Linan An

Fabrication of SiCN MEMS by photopolymerization of pre-ceramic polymer☆

This paper describes the use of photopolymerization of a liquid polysilazane as a novel, versatile and cost-effective means of fabricating SiCN ceramic MEMS. SiCN is a new class of polymer-derived ceramics whose starting material is a liquid-phase polymer. By adding a photo initiator to the precursor, photolithographical patterning of the pre-ceramic polymer can be accomplished by UV exposure. The resulting solid polymer structures are then crosslinked under isostatic pressure, and pyrolyzed to form an amorphous ceramic capable of withstanding over 1500°C. By adding and curing successive layers of liquid polymer on top of one another, multi-layered ceramic MEMS can be easily fabricated. The use of photopolymerization can also be used to make thin, membrane-like ceramic structures. Key issues concerning the fabrication process are discussed. By combining photopolymerization with other in-house developed techniques such as polymer-based bonding and flip-chip bonding, three SiCN MEMS devices for high-temperature applications have been fabricated: an electrostatic actuator, a pressure transducer, and a combustion chamber. These represent a wide range of MEMS, demonstrating the versatility of this technique.

Fabrication of SiCN ceramic MEMS using injectable polymer-precursor technique

In this paper, a novel and cost-effective technology for the fabrication of high-temperature MEMS based on injectable polymer-derived ceramics is described. Micro-molds are fabricated out of SU-8 photoresist using standard UV-photolithographic processes. Liquid-phase polymers are then cast into the molds and converted into monolithic, fully-dense ceramics by thermal decomposition. The resultant ceramic, based on the amorphous alloys of silicon, carbon and nitrogen, possess excellent mechanical and physical properties for high-temperature applications. This capability for micro-casting is demonstrated in the fabrication of simple single-layered, high aspect ratio SiCN microstructures. A polymer-based bonding technique for creating more complex three-dimensional structures is also presented.

DAMAGE-RESISTANT ALUMINA-BASED LAYER COMPOSITES

A new philosophy for tailoring layer composites for damage resistance is developed, specifically for alumina-based ceramics. The underlying key to the approach is microstructural control in the adjacent layers, alternating a traditional homogeneous fine-grain alumina (layer A ) for hardness and wear resistance with a heterogeneous alumina : calcium-hexaluminate composite (layer C ) for toughness and crack dispersion, with strong bonding between the interlayers. Two trilayer sequences, ACA and CAC , are investigated. Hertzian indentation tests are used to demonstrate the capacity of the trilayers to absorb damage. In the constituent materials, the indentation responses are fundamentally different: ideally brittle in material A , with classical cone cracking outside the contact; quasi-plastic in material C , with distributed microdamage beneath the contact. In the ACA laminates, shallow cone cracks form in the outer A layer, together with a partial microdamage zone in the inner C layer. A feature of the cone cracking is that it is substantially shallower than in the bulk A specimens and does not penetrate to the underlayer, even when the applied load is increased. This indicates that the subsurface microdamage absorbs significant energy from the applied loads, and thereby “shields” the surface cone crack. Comparative tests on CAC laminates show a constrained microdamage zone in the outer C layer, with no cone crack, again indicating some kind of shielding. Importantly, interlayer delamination plays no role in either layer configuration; the mechanism of damage control is by crack suppression rather than by deflection. Implications for the design of synergistic microstructures for damage-resistant laminates are considered.

Silicoboron-carbonitride ceramics: A class of high-temperature, dopable electronic materials

The structure and electronic properties of polymer-derived silicoboron–carbonitride ceramics are reported. Structural analysis using radial-distribution-function formalism showed that the local structure is comprised of Si tetrahedra with B, C, and N at the corners. Boron doping of SiCN leads to enhanced p-type conductivity (0.1 Ω−1 cm−1 at room temperature). The conductivity variation with temperature for both SiCN and SiBCN ceramics shows Mott’s variable range hopping behavior in these materials, characteristic of a highly defective semiconductor. The SiBCN ceramic has a low, positive value of thermopower, which is probably due to a compensation mechanism.

Control of calcium hexaluminate grain morphology in in-situ toughened ceramic composites

The influence of processing conditions on the morphology of calcium hexaluminate (CA6) grains in Al2O3: 30 vol% CaO·6Al2O3 (CA6) ceramic composites was investigated. Specimens were prepared by in-situ reaction sintering using precursor powders of alumina, and either calcium carbonate or calcium oxide. In some samples, 1 vol% anorthite glass was added as a sintering aid. X-ray diffraction was used to study the phase development in the as-calcined and sintered states. The resultant microstructures were characterized using both scanning electron microscopy (SEM), and imaging secondary ion mass spectrometry (SIMS). It was found that the CA6 grains developed a platelike morphology when CaCO3 was used as the starting calcium-rich powder. In contrast, samples prepared using CaO resulted in equiaxed CA6 grains. This result was observed to be independent of the anorthite glass addition. The findings are rationalized in terms of distinct CA6 reaction mechanisms, resulting from differences in the reactivity of the powders during the early stages of calcining.

Soluble and meltable hyperbranched polyborosilazanes toward high-temperature stable SiBCN ceramics.

High-temperature stable siliconborocarbonitride (SiBCN) ceramics produced from single-source preceramic polymers have received increased attention in the last two decades. In this contribution, soluble and meltable polyborosilazanes with hyperbranched topology (hb-PBSZ) were synthesized via a convenient solvent-free, catalyst-free and one-pot A2 + B6 strategy, an aminolysis reaction of the A2 monomer of dichloromethylsilane and the B6 monomer of tris(dichloromethylsilylethyl)borane in the presence of hexamethyldisilazane. The amine transition reaction between the intermediates of dichlorotetramethyldisilazane and tri(trimethylsilylmethylchlorosilylethyl)borane led to the formation of dendritic units of aminedialkylborons rather than trialkylborons. The cross-linked hb-PBSZ precursors exhibited a ceramic yield higher 80%. The resultant SiBCN ceramics with a boron atomic composition of 6.0-8.5% and a representative formula of Si1B(0.19)C(1.21)N(0.39)O(0.08) showed high-temperature stability and retained their amorphous structure up to 1600 °C. These hyperbranched polyborosilazanes with soluble and meltable characteristics provide a new perspective for the design of preceramic polymers possessing advantages for high-temperature stable polymer-derived ceramics with complex structures/shapes.

Application of microforging to SiCN MEMS fabrication

Ceramics and polymers are attractive candidate materials for MEMS applications because of the wide range of properties that can be obtained, and the promise of improved performance as compared to the existing materials set for MEMS. A challenge in the fabrication of ceramic MEMS is prohibiting cracking that can occur during processing. For example, this is significant in the development of a microcasting fabrication technique from a polymer precursor for silicon carbonitride (SiCN) MEMS. In this case, shrinkage mismatch between the SiCN structure and the microfabricated mold during thermal processes leads to significant stresses that can crack the ceramic structure. Here, we propose an approach to overcome this problem that relies on demolding prior to the large shrinkage mismatch thermal processes, which itself is a nontrivial challenge. To this end, we propose and describe a microforging process that facilitates demolding and show representative results for numerous SiCN ceramic microstructures.

Highly stable anion exchange membranes based on quaternized polypropylene

A series of novel quaternized polypropylene (PP) membranes with ‘side-chain-type’ architecture was prepared by heterogeneous Ziegler–Natta catalyst mediated polymerization and subsequent quaternization. Tough and flexible anion exchange membranes were prepared by melt-pressing of bromoalkyl-functionalized PP (PP-CH2Br) at 160 °C, followed by post-functionalization with trimethylamine (TMA) or N,N-dimethyl-1-hexadecylamine (DMHDA) and ion exchange. By simple incorporation of a thermally crosslinkable styrenic diene monomer during polymerization, crosslinkable PP-AEMs were also prepared at 220 °C. PP-AEM properties such as ion exchange capacity, thermal stability, water and methanol uptake, methanol permeability, hydroxide conductivity and alkaline stability of uncrosslinked and crosslinked membranes were investigated. Hydroxide conductivities of above 14 mS cm−1 were achieved at room temperature. The crosslinked membranes maintained their high hydroxide conductivities in spite of their extremely low water uptake (up to 56.5 mS cm−1 at 80 °C, water uptake = 21.1 wt%). The unusually low water uptake and good hydroxide conductivity may be attributed to the “side-chain-type” structures of pendent cation groups, which probably facilitate ion transport. The membranes retained more than 85% of their high hydroxide conductivity in 5 M or 10 M NaOH aqueous solution at 80 °C for 700 h, suggesting their excellent alkaline stability. It is assumed that the long alkyl spacer in the ‘side-chain-type’ of 9 carbon atoms between the polymer backbone and cation groups reduces the nucleophilic attack of water or hydroxide at the cationic centre. Thus, PP-based AEMs with long “side-chain-type” cations appear to be very promising candidates with good stability for use in anion exchange membrane fuel cells (AEMFCs).

Optical properties of single-crystalline α-Si3N4 nanobelts

The optical properties of single-crystalline α-Si3N4 nanobelts synthesized via catalyst-assisted pyrolysis of polymeric precursor were characterized by absorption, photoluminescence (PL) and photoluminescence excitation (PLE). The optical absorption spectrum showed that the nanobelts exhibited indirect absorption behavior with optical band gap of ∼5.0eV. Three broad peaks centered at 1.8, 2.3, and 3.0eV were observed from the room-temperature PL spectrum of the nanobelts. The PLE spectra suggested the existence of multifold energy levels within the gap. A qualitative model was proposed to explain the observed absorption, PL and PLE spectra.

Ultraviolet photoluminescence from 3C-SiC nanorods

An intensive sharp photoluminescence at 3.3eV is observed from single-crystal 3C-SiC nanorods. Structural characterization reveals that the nanorods contain a fairly large amount of threefold stacking faults. We tentatively attribute the emission to these stalking faults, which structurally resemble 6H-SiC nano-layers of 1.5nm embedded in a 3C-SiC matrix. The emission mechanism is discussed in terms of spontaneous polarization at the stacking faults.