Kitchen microwave

The domestic microwave oven is a serendipitous invention. Percy Spencer was working for Raytheon, a company heavily involved with radar during World War II, when he noticed the heat generated by a radar antenna. In 1947 an appliance called a Ra-darange appeared on the market for food processing. The first kitchen microwave oven was introduced by Tappan in 1955. Sales of inexpensive domestic ovens now represent a multibillion-dollar (euro) annual market. 

Chen, S.T., Chiou, S.H. and Wang, K.T., Preparative scale organic-synthesis using a kitchen microwave-oven, /. Chem. Soc., Chem. Commun., 1990, 807. 

Bose and coworkers have performed known chemical processes in various kitchen microwave devices to explore microwave chemistry at a larger scale [33]. Representative examples for multigram-scale synthesis without optimization of reaction conditions have been presented, such as the rapid preparation of 500-800 g of acetylsalicylic acid (Aspirin). Another valuable pharmaceutical compound, Tylenol (acetaminophen, paracetamol), could be... 

Figure 3 Microwave-assisted organic synthesis, (a) A domestic kitchen microwave with local "hot spots." (b) A single-mode microwave reactor for organic synthesis, (c) Comparison of surface temperature between microwave and conventional heating.

Classical sources for microwave generation of GHz and THz frequencies were klystrons or magnetrons. A magnetron microwave source is exactly what is present in your kitchen microwave. 

In the magnetron, electrons travel from a wire cathode to the anode walls of an evacuated chamber, forced into a circular path by a magnetic field. The anode wall contains hollow resonator cavities. The resonant frequency of the magnetron is determined by the resonator cavity dimensions. In your kitchen microwave, the frequency is tuned to a rotational absorption band in water, so water-containing samples absorb this frequency and heat up. 

Chemists first tried using domestic kitchen microwave ovens to speed up chemical reactions. They found that they were able to accelerate reactions, increase yields, and initiate otherwise impossible reactions. The results were often unsatisfactory, however, owing to uneven heating, lack of reproducibility, and the possibility of explosions. The power output of a typical kitchen microwave oven carmot be adjusted. The oven cycles between periods of full power and periods of zero power. This means that the amoimt of microwave energy being transmitted into an experiment cannot be controlled precisely. 

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