Table of Contents
Introduction
In the field of technical ceramics, the targeted influencing of material properties through controlled sintering atmospheres plays a central role. The focus here is on the “green body” – the unsintered blank dried after shaping – as it reacts particularly sensitively to temperature, atmospheric composition and sintering parameters. For research, development and process optimization, the method of simultaneous thermal analysis (STA) has established itself as a particularly effective tool for quantitatively characterizing and interpreting these reactions [1][2][3].
Green body and sintering atmosphere
The green body consists of compacted but not yet sintered ceramic powder. Its subsequent density, microstructure and mechanical quality are decisively influenced by the sintering conditions. The sintering atmosphere (e.g. oxidizing, reducing, inert, defined moist or dry) controls in particular:
- Decomposition of binders and additives
- Redox reactions of sensitive components
- Pore formation and closure
- Grain growth and development of secondary phases [4][5]
Simultaneous thermal analysis: methodology and advantages
Basic principle of the STA
The STA combines thermogravimetry (TGA) and differential scanning calorimetry (DSC) in one measurement run under exactly identical conditions [1][2][3]. Thermogravimetry (TG) measures mass losses or gains (e.g. through evaporation, decomposition or oxidation), while differential scanning calorimetry (DSC) measures the associated heat flow ( endo– or exothermic effects). This simultaneous recording of both data streams makes it possible to clearly assign an energetic process to each mass loss – and vice versa.
Key advantages of simultaneous measurement
The simultaneous recording of mass and heat changes offers several decisive advantages:
Direct correlation of thermal processes: Simultaneous recording enables the simultaneous recording of mass losses (e.g. due to outgassing of moisture, degradation of organic additives or decomposition processes) and the measurement of endo- or exothermic effects (e.g. phase transformations, chemical reactions, melting and crystallization processes) [1][2][3].
Avoidance of artifacts: If TG and DSC are carried out on separate samples or at different times, even the smallest differences in sample properties, atmosphere control or temperature program can lead to contradictory results. Simultaneous measurement guarantees identical conditions for both signals and therefore exact reproducibility.
Atmosphere control: The differentiation of atmosphere-specific reaction processes through targeted atmosphere control (N₂, O₂, Ar, H₂, CO₂ mixtures) is made possible. Various gas mixtures, humidity and pressure controls are possible [1][2][3].
Influence of different sintering atmospheres
The choice of sintering atmosphere (oxidizing, inert, reducing) significantly influences the chemical reactions in the green body, the type and time of decomposition and the release, reaction or binding of gases.
Effects on the mass change (TGA signal)
Oxidizing atmosphere (e.g. air, O₂): Under oxidizing conditions, there is a clear, usually gradual loss of mass due to the complete combustion of organic binders and additives. At the same time, crystalline water-containing components are released through dehydration processes. In some cases, an increase in mass can even be observed due to oxidation of surfaces or secondary elements such as metal particles.
Inert atmosphere (e.g. N₂, Ar):
- Organic components are thermally decomposed, often leaving more residue (pyrolysis coke) in the green body
- Slower loss of mass, possible occurrence of several superimposed decomposition stages
Reducing atmosphere (e.g. H₂, CO): In reducing atmospheres, a selective reduction of oxides takes place, whereby a significant reduction in mass can occur in metals or mixed systems due to the release of oxygen. Any pyrolysis coke present can be broken down in the presence of hydrogen and leads to gas formation, while it remains in the material under other reducing conditions.
Effects on the heat change (DSC signal)
Oxidizing atmosphere: Under oxidizing conditions, characteristic exothermic peaks occur due to the combustion of organic binders. At the same time, endothermic effects can be observed due to the melting of additives or the release of crystal water. Other exothermic reactions are also possible, for example through oxidation of metal particles or specific phase transformations in the ceramic material.
Inert atmosphere:
- Predominantly endothermic effects due to thermal decomposition (pyrolysis) of the organic components
- Reduction of exothermic peaks due to lack of combustion
Reducing atmosphere: Reducing atmospheres exhibit both exothermic and endothermic effects, which are highly dependent on the respective material system. Characteristic is a shift in the typical transformation temperatures compared to oxidizing or inert conditions, which is due to the changed reaction kinetics under reducing conditions.
Comparison of typical measurement curves
| Sintering atmosphere | Mass change (TGA) | Heat change (DSC) |
|---|---|---|
| Oxidizing | Significant weight loss, fast | Strong exothermic peaks |
| Inert | Reduced mass loss, slower | Weaker, mostly endothermic |
| Reducing | Chemoselective changes | Mixed exo-/endothermic |
Scientific findings and applications
Current scientific publications show how, for example, the sintering kinetics and the behavior of secondary phases on the green body can be derived in situ [4]. The influence of atmospheres on the formation of compressive properties, grain structure or microstructure development can also be quantified excellently using STA, as recent work on aluminum oxide, zirconium oxide and piezoceramics shows [4][5].
Exemplary findings:
Oxidizing atmospheres often promote the elimination of organic binders, as combustion takes place at lower temperatures and more completely. However, they can also lead to undesirable phase transformations, especially if oxygen-sensitive components are present in the ceramic system.
Reducing or inert atmospheres:
- Allow the targeted management of secondary phases through controlled redox conditions
- Often have a decisive influence on the pore structure through altered decomposition kinetics
Atmosphere change during the sintering process represent a particularly interesting possibility, as they can be actively used for microstructure control. Various process steps can be specifically optimized through time-controlled changes in the composition of the atmosphere [4][5].
Practical example of process identification
| Process in the green body | TG (mass) | DSC (heat flow) | interpretation |
|---|---|---|---|
| Removal of organic components | Mass loss (steps) | exothermic peak | Combustion/degradation of binding agents |
| Phase transition | no change in mass | endo-/exothermic effect | Crystal structure change without loss of substance |
| Reduction of an oxide | Mass loss | exothermic/endothermic depending on the reaction | Oxygen release, energetics of reduction |
Advantages for sintered atmosphere research
STA offers decisive advantages for the characterization of green bodies under different sintering atmospheres:
- Time and sample savings: Since both signals are obtained simultaneously from the same sample, less sample material is required and experimental effort is reduced
- Comparison and optimization: Different sintering atmospheres can be investigated directly in comparison, for example to optimize the oxidation sensitivity or the elimination of organic binders [1][2][3][6]
- Understanding complex processes: The overlapping of several processes is typical for technical ceramics. With STA, these processes can be better differentiated and correlated
- Better comparability: Especially when screening atmospheric influences or material batches due to identical measurement conditions
Technology transfer and practical relevance
The targeted application of simultaneous thermal analysis is a key technology for the trouble-free, reproducible production of high-performance technical ceramics. It enables the efficient development and optimization of sintering processes, adapted to individual requirements and material systems [1][2][3].
Laboratories and research institutions use this data to:
- Determine optimum sintering processes (freedom from defects, homogeneity)
- Enable atmosphere-guided material modification (e.g. targeted pore design, residual carbon management)
- Validate process scaling
Conclusion
The combination of innovative sintering atmosphere control and the analytical strength of simultaneous thermal analysis allows in-depth process and material characterization based on real transition and reaction data of the green body. The simultaneous measurement of TG and DSC provides decisive added value: it enables a comprehensive and reliable interpretation of thermal processes, improves reproducibility and saves time and resources – an invaluable advantage for research, development and quality assurance in the field of technical ceramics.
The STA illustrates how strongly the sintering atmosphere influences the thermal properties and reaction processes of green bodies and thus provides the basis for the efficient and safe development of ceramic materials. The use of STA opens up the full potential of modern ceramic development under variable atmospheric conditions – effectively, precisely and scientifically sound.
References
- [1] https://www.linseis.com/messgeraete/thermische-analyse/sta-simultane-thermische-analyse/
- [2] https://www.linseis.com/methoden/simultane-thermische-analyse-tga-dsc/
- [3] https://linseis.co.kr/wp-content/uploads/2018/07/LINSEIS_Produktbroschüre_DEU_v4.compressed.pdf
- [4] A. Klimera, Festigkeitssteigerung von Aluminiumnitrid-Keramiken, Dissertation University of Würzburg, https://opus.bibliothek.uni-wuerzburg.de/files/2243/Festigkeitssteigerung_von_Aluminiumnitrid_Keramiken_A_Klimera.pdf
- [5] https://www.db-thueringen.de/servlets/MCRFileNodeServlet/dbt_derivate_00012010/ilm1-2007000122.pdf
- [6] https://www.epe.ed.tum.de/es/forschung/messtechnik/thermogravimetrische-analyse/
