A recreational structure, such as a trampoline frame, can be quickly and reliably assembled using a sleeve-joint coupling having a \u201cT\u201d configuration. -% and its use for the preparation of dental materials. The black matrix is formed by a black coloring resin provided at upper surfaces of pixels on the pixel sheet, the gap between the pixels, and the outer periphery defined by the outermost pixels. Each of the segments includes a pair of spaced-apart radial stanchions supporting a generally rectangular frame to which a corresponding panel is attached. Additional doped regions formed in the structures may improve over-voltage protection characteristics.