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Metallurgical-grade silicon powder (10 m2/g surface area) has been porosified using a metal-assisted chemical etch process based on hydrofluoric acid-ferric chloride chemistry. By controlling the reagent concentrations and ratios, the degree of porosification has been varied. Initiating the reaction at temperatures below 0°C, typically between −15°C and −25°C, yields etched powders with spectacularly increased surface area and pore volume (porosity). The reduced temperature, and its subsequent control, favors pore nucleation and propagation while minimizing bulk chemical etching. Using this process, mesoporous powders with surface areas up to 480 m2/g and pore volumes up to 0.52 ml/g have proved easily achievable at the 10 g batch level.

Porous silicon membranes formed by anodization and electrochemical detachment from the parent wafers are rapidly oxidized at room temperature by immersion in lye solutions comprising methanol and sodium hydroxide (< 0.1 M). Coverage of the silicon skeleton by an oxide, the thickness of which depends on immersion time, lye concentration and alcoholic ratio, is evinced by reductions in surface area, pore volume and size, together with a color change and a susceptibility to etching in dilute hydrofluoric acid. High-porosity membranes can be almost completely converted to white mesoporous silica after immersion times of only a few minutes. Controlled oxidation and hydrofluoric acid etching is used to significantly enlarge the pores.Passivation of the internal surface of porous silicon (pSi), where the reactive hydride-terminated bonds are replaced with a more functionally-stable species, is a pre-requisite for the majority of perceived applications (with the notable exceptions of pSi-based energetics and hydrogen storage). Of the passivation techniques that can be readily-applied to pSi membranes and powders, thermal oxidation has been the most common; under static sample/furnace conditions, this has been shown to be highly-exothermic and can result in a large degree of sintering and pore collapse. 1 The classical lattice expansion model predicts that both pore volume and size would be decreased on oxidation of the silicon network, although this could be compromised by pore closure/collapse. The unexpected increase in average pore size actually observed 1,2 suggests that sintering may be predominant (especially-so for large batch sizes), with negative implications for both process scale-up and maximum loading of an active compound into the oxidized pSi structure.Liquid-phase oxidation shows promise as an alternative to thermal oxidation, with oxidants such as dimethyl sulfoxide, 3 nitric acid, 4 and hydrogen peroxide 5 being effective; sodium nitrite has also been used, with the associated oxidative lattice expansion facilitating trapping of Cobamine and Rhodamin G. 6 In the present study, we discuss the liquid-phase oxidation of 'free-standing' (detached from parent substrate) pSi membranes using the chemistry surrounding dilute lye (sodium hydroxide) and alcohol, at room temperature.Aqueous sodium hydroxide (NaOH), in typically molar concentration, can be used to etch pSi completely from the surface of a wafer, 7 a particularly useful characteristic for applications where the porous layer is considered as sacrificial. Rates of pSi corrosion in NaOH and potassium hydroxide (1 M solutions), and associated activation energies, have been determined by weight loss measurements, at various temperatures, with additives such as ethanol/methanol and isopropyl alcohol being used to either increase or reduce the degree of etching. 8,9 Purely aqueous NaOH solutions with molarities as low as 0.05 M have been used to etch the (low porosity) parasitic outer layer present on the surface layer of pSi prepared from p + wafers, thus ...

Shock‐tube detonators, used in the mining industry to initiate charges of blasting explosives, often contain pyrotechnic delay‐lines for controlling the sequence of blasts. Traditionally, delay‐lines consist of pyrotechnic compounds of toxic metals such as lead which are pressed into the metallic tubing. Since lead is well‐known to be a hazard both to people and the environment, lead‐free alternatives were investigated to create a delay‐line composition. Further, to create delay‐lines cheaply and reproducibly, a composition was developed that could be printed onto a substrate. Screen printing was investigated for this purpose. The pyrotechnic ink that we developed consists of two inert inks (a fuel and an oxidiser) so that it can be stored and transported safely and conveniently without being classified as an energetic. After being transported to its destination, the two inks can be mixed to form an energetic (wet) ink, printed, and used as a delay‐line when dry. A silicon‐bismuth trioxide composition bonded together using nitrocellulose was found to be a suitable ink for screen printing. The burning rate was measured as 52±7 mm s−1 for a thickness of 100 μm. This thickness reduces the raw delay‐line material by over 90 % compared to traditional 3D delay‐lines. The target burning rate was 24 mm/s, but the measured rate is sufficiently close to this that changes in geometry could meet this requirement. The critical thickness for a self‐sustainable burn was measured as 40±5 μm. The thermal output and viscosity of the ink were determined as functions of the particle size. The viscosity of the ink was determined as 7.04±0.05 Pa s, which is suitable for screen printing. Milling was used to reduce the particle size below 1 μm. Ignition of the printed delay‐line using shock tubing was achieved through overprinting with a shock‐sensitive ink. The burning rate of a delay‐line is one of its most defining properties when used in mining applications. Thus the effect on the delay‐line's burning rate of the pyrotechnic composition printed thickness, and the ink solvent was investigated. We found that the burning rate could be modified by changing the silicon content in the pyrotechnic, and to some extent, by adding aluminum. Adding a third, slowly evaporating solvent, to the ink vehicle was found to reduce the critical thickness to less than 40 μm, decrease the burning rate to 46±7 mm s−1, and enhance the ink's flow properties. We found that the width of printed lines did not affect the burning rate for widths between 1–5 mm. This is an interesting result and could be used to determine the most cost‐effective shape to produce printed delay‐lines. Ignition of the printed delay‐line using shock tubing was achieved through overprinting with a shock‐sensitive ink.