Search results

The purpose of this paper is to study tuneable positive temperature coefficient (PTC) effect in polymer-wax-carbon composite resistors. The resistivity dependence on temperature of composite resistors made of carbon fillers dispersed in an organic matrix is known to be strongly affected by the matrix thermal expansion. High PTC effects, i.e. essentially switching from resistive to quasi-insulating behaviour, can be caused by phase changes in the matrix and the assorted volume expansion, a behaviour that has been previously shown with both simple organic waxes and semi-crystalline polymers. However, waxes become very liquid on melting, possibly resulting in carbon sedimentation, and tuneability of semi-crystalline polymers is limited.

The authors therefore study a ternary polymer-wax-conductor (ethylcellulose-octadecanol-graphite) composite resistor system, where polymer and wax fuse to a viscous liquid on heating, and re-solidify and separate by crystallisation of the wax on cooling.

It is shown that with appropriate formulation, the resulting resistors exhibit strong PTC effects, linked with the melting and crystallisation of the wax component. The behaviour somewhat depends on sample history, and notably cooling speed.

The phase equilibria and transformation kinetics of the polymer-wax system (including possible wax polymorphism), as well as the exact mechanism of the conductivity transition, remain to be investigated.

As many compatible polymer-wax systems with different melting/solidification behaviours are available, ternary polymer-wax-conductor composite PTC resistors allow a high tuneability of properties. Moreover, the high viscosity in the liquid state should largely avoid the sedimentation issues present with binary wax-conductor systems.

Deterioration in natural stone is associated with many decay mechanisms and often the inherent composition of the materials themselves. Sandstone varies considerably but they all require a cementing matrix to bind amongst others, the silica (SiO₂) particles together (Reading, 1989). In calcareous sandstones and limestones this binding matrix is principally calcium carbonate based (Muir, 2006; Reading, 1989; McMillan et al., 1999) in the form of calcite (CaCO₃). Friable sandstone substrates and stones suffering from “surface dissolution” or disaggregation (Muir, 2006; Smith et al., 1992) have been traditionally consolidated utilising a host of chemical compounds that had, in many cases negative effects on their long-term performance (Muir, 2006). A principle issue amongst many was moisture entrapment and irreversibility of the consolidants adopted. The paper aims to discuss these issues.

This paper investigates the effect of microbial induced calcite precipitation (MICP) as a natural treatment for the conservation of historic natural stone substrates. Sporosarcina pasteurii has been proven as a bacterium that can perform MICP effectively in extreme conditions making it the preferred bacterium for the MICP process within this study. Surface treatment experiments were analysed by measuring the mass increase and surface changes using scanning electron microscopy (SEM).

The surface treatments showed a noticeable mass increase and observable deposition when viewed using a SEM microscope. Bio cementation of loose sand particles was observed and the degree of cementation was determined using a Moh's hardness test.

Recommendations for further work to improve this study are: use an increased Sporosarcina pasteurii cell optical density which would provide a greater calcite output. Carry out a paired comparison initial surface absorption test (BS 1881: Part 208, 1996 or ASTM C 1585-04, 2004). To be carried out on untreated control and MICP samples which would determine the pore blocking effect and surface repair capability of the treated samples.

A method for obtaining optimal results in terms of surface treatment would involve reducing the time between mixing and application, this would require having the two reaction constituents mixed only seconds before use. Using a late mix spray application system has the potential to allow the two mixtures to combine in the spray nozzle whilst exiting the apparatus.

This paper investigates a safe, natural process for stone repair.

A new family of thermoplastic binders has been developed for usage in fused deposition of ceramics (FDC). Mixtures were formulated consisting of a base binder, tackifier, wax, and plasticizer. The resultant formulation was chosen based on mechanical, rheological, and thermal property requirements. A formulation consisting of 100 parts base binder (by weight), along with 20 parts tackifier, 15 parts wax, and five parts plasticizer exhibited an optimized compromise of mechanical, rheological, and thermal properties. This formulation was compounded with 55 vol. per cent lead zirconate titanate (PZT) powder, and extruded into filaments with a diameter of 1.75mm and a length of approximately 50 (+/‐10) cm. The resulting filaments were used to fabricate functional piezoelectric ceramic devices via FDC. The binder development process is described, along with the associated mechanical, rheological, and thermal property data.

The purpose of this paper is to study the functionality of additively manufactured (AM) parts, mainly depending on their dimensional accuracy and surface finish. However, the products manufactured using AM usually suffer from defects like roughness or uneven surfaces. This paper discusses the various surface quality improvement techniques, including how to reduce surface defects, surface roughness and dimensional accuracy of AM parts.

There are many different types of popular AM methods. Unfortunately, these AM methods are susceptible to different kinds of surface defects in the product. As a result, pre- and postprocessing efforts and control of various AM process parameters are needed to improve the surface quality and reduce surface roughness.

In this paper, the various surface quality improvement methods are categorized based on the type of materials, working principles of AM and types of finishing processes. They have been divided into chemical, thermal, mechanical and hybrid-based categories.

The review has evaluated the possibility of various surface finishing methods for enhancing the surface quality of AM parts. It has also discussed the research perspective of these methods for surface finishing of AM parts at micro- to nanolevel surface roughness and better dimensional accuracy.

This paper represents a comprehensive review of surface quality improvement methods for both metals and polymer-based AM parts.