advancements in pressure-assisted sintering technology for low temperature co-fired ceramics (LTCC)
(2016)
Steadily increasing demands on design and dimensional accuracy of ceramic multilayer modules, as well as the processing of new materials, require continuous improvements of manufacturing technology, especially thermal processes. The capabilities of pressure-assisted sintering (PAS) for the manufacturing of highly integrated low temperature co-fired ceramics (LTCC) multilayer have been considerably extended in the last years by procedural and device-related advancements. A lambda probe has been integrated in a sintering press prototype to monitor and control the process atmosphere. Thereby, the development of oxygen partial pressure during binder burnout of real modules can be observed. On the other hand, the oxygen partial pressure can be regulated during densification, for example to prevent diffusion of silver from circuit paths into the surrounding LTCC. Thin-film capable surfaces can be produced without post-processing by using setter plates made of glass-like carbon in an advanced PAS process under nitrogen. A newly developed advancement of this approach enables in-situ hot-embossing of LTCC during PAS by using structured glass-like carbon molds. The prototype press is further extended by a sensitive displacement transducer for monitoring the thickness shrinkage of real modules with an edge length of up to 8 inch.