Recent Progress in Applications of the Cold Sintering Process for Ceramic–Polymer Composites

Three-dimensional structures in a multi-layer ceramic substrate are important in realizing ceramic-based meso- and micro-systems. During lamination and/or co-firing, three-dimensional structures, especially those with suspended structures, tend to deform and sag due to the intrinsic nature of the green (un-fired) ceramic material. Fabrication of three-dimensional structures with well-controlled dimensional stability and mechanical integrity remains a challenge. This paper discusses the challenges in fabricating structures in a multi-layer ceramic substrate. An overview is provided of the current state of the art in patterning and lamination techniques for the fabrication of these three-dimensional structures.

Overview on fabrication of three-dimensional structures in multi-layer ceramic substrate

Shaping ceramics into complex and intricate geometries using cost-effective processes is desirable in many applications but still remains an open challenge. Inspired by plant seed dispersal units that self-fold on differential swelling, we demonstrate that self-shaping can be implemented in ceramics by programming the material's microstructure to undergo local anisotropic shrinkage during heat treatment. Such microstructural design is achieved by magnetically aligning functionalized ceramic platelets in a liquid ceramic suspension, subsequently consolidated through an established enzyme-catalysed reaction. By fabricating alumina compacts exhibiting bio-inspired bilayer architectures, we achieve deliberate control over shape change during the sintering step. Bending, twisting or combinations of these two basic movements can be successfully programmed to obtain a myriad of complex shapes. The simplicity and the universality of such a bottom-up shaping method makes it attractive for applications that would benefit from low-waste ceramic fabrication, temperature-resistant interlocking structures or unusual geometries not accessible using conventional top-down manufacturing.

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Bio-inspired self-shaping ceramics.

This paper presents a wireless capacitive pressure sensor based on LTCC (low temperature co-fired ceramic) technology, where the design, fabrication, and measurement of the sensor is demonstrated and discussed. Differ from traditional LTCC process flow, a unique process of screen-printing sacrifice layer has been introduced to avoid deformation of the capacitive embedded cavity during lamination or sintering, which leads to a better performance of the sensor. A greater sensitivity of the sensor, comparing with its predecessors, is showed during measurement. Finally ways for future optimization are proposed.

Wireless LTCC-based capacitive pressure sensor for harsh environment

An electrostatic capacitor has been widely used in many fields (such as high pulsed power technology, new energy vehicles, etc.) due to its ultrahigh discharge power density. Remarkable progress has been made over the past 10 years by doping ferroelectric ceramics into polymers because the dielectric constant is positively correlated with the energy storage density. However, this method often leads to an increase in dielectric loss and a decrease in energy storage efficiency. Therefore, the way of using a multilayer structure to improve the energy storage density of the dielectric has attracted the attention of researchers. Although research on energy storage properties using multilayer dielectric is just beginning, it shows the excellent effect and huge potential. In this review, the main physical mechanisms of polarization, breakdown and energy storage in multilayer structure dielectric are introduced, the theoretical simulation and experimental results are systematically summarized, and the preparation methods and design ideas of multilayer structure dielectrics are mainly described. This article covers not only an overview of the state-of-the-art advances of multilayer structure energy storage dielectric but also the prospects that may open another window to tune the electrical performance of the electrostatic capacitor via designing a multilayer structure.

Recent Advances in Multilayer-Structure Dielectrics for Energy Storage Application.

Carbon burnout and densification of self-constrained low temperature co-fired ceramic (LTCC) are investigated using thermal analysis techniques. Slow heating rates and holding at a temperature higher than initial crystallization temperature of the glass component show evidence of retarding the densification of the self-constrained LTCC. Based on these results, it is proposed that the fabrication of embedded structures in a multi-layer self-constrained LTCC platform could be achieved by controlling carbon burnout with a multi-step co-firing profile, which can ensure complete carbon burnout without affecting the densification of LTCC structures. Using this approach, fabrication of an embedded cavity with dimensions of 10 mm × 10 mm × 0.50 mm in a self-constrained LTCC platform is demonstrated.

Carbon burnout and densification of self-constrained LTCC for fabrication of embedded structures in a multi-layer platform

The deformation behaviors of suspended low temperature co-fired ceramic (LTCC) laminates over a cavity and the evolution of open porosity of LTCC are studied for the fabrication of embedded structures in a multi-layer LTCC platform using carbon material. The effects of the type of LTCC materials (self-constrained and unconstrained LTCC), cavity width, laminate thickness, and lamination conditions on the deformation of the suspended LTCC laminate over a cavity are studied. For suspended three-layers and six-layers LTCC laminates over cavity width ranges from 10 to 25 mm, the self-constrained LTCC laminates were more dimensionally stable (sagged by less than −120 μm) after sintering as compared to the unconstrained LTCC. The evolution of open porosity and the distribution of open pores in the self-constrained LTCC with changes in sintering temperature and laminate thickness are also studied for process optimization.

Study of deformation and porosity evolution of low temperature co-fired ceramic for embedded structures fabrication

With recent development, multi-functional platform with substrate based on ceramics composite technology, in particular low temperature co-fired ceramic (LTCC) multilayer technology, is gaining advantages over conventional silicon technology. LTCC offers ease of integration for both homogenous and heterogeneous structures and materials. Its intrinsic properties and unique fabrication process makes 3-D meso and micro-system possible. There is, however, among the functionalities, optical access is desirable for most of the micro-electronic devices and micro-systems. Therefore, a process of integrating glass layer is always required. Glass layer could be fabricated directly on a fired ceramics composite substrate or ideally be co-sintered. In this paper, preliminary investigation on direct thermal glass bonding process on a fired LTCC substrate is presented. Bonding interfacial integrity was characterized using nondestructive scanning acoustic microscopy. At a temperature of 720degC for 30 minutes, bonding began to establish over 7 - 8 % of the interfacial area for a glass chip size of 10 mm times 10 mm. At 800degC, the bonded area reached approximately 98%. Glass sagging measurement over a number of open cavities was also conducted. Average sagging of about 42 mum was measured on a Oslash5 mm open cavity. Visible light transmittance for the open cavity after glass bonding was in the range of 89 - 92% which is closed to the reference "as received" glass of 92%. Cross-sectional micrograph at bonding interface showed a continuous metallurgical glass fused bond to the LTCC surface. Destructive shear strength test on glass/LTCC bonding was conducted. Failure was observed in the glass chip with an average shear apparent strength of about 2.5 MPa. Further investigation is required to eliminate edge deformation and to minimize sagging for large cavity.

Integration of Glass Layer for Meso and Micro-System Applications

Fabrication of embedded structures in a multi-layer low co-fired ceramic (LTCC) substrate is essential in realizing multi-functional platform. One of the major fabrication challenges is to preserve the designed dimensions of the embedded structures and to minimise distortion due to high lamination pressure and viscous flow of glass component during sintering. Fabrication techniques by employing fugitive material such as carbon could minimise distortion. Carbon powder could be formulated as paste or tape for incorporation by filling or drop-in respectively into the LTCC process to secure the fragile structure during the lamination and later the sintering process. Carbon could be completely burnt off under oxidizing environment and without any residue. This paper presents the current progress in developing a fabricaton process of realizing embedded structures in multi-Layer LTCC platform using carbon material as a supporting medium for the structures. To ensure complete carbon burnout from the LTCC platform containing embedded structures, the carbon burnout kinetic are determined by, thermogravimetry. The dependency of activation energy (Eiquest) over the extent of conversion is solved by, non-linear isoconversional methods. The activation energy is found to be high at the starting of the reaction, and has a gradual decreasing trend from 143.3 kJ/mol to 106.5 kJ/mol over a conversion of 0.1 to 0.9. The analysis of activation energy is also used to predict the carbon bum-off kinetic at isothermal condition for the development of multi-step sintering for embedded structure fabrication.

Y.M. Tan

Papers

Overview on fabrication of three-dimensional structures in multi-layer ceramic substrate

Carbon burnout and densification of self-constrained LTCC for fabrication of embedded structures in a multi-layer platform

Study of deformation and porosity evolution of low temperature co-fired ceramic for embedded structures fabrication

Integration of Glass Layer for Meso and Micro-System Applications

Process Development for Realization of Embedded Structures in Multi-Layer Ceramics Platform using Carbon Fugitive Material