New Trends in Ceramic Materials Properties Characterization
Advanced ceramic materials require novel and improved methods of synthesis, characterization, and investigation.
The continuing development of ceramic materials, along with an increasing number of industrial applications, is posing new challenges for the materials science industry. Modern technologies require novel and improved methods to synthesize, characterize, and investigate the properties of ceramics and composites.
Understanding the materials’ structure-properties relationship results in the production of advanced complex ceramics that extend beyond the traditional oxides that have been used for centuries. The standard techniques usually applied to alloys are currently finding their way into ceramics manufacturing as a result of improvements in physical properties; advanced ceramics often exhibit properties superior to those of metal-based systems.
Nanocomposites are defined as a ceramic matrix with one or more second phases (one-, two-, three-dimensional, amorphous particles) incorporated. Nanocomposite organic-inorganic materials are another fast-growing class of ceramics. The increased complexity of these materials, combined with the nano-sized particles, results in more properties variables and makes choosing the most suitable techniques to explore them a necessity.
The thermal and mechanical characteristics of the heterogeneous composites depend not only on the individual properties of the parent compounds, but on their morphology and particularly their surface area. A decrease in the particle size alone results in a stabilization of different structures and phases, as well as an increased number of functional groups on the surface (could result from the synthesis route or the subsequent adsorption of gases and vapors).
To interpret the surface energetics and other properties of such composite systems in detail, their composition must be characterized accurately. An extensive knowledge of all individual materials in the nanocomposite would allow manufacturers to understand and predict the nucleation, sintering behavior, and mass transport, and to choose the best synthesis route and consecutive treatment. The mechanical, thermal, and thermodynamic properties of ceramics and nanocomposites can be studied using a wide range of techniques.
Thermogravimetric analysis (TGA) is a preferred method of characterizing materials in order to determine the amount of impurities they contain, as well as to investigate how different thermal treatments would affect the materials’ stability and properties. Coupling TGA with evolved gas analysis instruments allows users to determine the amount of each gas released and the conditions of the decomposition, and to identify the composition of the compound to provide a more complete characterization of the complex composite materials.
For example, thermogravimetry proves to be a critical part of the characterization of two-dimensional carbon nanomaterials, which are used as a second phase dispersed in the ceramic matrix. In many cases, carbon atoms are included in more than one bonding state (e.g., carbon nanotubes containing amorphous carbon, fullerenes, etc.) with a different thermal stability and/or functional groups on the surface and varying oxygen content.
Thermomechanical analysis (TMA) provides important information related to the sintering behavior of ceramics and, more importantly, the effect of the added nanoparticles on the mechanical properties of the nanocomposite. Determining the exact composition and structure of the nanocomposites is crucial for applications where a small variation in the content might result in diminished, rather than enhanced properties.
Many ceramic materials are manufactured as loose powders, which necessitates the use of a vertical design for the TMA instruments (vs. horizontal). Another important application is related to densification, which occurs during a sintering process and allows manufacturers to obtain chemically, thermally and mechanically stable materials. Maintaining a constant and not too fast rate of shrinkage would additionally improve the properties of the materials. This process can be simulated during a TMA measurement by controlling the scanning rate simultaneously with the rate of shrinkage.
Incorporating Differential Scanning Calorimetry
Combining differential scanning calorimetry (DSC) and differential thermal analysis (DTA) techniques allows manufacturers to study the temperatures and enthalpies of transitions of high-temperature ceramic materials, including the melting temperature and heat of fusion above 2,000°C that cannot be obtained otherwise (e.g., yttria, zirconia). A simultaneous TGA and DSC/DTA technique provides more details about each occurring phenomenon, linking the weight change to the type of the phase transition.
Coupling a DSC with a manometric system has become an important tool to characterize the thermodynamics of nano-sized ceramics as related to particle size. As mentioned earlier, the high surface area results in compounds with higher energy than bulk materials. In order to compensate for the positive surface energy, the materials adsorb water vapors as both physi- and chemisorbed water, which affects their properties and applications. The coupled calorimetric-manometric techniques provide both the differential and integral heat of adsorption.
A standard two-dimensional DSC is generally used for heat capacity determination of solids. Replacing it with a three-dimensional Calvet-type sensor increases the rate with which the temperature difference can be measured, as well as the sensitivity of the measurement. Using a thermopile allows users to detect temperature differences between 10-3 and 10-7 K. (A thermopile consists of thermocouples wired in series.) The error of that type of measurement is usually less than 2%.
Thermal conductivity is also a critical characteristic of modern advanced ceramics. Efficient, accurate and repeatable measurement of thermal conductivity plays a key role in such material development. Using the modified transient plane source technique, for example, allows for a single-sided, non-destructive test of thermal conductivity with accuracies better than 5% and repeatability better than 1%. With test times under 5 seconds, thermal conductivity can be a quick check of a material’s performance throughout the R&D process.
High-temperature oxide melt solution calorimetry (isoperibol calorimetry) is a powerful tool and the most direct method to determine the thermodynamics of ceramics composites (e.g., enthalpies of formation, enthalpies of transition, enthalpies of mixing, etc.). It plays a crucial role in the direct determination of the surface and interface energetics of nanomaterials and the particle size-phase stability relationships. The isoperibol calorimeter consists of two identical thermopiles that provide high sensitivity and precision, which allows the measurement of a temperature difference of 10-3 K. The twin arrangement cancels out the effect of the environmental changes, as they affect both sides equally, thus increasing the stability. High-temperature oxide melt solution calorimetry has also been successfully used on non-oxides ceramics such as carbides, nitrides, silicides, and amorphous silicon oxycarbide polymer-derived ceramics (PDCs).
Many classic and newly enhanced techniques can be used in the development of new ceramic materials. Though researchers must carefully consider the best methods suitable for their unique applications, this is an exciting time to investigate and discover new materials that will continue to make the world a better place.
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