You've listened to them, you've played games off of them, you've loaded programs onto your computer with them, and you have probably even burned a few of them. What we're taking about are compact discs, better known as CDs.

CDs have become widely available and very cheap in this day and age. They are resistant against magnetic fields of any strength, heat, dust, small scratches, and most acids. In fact, it takes very strong acids to attack the aluminum layer of a CD, end even then the polymer layer that makes up the CD's main body will not be dissolved.

So what is this resilient material that is used to makes CDs?

In most cases, the main body of a CD is a polymer that was developed in 1953, called Makrolon. Polymers are chemical compounds that are made up of repeated units of small molecules. You can envision them like a LEGO structure made up of many LEGO building blocks. The chemical used for making CDs is, in fact, the poly carbonic acid ester of the 2,2-Bis(4-hydroxyphenyl)propane, a.k.a. Bisphenol-A (a much simples name, don't you agree).

The chemical groups that are found in Bisphenol-A differ from the standard methyl (-CH3) group and give the polymer desirable properties, such as better acid resistance and better hardness and strength.

Did you know? Chemistry is a major player in CD technology.

But let's put chemistry on hold for a moment so that we can talk about data. How does data storage happen on CDs?

The principle is the same as for gramophone records (remember those?). Fine grooves are created on the disc's surface. While these grooves are big and obvious on old vinyl records, they can not quite so easily be seen on CDs because of their microscopic size. Within the grooves are a series of data storage pits that have been made using heat created by a laser.

Did you know? CD's are like vinyl records; both storage devices hold data in grooves that spiral around the disc to guide the data reading device.

These pits are similar to the punch cards used in antiquity and encode the binary information that is used in computer language (0's & 1's). These pits are tiny; they have an average diameter of 1-3µm (that's 1 millionth of a meter) are separated by a distance of 1.6 µm. Since these pits are so small, much more data can be stored on discs than on the old records.

Lasers are used to read the data from a CD. When a laser scans the surface of a CD, the laser beam will be reflected away from the disc, captured by a detector, and converted into binary data interpretable by a computer. Because of the size of the data pits, the lasers used are within the red, longer wavelength range. The grooves on a CD serve as a guide for the laser beam to ensure that the data is being read in the correct order/direction.

Did you know? Longer wavelength laser beams (in the red range) are used to read data off of CDs and DVD. Blue lasers, which are in the shorter wavelength range, are used for HD-DVDs and Blu-ray discs.

MORE...We Need More!

Our data hungry society demands for even larger data storage than a 700 MB CD offers. There are movies, games, photographs, and music that all need to be stored! The Digital Versatile Disc, or DVD, was developed for this reason and is constantly being improved.

DVDs have a seven times higher storage density than CDs. The reason for this is that the pits of a DVD, which contain the same information as a CD, are only 0.3-1.3µm long and the grooves are closer together, at a distance of 0.74µm. This allows more information, about 5 GB, to be stored on a single disc. DVDs have a further advantage in that they use all three dimensions for storage. Up to 5 read/write Makrolon layers can potentially be created on each side of one disc, increasing the data storage capacity to approximately 20 GB.

In reality, today's top-model DVDs use three layers. High definition-DVDs (HD-DVDs), however, increase the storage capacity by reducing the pit size even further, until it is so small that standard infrared CD and red DVD lasers can not read it anymore. Blue lasers with wavelengths below 405 nanometers (0.4µm) must be used. The theoretical storage maximum using all sides and multiple layers of a HD-DVD is 60 GB.

Did you know? DVDs and HD-DVDs hold more data because data storage pits are smaller and packed in closer together

Blu-ray devices uses similar lasers but different polymers (chalcogenides) that have a higher information holding capacity per layer (25 GB instead of 15 GB for the HD-DVD). The maximum theoretical storage capacity exceeds 100 GB per disc. However, the Blu-ray discs are more delicate than CDs and DVDs. In fact, they have to be covered in a solid protective hull, similar to floppy discs, to avoid damage. Nevertheless, the new chemical polymer has provided an opportunity to advance data storage technology.

With society's increasing demand for data storage, new technologies are always being developed; Blu-ray is only one example. Powerful consortia stand behind the different technologies, and only the future will show which one will prevail and which ones will become derelict and obsolete.

References

Polymers:

"Create a Funky Hairstyle...With hair gel at your fingertips, you can!",

Punchcards and older data storage technologies:

"Holiday Shopping...The Science of Debit Cards",

Wavelengths:

"CLICK on Science...The Workings of a Remote Control",

HD-DVD vs. Blu-ray:

http://www.thelookandsoundofperfect.com

http://www.emedialive.com/Articles/ReadArticle.aspx?ArticleID=11392

Lars Rose

Lars Rose is a PhD candidate in high temperature Solid Oxide Fuel Cell research (that is sustainable energies), at the Department of Materials Engineering in the Faculty of Applied Science at the University of British Columbia (UBC), and at the National Research Council Canada, Institute for Fuel Cell Innovation (NRC-IFCI). He enjoys teaching fun stuff and is the current Media Relations and Human Resources coordinator of the outreach program Let's Talk Science at UBC. He enjoys writing science in a fun way for CurioCity, UBC Terry, the Science Creative Quarterly, Fuel Cell Today and Ubyssey.







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