X-ray diffractometers are the main instruments used for studying the crystallographic properties of matter. In this blog, we will give a few examples of the importance of crystallography in our daily lives.
Many substances found in nature, are crystalline. Crystals that appear in nature, as a result of volcanic activity, are formed under high pressure or crystallized from water.
Here, you see beautiful gypsum crystals that grew during thousands of years deep under the ground. They were found a few years ago, by accident, during mining activities in Naica Mexico. These crystals are extraordinarily large; they are meters long. Note the small human figure at the bottom right of the picture.
In most cases, however, the crystallites found in nature are much smaller in size. Most rocks, soils, and sands, consist of small submillimeter particles such as iron-containing rocks.
If you’d make a cross-section of a rock fragment in preparation for the optical microscope, you’d see the small crystallographic domains in the rocks. The crystallographic properties of such rocks can be investigated with an X-ray diffractometer (XRD) such as the Empyrean multipurpose X-ray diffractometer. The Empyrean is meant for the analysis of powders, thin films, nanomaterials, and solid objects.
A single crystal deflects X-rays into beautiful diffraction patterns. Bragg’s law determines at which angle a single crystal will give a diffraction signal. Also, polycrystalline materials or powders give diffraction patterns. Of the many small crystallites contained in the powder, only the ones with the right orientation will provide strong diffraction signals. Because the diffraction signal will come from multiple crystallites, the powder pattern can also be used to determine the constitution of mixtures.
The red trace that you see here is a diffractogram. It consists of many peaks recorded as a function of the diffraction angle. From the angular position of the peaks the different components of a mixture, also called the phases in the mixture, can be determined. From the relative intensity of the peaks, the relative abundance of the phases can be computed. A powder pattern is like a unique fingerprint of the material; such a diffractogram can also be obtained from solid objects such as rocks and metals. These objects consist internally out of many small crystallites and produce their own unique powder patterns. Powder diffractograms can be recorded for many of the substances that we find in the world around us. These materials determine the quality of our daily lives. Let’s have a look at the importance of understanding the crystallography of powders and other crystalline mixtures.
Cement, a boring material?
Cement is the main construction material for the buildings in which we live, since Roman times. Did you know that the workability, the setting time, and the final strength of concrete, are determined by the crystallographic properties of cement? To be more precise, the quality of the buildings we create is determined by the crystallographic phase changes during cement hardening – a process still not fully understood by today’s scientists!
Cement is made by heating limestone and other raw materials in the long rotary oven called a kiln. In the kiln, the substances undergo crystallographic changes at temperatures up to 1,400 degrees Celsius yielding a material called clinker, which is ground afterward and mixed with other constituents in order to create cement. The making of cement results in significant emission of carbon dioxide, CO2, one of the gases responsible for global warming. For each kilogram of cement, almost one kilogram of CO2 is produced, creating about 5 percent of the CO2 emissions from human activities. It is the second source of CO2 emission after power generation.
Of the total CO2 emission in the cement production process, the majority (60%) stems from limestone calcination, 30% comes from the fuel needed to heat the kiln. The final 10% is needed for grinding of the clinker, transport of the material through the plants, and so on. Attempts to reduce CO2 emission focus on two aspects:
- First make cement with less clinker. Industrial byproducts such as fly ash from power plants or slags from iron producing blast furnaces are used for this. These materials also have a cementitious effect.
- Secondly alternative fuels can be used for heating the kiln such as plastic waste, animal carcasses, or used car tires, but these also influence cement properties.
The understanding of the crystallographic properties of the cement is essential for producing cement with low CO2 emissions.
Optimizing iron ore in mining
Another important material in our daily lives is iron. The starting point for all iron is the ore which is dug from the ground in mines. The quality of the ore in a mine is never constant. It was determined millions of years ago when the rocks were formed. The classical and simple way for determining the quality of the ore is by visual inspection: compare the color of the unknown with a reference set. From such a visual inspection different parts of the ore body can be classified as low grade or high grade.
By determining the crystallography, however, a much finer classification of the ore body can be made. Using this approach allows one to much better sort the mined materials into different grades and mix them to create an intermediate that is much more constant in quality; it increases the profitability of the mining activities as less waste is created and it reduces the damage to the environment.
Let’s talk about stress
When you were in an airplane traveling, did you ever wonder why the windows in the plane are oval and not rectangular in shape? Airplanes and other machinery are subject to cyclic loads during operations like takeoff and landing. After many repeated loads cracks can form at the surface which can suddenly propagate through the whole assembly causing failure: the so-called metal fatigue. Metal fatigue was not fully understood when the first commercial jet airplanes were built. The de Havilland Comet was an example of such a jet airplane that was built in the 50s. After a successful introduction of the airplane, two of these planes crashed after more than one year of successful operation, several midair catastrophic accidents happened in a short period of time. All planes were grounded, and the investigation started.
The repeated loads on the airplane’s body were simulated by placing one of the remaining airplanes in a water tank – which was repeatedly pressurized and depressurized. After more than three thousand cycles the plane suddenly burst open. The investigation showed that a fatigue crack had occurred at the corner of a rectangular window. From the simulated stresses in the window frame, one could see that these stresses are much higher in rectangular corners than the rounded ones. So nowadays airplane windows have rounded corners.
A further improvement of the mechanical components in airplanes and other machines was obtained by deliberate generation of compressive residual stress in the surface of the metallic components, causing micro-cracks to stay closed and therefore reducing the chance of metal fatigue. Nowadays metal parts undergo treatment by shot peening, which adds this compressive stress to the top surface, and metal fatigue problems are largely overcome. Understanding crystallographic deformation, and the measurement by X-ray diffraction, are essential for making the safe and long-lasting machines that we use in our daily lives.
Again, another area: microelectronic devices such as computers and cell phones have also become an essential part in our daily lives especially for the youngest generation. Cell phones have become so small and powerful because of our understanding of crystallography. With this understanding, we have created smaller and more powerful batteries, as well as energy-efficient components such as the backlight of the screens in our cell phones. Cell phone backlights are made from gallium nitride (GaN), a semiconducting material. These backlights consist of many thin layers which should have the right crystallographic properties for a good working device. Let’s have a look at controlled crystal growth.
GaN backlights, like other microelectronic components, consist of many layers of different materials which are grown on single-crystal substrates in chemical vapor deposition reactors. Depending on the growth conditions in the reactor, such layers can be relaxed: there’s no relation with the crystal structure of the substrate or strained: the layer is deformed and matches the crystallographic structure of the substrate. These strained layers are essential for the correct functioning of the device. X-ray diffraction is used to probe the crystallographic quality of these layers. Well-produced LEDs result in energy-efficient long-lasting cell phone screens. Again, understanding crystallography is essential for our daily lives.
The growth and aging of the world population ask for the availability of pharmaceutical materials for everyone. Understanding the crystallography of pharmaceuticals is essential for the development and production of safe medicines. The rotating molecule is Thalidomide, a drug developed in the 50s, which was found to have adverse effects on unborn children. A crystallographic property common in organic molecules is polymorphism: the ability of the molecule to crystallize in different forms.
Here, you see two forms of indomethacin, a strong painkiller. We need to understand these crystallographic forms in order to make safe pharmaceuticals. By measuring the crystallography, we can also check the authenticity of the drug. Counterfeiting of pharmaceuticals is a widespread problem and is a potential threat to the safety of our population. Counterfeiting is less risky than narcotics trafficking.
Here you see diffractograms of alpha and gamma indomethacin. Since the two polymorphs have different crystal structures, both diffractograms are different. X-ray powder diffraction is the only tool to readily distinguish between different polymorphs of a compound.
Crystals in your food
Crystallography is also important for feeding our growing population. Fertilizers are essential nowadays for improving the yield of agriculture. Understanding the crystallography of soils and fertilizers helps to optimize the fertilizer for the crops which are to be grown.
Access to drinking water is a growing problem in many areas of the world. The water in our rivers is often too polluted, or used for irrigation, causing water shortages for the population downstream. Making drinking water from the sea so called desalination there’s a growing activity. Understanding the crystallography of the membranes and filters is important for building desalination plants with reduced power consumption. Finally, crystallographic substances are present in many food substances we take. Chocolate is a tasteful crystallographic substance. So, crystallography is not only essential for our daily lives, it also adds taste.
Written by: Martijn Fransen, Posted by: Malvern Panaltyical (www.materials-talks.com)