How Did Nuclear Zirconium Alloys Develop?

Zirconium alloys have a small thermal neutron capture cross-section (0.185b) and are surprisingly resistant to corrosion, so they are widely used in fission reactors, such as core-clad tubes, grids, and guide tubes in boiling water reactors, as well as pressure pipes and exhaust reactor vessels in pressurized water reactors.

Nuclear zirconium alloy

With the application of zirconium alloys in the nuclear energy industry, the zirconium industry has developed rapidly.

In the nuclear giant change reactor, nuclear fuel is fission reaction all the time. In the reaction, the neutron bombards the nucleus of U235, which splits into Ba140 and Kr93, and releases two or three neutrons at the same time; other U235 nuclei are bombarded by these neutrons and re-fission. This is the chain reaction of fission.

nuclear-reactor
Nuclear Reactor

A material with a large neutron capture cross section will absorb many neutrons when they hit the wall, reducing the efficiency of the chain reaction. Meanwhile, the chain reaction produces a lot of heat, which is removed by circulating cooling water (or other coolants) to avoid overheating and damage to the reactor. When metals come into contact with high-temperature water, they can be corroded (oxidized). Materials with poor corrosion resistance need to be replaced frequently, which increases the cost and easily leads to safety accidents. Therefore, as core-cladding and structural materials, zirconium alloys are required to have low neutron capture cross-section and excellent corrosion resistance, so the development of zirconium alloys should be attributed to the nuclear industry.

Origin of zirconium alloys

Initially, zirconium was not considered a suitable material for use in the nuclear industry, because studies have shown that zirconium’s effect on thermal neutron absorption can affect the efficiency of nuclear reactors. Later, researchers at the Oak Ridge Institute found that 2.5% of the hafnium in zirconium was responsible for its large thermal neutron capture cross-section.

Zirconium Alloy
Zirconium Alloy

Zirconium and hafnium are associated with ore and are generally difficult to separate. Until the 1850s, Admiral in the Naval Nuclear Propulsion project decided to use zirconium in the water-cooled reactor of the Nautilus Nuclear Submarine. Although zirconium had already been used for the project by that time, there were no strict standards for the use of zirconium, and the researchers only knew that improving the purity of zirconium would be good for the properties of the alloy. Some processes are used to purify strip zirconium, but it still contains small amounts of nitrogen, making it less resistant to corrosion at high temperatures. Finally, the researchers realized that purity was not the key to zirconium’s corrosion resistance, because they found that some zirconium materials containing impurities (such as tin, iron, chromium, and nickel) were more resistant to corrosion than higher-purity zirconium materials. Therefore, the development of zirconium alloys is put on the agenda.

Development of zirconium alloys

The first alloy, Zircaloy-1, contains 2.5% tin. It was found that the corrosion rate of Zircaloy-1 alloy was increasing and not consistent with the expected decrease. This was similar to a normal sponge zirconium material, so Zircaloy-1 was quickly abandoned.

At the same time, the researchers found that adding iron and nickel to the Zircaloy-2 could improve corrosion resistance. The tin content was reduced to 1.5% and 0.15% iron, 0.05% nickel and 0.10% chromium were added. It was found that Zircaloy-2 had the same mechanical properties as Zircaloy-1, but the high-temperature corrosion resistance of Zircaloy-2 was much better than that of Zircaloy-1. However, during the service of the pressurized water reactor, the alloy produces a lot of hydrides, resulting in hydrogen embrittlement.

By studying the binding technique, the researchers found that nickel greatly enhanced the hydrogen absorption capacity of zirconium alloys. The researchers removed the nickel from the Zircaloy-2, creating a Zircaloy-3. But Zircaloy 3 was quickly abandoned because its strength was too low. In addition, Zircaloy-3 produced many striated Fe-Cr binary intermetallic compounds when it was processed in the two-phase zone, so it could not provide sufficient corrosion resistance. The strength of Zircaloy-3 was still too low, although changes in the heat treatment process prevented the production of the striated compound.

Zircaloy-2 Composition
Zircaloy-2 Composition

The researchers compensated for the nickel by increasing the iron content by 0.22 percent and found that the corrosion resistance of the new alloy was similar to that of zircaloy-2, which had only half the hydrogen absorption rate. The new alloy quickly became a major part of the pressurized water reactor, the first Zircaloy-4.

Zirconium alloys for the nuclear industry have been developed into the third generation of products, which are used in various reactors.

The first generation is the standard zircaloy-4 and Zircaloy-2, whose composition and process requirements are specified in the ASTM standard. This generation zirconium alloy is still in use.

The second generation is low tin Zircaloy-4 and optimized Zircaloy-4. The tin content of low tin Zircaloy 4 decreased from 1.2% ~ 1.70% to 1.20% ~ 1.50%, and the carbon and silicon were controlled at 0.008% ~ 0.020% and 0.005% ~ 0.012%, and the cumulative annealing process parameters in the alpha phase after quenching in the beta phase were strictly controlled; the optimized zircaloy-4 is based on the low tin zircaloy-4, and the content of alloy elements and process parameters are more strictly controlled, so as to improve the uniformity of materials.

The third generation of zirconium alloy has excellent properties and is widely used as a fuel rod cladding tube and fuel assembly guide tube. NDA and MDA from Japan, HANA from South Korea and composite casings from Siemens are also examples of this generation of products.

Prospect of zirconium alloys

Zirconium alloys above 620℃ (depending on composition) convert to body-centered cubic β-zirconium. After the transformation, the mechanical properties and corrosion resistance of the alloy will be greatly reduced, and it cannot continue to maintain the safe operation of the nuclear reactor. The famous event is the accident at the Fukushima nuclear power plant in Japan. Affected by the big earthquake in eastern Japan, the reaction water of Fukushima nuclear power plant leaked, and the cladding temperature increased significantly. The zirconium alloy cladding softened quickly, and brittle material formed with the leakage of air, leading to the leakage of nuclear fuel. Large amounts of nuclear-contaminated water flow into the sea have caused great damage to the ecology of the world.

As a nuclear reactor cladding material, it needs to have a small thermal neutron capture cross-section, which leads to the zirconium alloy cannot be highly alloyed, so it is bound to be difficult to break through the zirconium alloy’s high-temperature performance. At present, countries attach great importance to this problem. On the one hand, they are trying their best to make a breakthrough in the high-temperature performance of zirconium alloy; on the other hand, they are looking for alternative products of existing fuel cladding, such as silicon carbide (SiC) composite material, molybdenum alloy, cobalt alloy and so on. Molybdenum alloys and cobalt alloys were originally intended as structural materials for fusion reactors. Although they do not have the same low thermal neutron absorption cross-section as zirconium alloys, they have excellent high-temperature stability.

Stanford Advanced Materials supplies high-quality zirconium alloys to meet our customers’ R&D and production needs. Please visit http://www.samaterials.com for more information.

What Are the Uses of Advanced Composite Ceramic Substrates in Missiles?

In the mid-1980s, the United States developed an aerospace aircraft program that required both high-temperature tolerance and light mass. For this purpose, a variety of new high-temperature materials were developed, including advanced resin matrix composites, metal matrix composites, ceramic matrix composites, and carbon/carbon composites. Ceramic material is the preferred material for missile radome because of its excellent mechanical, thermal and electrical properties. The radome is the most widely used ceramic matrix composite material in missile structure.

Missile radome

The missile radome is located at the front end of the missile. Its function is to protect the navigation antenna from damage so that the missile can effectively hit the target. It is not only an important part of the aerodynamic shape of the missile but also the protection device of the antenna. During the flight of the missile, the radome should not only withstand aerodynamic heating and mechanical overload, resist the erosion of rain, sand, and other adverse working conditions, but also meet the stringent requirements of electrical performance proposed by the missile control loop. Therefore, the missile radome material should have the following properties:

Missile-radome

  • Excellent dielectric properties

In the guidance system, the transmission efficiency and aiming error of the radome are very sensitive to the dielectric properties of the material and its relationship with temperature and frequency. It is required that the material has low dielectric constant (10) and dielectric loss, and the dielectric properties do not change obviously with temperature and frequency.

  • Good heat resistance and thermal shock resistance

The high Mach number of missile can make the radome of instantaneous heating rate is as high as above 120 ℃ / s, so the material is required to have good thermal shock resistance, and the molecular structure of the material is required to be stable when the temperature is raised, and the material properties (such as dielectric properties and mechanical properties) change little to ensure that the radome can work normally when the temperature is raised.

  • High strength structural properties

The strength of the radome material should be high and rigid enough to satisfy the mechanical stress and bending moment caused by the longitudinal or transverse acceleration of the aerodynamic forces in the spacetime of the missile flying at high speed.

  • Resistance to rain erosion

It plays a decisive role in the design allowable range of impact Angle and the sensitivity of aircraft in rain erosion.

  • Low-temperature sensitivity

The dielectric properties and strength properties of general materials change obviously when they work at high temperature. Therefore, the properties of the radome material, especially the dielectric properties and strength, are affected by the temperature change as little as possible.

Ceramic-based missile radome

Ceramic-based missile radome materials mainly include silicon nitride based, a silicon oxide based and phosphate-based materials. Silicon nitride ceramics have not only excellent mechanical properties and high thermal stability but also low dielectric constant. Its decomposition temperature is 1900 ℃, and its erosion resistance is better than fused silica, and it can withstand 6 ~ 7 Ma rating of flight conditions. Silicon nitride ceramic composite radome is one of the main research targets in various countries, which has been identified as the most promising radome material by the test of the Georgia Institute of Technology. Yttria Stabilized Zirconia (YTZ), also known as yttria-zirconia, is the strongest ceramic material. This material offers the highest flexural strength of all zirconia-based materials, and the research on zirconia-based materials as missile radome is in progress.

Ceramic-based-missile-radome

  • Silica-based material

Because of the high flying Mach number of the missile and the relatively long heating time, if the radome of the medium-range missile is made of a single quartz ceramic material, it cannot meet the bearing requirement of thermal stress. In order to meet the requirements of medium and long-range ground-to-ground tactical and strategic missile radome, quartz glass, high-silica puncture fabric and orthogonal tri-directional quartz fabric reinforced silica matrix composites have been developed and successfully applied.

  • Phosphate-based materials

Phosphate matrix composite material is a kind of Russian characteristic permeable material, which is made by impregnating cloth or fabric with a phosphate solution and then curing under pressure. Aluminum phosphate has stable performance in 1500 ~ 1800 ℃. At present, such materials have been used in cruise missiles, anti-missile missiles, tactical missiles and space shuttles. The most obvious disadvantage of phosphate is that it is highly hygroscopic, so the surface of the composite material needs to be coated with an organic coating for moisture-proof treatment.

  • Silicon carbide ceramic matrix composites

Silicon carbide ceramic matrix composites have a series of excellent properties, such as low density, high-temperature resistance, ablation resistance, erosion resistance, and oxidation resistance, and it has a wide application prospect in the field of aerospace. Since the late 1980s, the United States has successfully developed a series of C/SiC, SiC/SiC ceramic matrix composites, which can be applied to the re-entry nose cone of missiles, the front end of wings and other heat-resistant structures.

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Zirconia Ceramic Structural Parts VS Auto Parts

As we all know, a car is a product with an extremely demanding operating environment and working conditions, so the various components that make up this giant must have very superior functions. The zirconia ceramic structure has been widely used in auto parts.

Zirconia ceramic structure parts with excellent performance just make up for the lack of metal materials, so they began to be accepted by the automobile. For example, it has a long vibration tolerance of 20 grams; as parts used in combustion and exhaust system, it can endure 50 ~ 60 ℃/S of thermal shock for a long time; due to strong mechanical reliability of the zirconia ceramic, the failure rate is usually between 10 and 5; it can also be mass produced and low in price, which is convenient for the formation of industrial management.

Zirconia Ceramic Structural Parts
Zirconia Ceramic Structural Parts

In recent years, scientists in the international special ceramics field have developed a large number of automobile special ceramics through hard research, and experiments and industrial applications have proved that the superior mechanical properties and high-temperature chemical properties of ceramic materials have far surpassed those of metal materials or other materials. At present, the applications of zirconia ceramic structure parts in auto parts industry are as follows.

Zirconia ceramic oxygen sensor

The zirconia ceramic oxygen sensor has high mechanical properties and reliability. As a component of clean exhaust, O2 concentration in automobile exhaust is measured, and the measured value is fed back to the gas and fuel supply system of the engine to keep the fuel always in full combustion state. Since all phases of the ceramic material are partially stabilized zirconia mixed with fully cubic, tetragonal and monoclinic crystals, the mechanical properties are superior during use and the heat generated by friction can be reduced.

Zirconia ceramic valve heater

In order to make the engine burn completely when starting, a heating device, the valve heater, is installed on the suction side of the engine, which is used to heat the air so that the fuel vaporizes and mixes completely. In order to control the temperature and improve the reliability of the device, the barium titanate ceramic PTC (thermistor) is used as the valve heater. After adopting the ceramic valve heater, the engine is in full combustion state when it starts, so as to improve thermal efficiency, energy saving, and purification and exhaust efficiency.

Zirconia ceramic engine

The application of special ceramics in the automobile has been popularized by the piston engine, and there will also be an auxiliary combustion chamber, piston head, cylinder liner, cylinder head, pressurized rotor, etc. Special ceramic materials such as silicon nitride, silicon carbide, and partially stabilized zirconia are also being considered for these parts.

Zirconia ceramic engine
Zirconia ceramic engine

Zirconia ceramic sensor

The shock absorber of the high-class car is a smart shock absorber which is developed by using the positive piezoelectric effect, inverse piezoelectric effect and electrostrictive effect of sensitive ceramics. The smart shock absorber, with its ability to recognize and self-regulate the road, minimizes the vibration of cars on rough roads, making them comfortable for the passenger.

Intelligent ceramic wipers

The intelligent ceramic windshield wiper is made of barium titanate, which can automatically sense rainfall and adjust the windshield wiper to the best speed. Some other ceramic sensing elements, such as thermal, pressure, humidity and magnetic ceramic materials, can also be sensitive to temperature, humidity, condensation, anti-freezing, etc. with automatic control and adjustment.

In addition, many parts, and small devices used in automobiles are made of special ceramic materials, such as the electronic buzzer, ultrasonic vibrator, heat absorbing glass, photocell, oil plug ring, oil seal, etc. These kinds of automobile products made of new special ceramic materials generally have high physical and chemical properties, such as anti-seismic, wear-resisting, anti-corrosion, high-temperature resistant, lightweight and easy to process and produce.

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