The beam fell on the crystal. Fun chemistry experience at home

ENTERTAINING EXPERIENCES AT HOME
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The beam fell on the crystal. Chemical experiments

Entertaining experiments in chemistry

Entertaining experiences at home / Chemistry experiments for children

Let's get down to making the semiconductor. Once you already succeeded - experiencewhen you turned an aluminum spoon into a rectifier. Now the experience is no less interesting, and with theoretical explanations. It is better to put it in a chemistry circle or in a school laboratory, and not because the experience is dangerous: you just most likely do not have the required substances at home.

First, preliminary experience. Prepare a solution of lead nitrate or acetate and pass hydrogen sulfide through it (work under draft!). Dry the precipitated lead sulfide PbS and check how it conducts electricity. It turns out that this is the most common insulator. So what about semiconductors?

Let's not rush to conclusions, but put the following, basic experiment. For him, you will have to prepare equal amounts, say, 15 ml, of a 3% solution of thiocarbamide NH₂C(S)NH₂ and 6% solution of lead acetate. Pour both solutions into a small glass. Using tweezers, introduce a glass plate into the solution and hold it vertically (or secure it in this position). Wearing rubber gloves, pour a concentrated lye solution into the glass almost to the top (carefully!) and stir very carefully with a glass rod, trying not to touch the plate with it. Heat the solution slightly until steam appears; Continue stirring. After about ten minutes, carefully remove the glass plate, wash it under running water and dry it.

And in this case you got lead sulfide - so what's the difference?

In the second experiment, the reaction proceeds slowly, and the precipitate does not precipitate immediately. If you watched the solution, you noticed that at first it turned cloudy and became almost like milk, and only then darkened - these intermediate compounds, decomposing, formed black lead sulfide. And it settles on the glass in the form of a thin black film, which consists of very small crystals that are visible only under a microscope. Therefore, the film appears very smooth, almost mirror-like.

Attach two electrical contacts to the film and pass the current. If the lead sulfide from the previous experiment behaved like a dielectric, now it conducts current! Put an ammeter in the circuit, measure the current and calculate the resistance: it will be higher than that of metals, but not so large as to serve as an obstacle to the passage of current.

Bring a lighted lamp very close to the plate and turn on the current again. You will immediately find that the lead sulfide resistance has dropped dramatically. A black film will behave in much the same way if it is simply heated. But if the conductivity increases during lighting and heating, then we are dealing with a semiconductor!

Why does lead sulfide have this property? We wrote down its formula as PbS, but the true composition of the crystals of this substance does not quite correspond to it. Some compounds, including lead sulfide, do not obey the law of constant composition. And they are all semiconductors. (The same, by the way, also applies to aluminum oxide, which rectified alternating current.)

In a PbS crystal, the order of arrangement of particles should, it would seem, be strictly repeated. But often, due to the fact that the concentrations of the solutions from which the crystals are obtained fluctuate, the order is disrupted. The influence of temperature and other external causes is felt. However, in a real crystal the ratio of sulfur and lead atoms is not exactly 1:1. Deviations from this ratio are very small, only about 0,0005. But this is enough for the properties to change significantly.

Lead and sulfur atoms are connected in a crystal by two electrons: lead donates them to sulfur. Well, when does the 1:1 ratio break down? If there is no sulfur atom next to the lead atom, the electrons will be free - they will serve as current carriers. And such cases are not as few as it might seem. Of course, the ratio of 1,0005:1 is almost equal to unity, but if you remember how many atoms there are in a crystal, then this minor difference will no longer seem so trivial to you.

The composition of lead sulfide can be adjusted. This is necessary in order to change its conductivity. When there are more sulfur atoms in the crystal, the conductivity decreases, and when there are fewer of them, more free electrons are formed and the conductivity increases. In short, by changing the ratio of sulfur and lead atoms, you can obtain the required conductivity. This experiment is not easy to carry out; If you do not dare to conduct an experiment, take my word for it that it will work.

Take a quartz tube and place a boat with lead sulfide in the sky. On the other hand, insert the same boat with lead into the tube and heat the tube very strongly so that the lead begins to evaporate. Sulfide in this case will absorb vapors, it will be enriched with lead, and its electrical conductivity will increase significantly.

It remains only to answer the question why lead sulfide is so sensitive to light. Light quanta impart energy to electrons, and in each specific case, rays with a certain wavelength are the most effective. For lead sulfide, this is infrared thermal radiation. That's why we advised you to bring the lamp closer to the film.

By the way, infrared receivers usually use an excellent semiconductor - lead sulfide.

Author: Olgin O.M.

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