We present a design study aimed at fabricating micromachined thermal emitters for infrared gas sensing with high emission efficiency in the 9-12 μm wavelength range. In order to achieve high optical radiance with relatively low power consumption and possibility of fast thermal modulation, a technology based on dielectric membranes composed by stacked silicon oxide and nitride with platinum heaters is adopted, that guarantees good thermal isolation and reduced thermal mass. The dielectric stack adopted is composed in such a way that its emissivity is enhanced in the wavelength range of concern and an optimized design of the emitter geometry is proposed based on finite element thermal simulations. Some preliminary experimental results are presented about the optical properties of the structural layer and its medium-term reliability when operated with Pt heaters at a temperature around 600°C.