3 Types of Zirconium Based Getter Materials

Zirconium-based getter material refers to the alloy with high absorption active gas characteristics formed by adding other elements based on zirconium.

Before sealing in vacuum tubes and devices, the material must be heated and activated under vacuum conditions for fast and effective gettering. The function of activation is to remove the passivation film formed on the surface during the manufacturing and storage process to expose the fresh surface, which is conducive to the overall gettering, so as to achieve the purpose of absorbing a large amount of oxygen, nitrogen, hydrogen, carbon monoxide, carbon dioxide, hydrocarbons, and water vapor.

Zirconium-aluminum alloy, zirconium-graphite, and zirconium-vanadium-iron alloy are widely used zirconium-based getter materials today.

Zirconium-aluminum alloy getter

Zirconium-aluminum alloy getter can be made into ring-shaped material and composite strip-shaped material.

(1) Ring-shaped material. The material has poor gettering performance at room temperature and is usually not used for gettering at room temperature. This material is commonly used in electronic tubes, various vacuum devices, special lamps, inert gas purification, zirconium-aluminum getter pumps, etc.

(2) Composite strip material. The advantage is that the amount of mercury can be accurately controlled, and it does not decompose or generate mercury vapor below 500°C, thereby greatly reducing environmental pollution, preventing workers from mercury poisoning, and improving lamp quality and life. It has been widely used in fluorescent lamps and energy-saving lamps.

ZR1422 Zirconium Aluminum Alloy, ZrAl Alloy
ZR1422 Zirconium Aluminum Alloy, ZrAl Alloy

Zirconium graphite getter

Zirconium graphite getter is often used in high-reliability and long-life vacuum tubes and devices for long-term operation and storage, such as traveling wave tubes, X-ray tubes, trigger tubes, ceramic tubes, and laser tubes.

Zirconium-vanadium-iron alloy getter

Zirconium-vanadium-iron alloy getter is a low-temperature activated getter material composed of zirconium, vanadium, and a small amount of iron. It is divided into two types:

(1) Zirconium vanadium ferroalloy getter material, smelted by 70%zr+24.6%V+5.4%Fe in electric arc furnace or medium frequency induction furnace under vacuum or filled with inert gas, then crushed, pulverized, and then pressed into getter elements.

(2) (Zirconium vanadium ferro)/zirconium getter material. It is made by adding the zirconium vanadium ferroalloy powder prepared in (1), adding zirconium powder in a certain proportion, mixing evenly, and then pressing, high temperature and high vacuum sintering and other processes. into a suction element. Product forms are powder, flakes, rings, and strips.

These two zirconium-vanadium-iron alloy getters are low-temperature activated getters, and the activation process is as follows: the temperature is 400-600°C, the vacuum degree is 10-2-10-4Pa, and the maintenance is 10-30min. The working temperature is from room temperature to 350℃.

Zirconium-vanadium-iron alloy getter is widely used in stainless steel vacuum insulated cups (bottles), solar vacuum water heaters, high-efficiency oil-insulated pipes, and vacuum tube containers that are only allowed to operate at 500°C.

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Zirconium Carbide and Its Composite Functional Materials

Introduction

Zirconium carbide, with the chemical formula ZrC, has a theoretical carbon content of 11.64%. It belongs to the typical NaCl type face-centered cubic structure. The atomic radius ratio of C atoms and Zr atoms is 0.481, which is less than 0.59, forming a simple interstitial phase. The Zr atoms form a compact cubic lattice, and the C atoms are located in the octahedral interstitial positions of the lattice.

The melting point of zirconium carbide is 3540℃, the theoretical density is 6.66g/cm3, and the thermal expansion coefficient is 6.7×10-6℃-1. It is insoluble in hydrochloric acid, but soluble in nitric acid. Zirconium carbide is a key material for the preparation of high-performance cemented carbide, aerospace, atomic energy, textiles, electronics, coatings, hard films and metallurgical automation and other high-tech fields.

ZR1394 Zirconium Carbide (ZrC) Powder

Advantages

Zirconium carbide has the advantages of high surface activity, high temperature resistance, oxidation resistance, high hardness, good thermal conductivity, good toughness, etc., and has the characteristics of efficient absorption of visible light, a reflection of infrared rays, and energy storage. It is an important high-temperature structural material.

Using ultra-high-purity zirconium dioxide and high-purity carbon black as raw materials, and applying core technology and alloying and sintering technology to prepare, can ensure the purity, low oxygen content, and low free carbon of zirconium carbide powder. The prepared ZrC powder has densified grains, stable phase composition, uniform particle size, and stable quality.

Application

1. Zirconium carbide is added to rubber, plastics, polyethylene, acrylonitrile-butadiene-styrene copolymer ABS plastics, transparent plastics, resins, polyurethane materials, and other materials for manufacturing related products. As an additive, zirconium carbide can greatly improve the strength, high-temperature resistance, and drop resistance of plastics and related materials.

2. Adding a certain proportion of zirconium carbide to Zr-Ti alloy, C/C-(Zr-Ti-C-B/SiC) composite material, and Zr-Ti-C-B ceramic material can be made into a ceramic coating resistant to 3000℃ ablation and its composite materials. The composite material made in this way exhibits superior ablation resistance and thermal shock resistance and is a new type of material for key components of hypersonic aircraft, which is now widely used in the military and aerospace fields.

3. Zirconium carbide has the characteristics of heat absorption and heat storage. Therefore, it can be used to manufacture solid propellants in rocket engines, to produce metal zirconium and zirconium tetrachloride, and as abrasive.

4. Zirconium carbide is used for U-shaped ZrC-graphite composite ceramic combined heating element. This heating element has high heating efficiency, good energy saving effect, small occupied volume, low cold end temperature, and stable electrical performance; under vacuum, neutral or reducing atmosphere, it can provide a high-temperature environment above 2000 ℃; it has good It has excellent thermal shock resistance, high thermal efficiency, and fast heating rate, and can be raised from room temperature to 2000 ° C in 120 minutes; it can be used for thermal shock resistance test of ultra-high temperature refractory materials.

5. Zirconium carbide is used for zirconium carbide composite ceramic sensors. This sensor has high mechanical strength, is not easy to deform and volatilize at high temperatures, and has stable electrical performance and long service life; in a vacuum or protective atmosphere, it can more accurately measure ultra-high temperature ambient temperature below 3000 °C; it is the temperature sensing element with the highest temperature measurable in the contact sensor.

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Zirconium – A Vacuum Material

Properties of Zirconium

Zirconium easily absorbs hydrogen, nitrogen, and oxygen; zirconium has a strong affinity for oxygen, and oxygen dissolved in zirconium at 1000°C can significantly increase its volume. The surface of zirconium is easy to form an oxide film with luster, so its appearance is similar to that of steel. Zirconium is resistant to corrosion but is soluble in hydrofluoric acid and aqua regia. At high temperatures, zirconium can react with non-metallic elements and many metal elements to form solid solutions. Zirconium has good plasticity and is easy to be processed into plates, wires, etc. Zirconium can absorb a large amount of oxygen, hydrogen, nitrogen, and other gases when heated, and can be used as a hydrogen storage material. The corrosion resistance of zirconium is better than that of titanium, and it is close to niobium and tantalum. Zirconium and hafnium are two metals with similar chemical properties that are symbiotic together and contain radioactive substances.

Applications of Zirconium

Like lithium and titanium, zirconium can strongly absorb nitrogen, hydrogen, oxygen, and other gases. When the temperature exceeds 900 degrees Celsius, zirconium can absorb nitrogen violently; under the condition of 200 degrees Celsius, 100 grams of metal zirconium can absorb 817 liters of hydrogen, which is equivalent to more than 800,000 times that of iron. This characteristic of zirconium makes it widely used in the electric vacuum industry. People use zirconium powder to coat the surface of the anode and other heated parts of electric vacuum components and instruments to absorb residual gas in vacuum tubes. The high vacuum tubes and other electric vacuum instruments made in this way have high quality and long service life.

high vacuum tubes

Zirconium has a small thermal neutron capture cross-section and has outstanding nuclear properties, so it is an indispensable material for the development of the atomic energy industry and can be used as a reactor core structural material. Zirconium powder is easy to burn in the air and can be used as a detonator and smokeless powder. Zirconium can be used as an additive for deoxidation and desulfurization of high-quality steel and is also a component of armor steel, cannon steel, stainless steel, and heat-resistant steel.

Zirconium can also be used as a “vitamin” in the metallurgical industry to exert its powerful deoxidation, nitrogen removal, and sulfur removal effects. Adding 1/1000 zirconium to steel will increase the hardness and strength amazingly; zirconium-containing armored steel, stainless steel, and heat-resistant steel are important materials for the manufacture of defense weapons such as armored vehicles, tanks, cannons, and bulletproof panels. When zirconium is mixed into copper and drawn into copper wire, the conductivity is not weakened, while the melting point is greatly improved, which is very suitable for high-voltage wires. Zirconium-containing zinc-magnesium alloy is light and resistant to high temperatures, and its strength is twice that of ordinary magnesium alloys. It can be used in the manufacture of jet engine components.

Zirconium powder is characterized by a low ignition point and fast burning speed and can be used as a primer for detonating detonators, which can explode even underwater. Zirconium powder plus oxidant is like adding fuel to the fire, it burns with strong light and dazzling, and it is a good material for making tracer and flare.

Zirconium alloys and their applications

Zirconium alloy is a non-ferrous alloy composed of zirconium as the matrix and other elements are added. The main alloying elements are tin, niobium, iron, and so on. Zirconium alloy has good corrosion resistance, moderate mechanical properties, low atomic thermal neutron absorption cross-section in high temperature and high-pressure water and steam at 300-400 °C, and has good compatibility with nuclear fuel. In addition, zirconium alloy has excellent corrosion resistance to various acids, alkalis, and salts, and has a strong affinity with oxygen, nitrogen, and other gases, so it is also used in the manufacture of corrosion-resistant parts and pharmaceutical machinery parts. For example, it is widely used as a non-evaporable getter in the electric vacuum and light bulb industries.

zirconium alloy

There are two types of zirconium alloys produced on an industrial scale: the zirconium-tin series and the zirconium-niobium series. The former alloy grades are Zr-2 and Zr-4, and the typical representative of the latter is Zr-2.5Nb. In zirconium-tin alloys, the alloying elements tin, iron, chromium, and nickel can improve the strength, corrosion resistance, and thermal conductivity of the corrosion-resistant film, and reduce the sensitivity of the surface state to corrosion. Usually, Zr-2 alloys are used in boiling water reactors, and Zr-4 alloys are used in pressurized water reactors. In zirconium-niobium-based alloys, the corrosion resistance of the alloy is the best when the addition amount of niobium reaches the solid solution limit of the crystal structure of zirconium at the service temperature. Zirconium alloy has isomorphous transformation, the crystal structure is body-centered cubic at high temperature, and hexagonal close-packed at low temperature. Zirconium alloy has good plasticity and can be made into pipes, plates, bars and wires by plastic processing; its weldability is also good and can be used for welding.

Other Zirconium Compounds

Zirconium dioxide and zircon are the most valuable compounds in refractory materials. Zirconium dioxide is the main material of new ceramics and cannot be used as a heating material that resists high-temperature oxidation. Zirconium dioxide can be used as an additive for acid-resistant enamel and glass, which can significantly improve the elasticity, chemical stability, and heat resistance of glass. Zircon has a strong light reflection performance and good thermal stability and can be used as sunscreen in ceramics and glass. Zirconium can absorb a large amount of oxygen, hydrogen, ammonia, and other gases when heated, and is an ideal getter. For example, zirconium powder is used as a degassing agent in electronic tubes, and zirconium wire and zirconium sheets are used as grid supports and anode supports.

Powdered iron mixed with zirconium nitrate can be used as glitter powder. Zirconium metal is used almost exclusively as the cladding for uranium fuel elements in nuclear reactors. It is also used to make photographic flashes, as well as corrosion-resistant containers and pipes, especially hydrochloric and sulfuric acids. Zirconium chemicals can be used as crosslinking agents for polymers.

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Applications of Zirconium Silicate Grinding Media

Zirconium Silicate is a high-quality and inexpensive opacifier with a high refractive index of 1.93-2.01 and chemical stability. It is widely used in the production of various ceramics. Besides, Zirconium Silicate has a high melting point, so it is also widely used in refractory materials, zirconium ramming materials for glass furnaces, casting materials, and spray coatings.

The zirconium silicate media ball is one of its kind, offering users the highest quality and superior grinding levels with improved abrasion resistance, better cost-effectiveness and lower overall contamination rates. Zirconium silicate beads are formulated in strict quality-controlled laboratory containers, in which they undergo specialized instillation techniques, followed by high-temperature sintering and final surface treatment. Compared to other alternative grinding media options such as glass beads or alumina, this ultra-hard media is an ideal solution for grinding special and complex products.

Zirconium Silicate Grinding Media
Zirconium Silicate Grinding Media

The basic characteristics of a good quality zirconium silicate grinding media are that they are high in density, shiny and smooth in appearance, and consist of a uniform solid spherical shape which in turn assures better efficiencies, decreased media wear, and a much longer life span respectively. Additional specialized techniques such as solidifying the media from surface to center result in further strengthening of the molecular structure of ZrSi beads. Zirconium silicate media balls exist in varying sizes and diameters in accordance with each buyer’s prerequisites.

ZrSi04 applications and uses are tremendous and widespread from everyday products such as paints and inks to ceramics, pharmaceuticals, and even in controlled quantities within edible food materials. Zirconium Silicate grinding media plays an integral role as an emulsion agent in order to achieve a ceramic glaze in refractory’s and on cutlery etc. Also being chemically inert and nonreactive allows ZiSi04 media to be used for grinding plastic on a mass level and at economical costs. Moreover, zirconium casting refractories of all kinds utilize this media for operational purposes within glass melting furnaces, cement production and heat/fire resistant porcelain among many others. On a generalized level, Zirconium Silicate grinding media performs numerous operations including mold cleaning of stainless steel, plastic as well as nonferrous materials, mechanical polishing, buffing and eventual after-cleaning processing.

On an overall rating scale, the benefits of this industrial product being extremely dense and strong results in creating an ideal surface roughness and metallic depth with a much lower breakage or contamination rate comparatively. These attributes in turn render Zirconium Silicate milling balls suitable for application on all types of materials and within both wet and dry environments easily.

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Two Surface Treatment Technologies for Zirconium Materials

The surface of the zirconium rod and zirconium alloy must be clean and smooth before joining, heat treatment, electroplating and forming. This article introduces 2 types of surface treatment methods for zirconium materials.

  1. Surface decontamination

Grease, oil, and lubricants produced during zirconium machining or other processing can be removed in a number of ways. Commonly used cleaning methods are

1) cleaning with alkaline or milky detergent in a soaking tank;

2) cleaning with ultrasonic vibration;

3) rinsing with acetone or trichloroethylene or steam degreasing and

4) cleaning with other cleaning agents.

Small stains can also be removed by hand wiping with some solvents such as acetone, alcohol, trichloroethylene, or a trichloroethylene substitute. In the electrolyte system, if the voltage and current can be controlled to avoid anodic polarization or spark discharge and pitting, positive or negative polarity decontamination can be used. Before heat treatment and bonding, the surface of the zirconium material must be cleaned to prevent metal contamination and the resulting deterioration of ductility.

Surface Treatment Technologies for Zirconium Materials

  1. Blast cleaning

Mechanical decontamination methods such as sandblasting, shot blasting, and evaporative cleaning can remove dirt and lubricants from zirconium and hafnium surfaces. Alumina, silicon carbide, silica and steel grit are ideal media for mechanical decontamination. The decontamination medium used should be replaced regularly to avoid increased workload due to particle passivation.

Grinding or shot peening may cause residual compressive stress and thermal deformation on the surface of the material, especially the surface of the sheet. Hot deformation may also occur during subsequent rolling and profile machining.

Blast cleaning is not a substitute for pickling. Blast cleaning cannot remove surfaces contaminated with interstitial elements such as carbon, oxygen, and nitrogen. In general, blast cleaning followed by pickling can ensure the complete removal of surface contamination and cold-worked layers, resulting in a smooth, shiny metal surface.

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Application of Zirconium Silicate in Ceramic Industry

Introduction of Zirconium Silicate

Zirconium silicate is a non-toxic, odorless white powder. It is usually made of natural high-purity zircon sand concentrate, which needs to be processed by ultra-fine grinding, iron removal, titanium processing, surface modification treatment and other processes. Zirconium silicate powder is a high-quality and inexpensive ceramic glaze opacifier, brightener, anti-seepage agent and stabilizer.

Main roles of Zirconium Silicate

  1. Improve the hardness of ceramic glaze

Zirconium silicate has good chemical stability, and can significantly improve the separation performance of ceramic glaze and improve the hardness of ceramic glaze;

  1. Whitening effect

Zirconium silicate powder can whiten ceramic glazes.

Application of zirconium silicate in the traditional ceramic industry

Zirconium silicate is mainly used for high-temperature opaque glaze in daily ceramics and sanitary ware.

Application of zirconium silicate

Many companies add a small amount of zirconium silicate or zircon powder to polished tiles and glazed products to increase stability.

Another function of zirconium silicate in the traditional ceramic glaze is to increase the hardness of the ceramic glaze and improve its wear resistance of the glaze. Zirconium silicate is generally used in raw glazes with little or no zircon powder. Compared with zircon powder, zirconium silicate powder is finer and brighter.

Engobe generally uses zirconium silicate, which can increase the whiteness of the engobe and adjust its expansion coefficient and stability.

Zirconium silicate has a good effect when added to the medium and high-temperature glaze of raw materials. A certain amount of zirconium silicate is generally added to the high-gloss and matt glazes of sanitary ware and glazed porcelain tiles.

Conclusion

Zirconium silicate is a high-quality and inexpensive opacifier, which is widely used in various architectural ceramics, sanitary ceramics, daily-use ceramics, and first-class ceramics. Zirconium silicate has also been further used in the production of color picture tubes in the TV industry, emulsified glass in the glass industry, and enamel glaze production. Zirconium silicate is also widely used in refractory materials, glass furnace zirconium ramming materials, castables and spray coatings due to its high melting point.

How to Ensure the Welding Quality of Zirconium Alloy

In the previous article, we introduced the basic properties of zirconium alloys and the problems that easily occur during the welding process. Next, we will explain how to ensure the welding quality of zirconium alloys and some precautions.

Precautions for zirconium alloy welding

  • In the welding prefabrication stage of a large number of welds, a special closed clean place needs to be set up, and strict control of environmental dust pollution and air humidity. For example, when entering the construction site, measures such as wearing clean labor insurance shoes must be worn to ensure the cleanness of the welding environment. In the outdoor installation environment, make a temporary operating room to achieve clean conditions.
  • Strengthening the requirements for the weld joint groove and within 70mm of both sides of the groove and the cleanliness of the surface of the welding wire is an important factor to ensure the welding quality.
  • In the welding process of zirconium alloy, pores are the most prone to defects, and it is mostly concentrated near the fusion line and the centerline of the weld. The most critical steps to prevent the occurrence of welding porosity defects are to strengthen the control of the cleanliness and humidity of the welding environment, and to enhance the cleaning of the bevel and the surface of the welding material, so as to improve the quality of the internal and external protection of high purity argon in the weld zone.
  • The zirconium alloy has a low thermal expansion coefficient, a small amount of thermal deformation, and a small volume change during phase change. It has a low content of impurities such as sulfur, phosphorus, and carbon, so there is no obvious tendency to form cracks during welding. However, when the welding seam absorbs a certain amount of oxygen, nitrogen and hydrogen gas impurities, the performance of the welding seam and the heat-affected zone will become brittle. If there is stress in the weld in the peer group, a cold crack will occur. In addition, the hydrogen atoms have the property of diffusing and accumulating to the high-stress parts in the heat-affected zone at a relatively low temperature, which promotes the formation of relatively weak links in these parts, which may lead to the occurrence of delayed welding cracks.
  • In the welding test, manual tungsten argon arc welding with low welding line energy and convenient gas welding protection should be selected; The larger-diameter welding torch nozzle, the outer surface of the weld seam, and the internal argon filling method of the pipe are used for air isolation to achieve the purpose of the weld seam not being oxidized and absorbing harmful gases.
  • The filler wire used for zirconium alloy welding should be selected according to the principle of matching the composition of the base metal. The surface of the welding wire must be free from defects such as heavy skin, cracks, oxidation, and metal or non-metallic inclusions. The welding wire should be cleaned and dried before use.
  • Zirconium alloy tungsten arc welding requires high-purity argon with a purity of not less than 99.999%, and its impurity content meets the requirements of the current GB / T4842 standard. Due to the extremely high requirements for the purity of the welding protective gas, the welding process needs to be continuously inflated and cannot be interrupted halfway, otherwise, the argon filling must be replaced again. The method of using an ordinary single bottle of argon direct gas supply cannot meet the protection requirements. Multiple bottles of argon gas need to be connected in series to increase the gas supply capacity, and multiple welders can be operated simultaneously by dividing the cylinder.
  • Because zirconium alloys are active at high temperatures, relying solely on the argon gas supplied by the argon arc welding torch nozzle to protect the molten pool and high-temperature bead and heat-affected zone during welding cannot guarantee the welding quality. In order to ensure that the requirements for gas isolation in high-temperature areas and prolonged argon protection time are met, special external gas protection devices for pipes must be added to provide high-purity argon isolation protection for weld pools, high-temperature weld beads and heat-affected zones at high temperatures.

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Problems Prone to Welding of Zirconium Alloys at High Temperatures

Zirconium is an expensive corrosion-resistant metal material with excellent resistance to corrosion by acids and alkalis. In some media, it even exceeds metals with good corrosion resistance such as niobium and titanium. Zirconium alloys have been gradually used in recent years as structural materials for equipment and pipelines in the chemical industry due to their good corrosion resistance.

The commonly used zirconium alloy grades include Zr702 (UNSR60702), Zr704 (UNSR60704), and Zr705 (UNSR60705). Among them, Zr702 (UNSR60702) is widely used in chemical projects.

Basic characteristics of zirconium alloy

Zirconium alloy has good welding performance, stable chemical properties at room temperature, and outstanding corrosion resistance. However, its high-temperature chemical properties are lively and have a strong affinity for the pollution of oxygen, nitrogen, and hydrogen in the ambient gas, and dust and humidity in the operating environment. As the temperature rises, its chemical activity sharply increases, and it forms ZrH2 with hydrogen at 200 ℃; it can form ZrO3 with oxygen at 300 ℃; it reacts with oxygen in the air above 550 ℃ to form a porous brittle oxide film; at 600 ° C, zirconium absorbs nitrogen to form ZrN; it absorbs oxygen and severely embrittles the material at above 700 ℃. As the temperature increases, its absorption capacity and reaction speed increase. Therefore, the high temperature environment and welding seams generated by welding are the keys to restrict chemical equipment.

The excellent corrosion resistance of zirconium alloys comes from the oxide film formed on its surface and depends on the integrity and robustness of the oxide film. When zirconium alloy absorbs a certain amount of oxygen, nitrogen, hydrogen and other gas impurities, its mechanical properties and corrosion resistance will drop sharply. Therefore, strengthening the protection of environmental dust, humidity and heat-affected zone surfaces and the back of welds is a key element of quality control during welding.

Problems prone to welding of zirconium alloys

High temperature is the natural enemy of zirconium alloys with great changes in corrosion performance. Zirconium generally reacts easily with the atmosphere at high temperatures. It starts to absorb oxygen at 200 ℃, hydrogen at 300 ℃, and nitrogen at 400 ℃. The higher the temperature, the more intense the reaction. Because zirconium is active against oxygen, nitrogen and hydrogen, it must be protected with a high-purity inert gas or welded in a good vacuum chamber.

During zirconium welding, the weld seam and heat-affected zone are easily polluted by oxygen, hydrogen, nitrogen and other elements in the air, forming hard and brittle compounds, and producing a brittle needle-like structure, which increases the hardness and strength of the welded joint , while the plasticity declines, and the corrosion resistance is also greatly reduced. Therefore, zirconium welding should fully protect the molten pool, weld and heat-affected zone to completely isolate the air.

The welding of zirconium alloys is generally performed by the welding method of tungsten inert gas shielded arc. Other welding methods include electron beam welding, plasma arc welding and resistance welding. Its welding performance is close to that of titanium metal welding. Due to the small thermal expansion coefficient and elastic modulus of zirconium, the welding deformation and weld residual stress are relatively small. It is recommended that the stress relief time of the weld at 1100 ° F (594 ℃) be 1 hour/inch thickness.

Another major problem of zirconium welding is that the weld is prone to soften too much and cause the weldment to be distorted. When welding zirconium, the welding piece should be properly fixed and double-sided welding should be used as much as possible. Except for titanium, niobium, silver, and vanadium, zirconium cannot be directly welded to other metals. Therefore, choosing a clean operating environment and strengthening the isolation and protection of welds and heat-affected zones are the keys to ensuring the quality of zirconium alloy welding.

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Basic Info | Toughening Methods of Zirconia Ceramics

Zirconia (ZrO2) ceramics are special ceramics with unique physical and chemical properties, and their applications in electronic ceramics, functional ceramics and structural ceramics have developed rapidly. However, the fatal shortcomings of zirconia ceramic materials are brittleness, low reliability, and low repeatability, which seriously affect its application range. Only by improving the fracture toughness of zirconia ceramics, strengthening the material and improving its reliability and service life, can zirconia ceramics truly become a widely used new material.

Toughening technology of zirconia ceramics has been a hot spot in ceramics research. At present, ceramic toughening methods mainly include phase change toughening, particle toughening, fiber toughening, self-toughening, diffusion toughening, synergistic toughening, and nano-toughening, etc.

Phase toughening

Phase toughening refers to the metastable tetragonal phase t-ZrO2 undergoing a phase change under the action of the stress field at the crack tip, forming a monoclinic phase, resulting in volume expansion, thereby forming compressive stress on the crack, hindering crack growth, and increasing the role of toughness. In addition, external conditions (such as laser shock, fatigue fracture toughness, low temperature, grain size and content, critical transition energy, etc.) have a great effect on the phase toughening of zirconia ceramics. If the phase transition produces large stress and volume changes, the product is prone to fracture. Therefore, the influence of external factors on the phase toughening of zirconia ceramics should be avoided during production.

Particle toughening

Particle toughening refers to the method of using particles as a toughening agent and adding it to ZrO2 ceramic powder. Although its effect is not as good as whiskers and fibers, if the particle type, particle size, content and matrix material are properly selected, there is still a certain strong effect. The advantage is that it is simple and easy to implement, and it will also improve the high-temperature strength and high-temperature creep performance while toughening. The toughening mechanism of particle toughening mainly includes the refinement of matrix grains and crack-turning bifurcation.

Fiber toughening

The principle of fiber and whisker toughening is that the crystal close to the crack tip adds closing stress to the crack surface due to deformation, offsets the external stress at the crack tip, and passivates the crack propagation, thereby strengthening the toughness. In addition, when cracks are propagated, the frictional force must be overcome when the columnar crystals are pulled out, which also plays the role of toughening.

Self-toughening

Due to the existence of columnar crystals, cracks will be deflected during the fracture process of zirconia ceramics, which will change and increase the path of crack growth, thereby passivating the cracks, increasing the crack growth resistance and achieving toughening.

Diffuse toughening

Diffusion toughening mainly refers to the toughening of the ceramic matrix by the tetragonal ZrO2 particles. In addition to the phase toughening mechanism, there is also a diffusion toughening mechanism of the second phase particles. Before cracks propagate, the internal residual strain energy of the ceramic itself must first be overcome to achieve the purpose of toughening.

Microcrack toughening

Micro-crack toughening refers to adding a tough material at the crack stress tip to cause micro-cracks to achieve the purpose of dispersing stress, reducing the force of crack advance, and thereby increasing the toughness of the material. When a material undergoes a phase transition, it often results in residual strain energy effects and microcracks. Therefore, the effect of phase transition toughening is significant.

Composite toughening

Composite toughening refers to the simultaneous use of several toughening mechanisms during the actual toughening of ZrO2 ceramics, thereby improving the toughening effect of ZrO2 ceramics. In the actual application process, the specific toughening mechanism is selected according to the different properties of the zirconia ceramic material to be prepared.

Zirconia Toughened Alumina

Nano toughening

At present, there are three main academic viewpoints of nano-toughening, namely: the theory of refinement, trans-crystalline, and “pinning”.

  • The refinement theory believes that the introduction of nano-phases can suppress the abnormal growth of the matrix grains, refine the matrix structure uniformly, and improve the strength and toughness of the nano-oxide ceramic composites.
  • The trans-crystalline theory holds that in nanocomposite materials, the matrix particles are densified with the nanoparticles as the core, and the nanoparticles are encapsulated inside the matrix grains to form an “intracrystalline” structure. In this way, the effect of the main grain boundary can be weakened, transgranular fracture is induced, and transgranular fracture instead of intergranular fracture occurs when the material is fractured, thereby improving the strength and toughness of the nano-zirconia ceramic composite material.
  • The “pinning” theory believes that the nanoparticles existing in the grain boundaries of the matrix produce a “pinning” effect, which limits the occurrence of grain boundary slippage, pores, and creep. The enhancement of grain boundaries leads to the improvement of the toughness of nano-zirconia multiphase ceramic.

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What Are the New Sintering Methods of Zirconia Ceramics?

With the continuous development of science, sintering methods of zirconia ceramics are continuously introduced.

Electric field sintering

Electric field sintering refers to the sintering of the ceramic body under the action of a DC electric field. Some high-curie-point ferroelectric ceramics, such as lithium niobate ceramics, apply a DC field to both ends of the green body at their sintering temperature. After cooling to a temperature below the Curie point (Te-1210 ℃) and removing the electric field, you can obtain Piezoelectric ceramic samples.

Sintering process

 

Ultrahigh pressure sintering

Ultra-high pressure sintering is sintering at a pressure of several hundred thousand atmospheres or more. Its characteristics are that it cannot only make the material reach high density quickly, and have fine grains (less than 1um), but also change the crystal structure and even the atomic and electronic states, so that the material cannot be reached under the usual sintering or hot-pressing sintering process Performance, and can synthesize new artificial minerals. This process is relatively complicated and requires higher mold materials, vacuum sealing technology, and fineness and purity of raw materials.

Activated sintering

The principle of activated sintering is to use some physical or chemical methods to make the atoms or molecules of the reactants in a high-energy state before or during sintering. With the instability of this high-energy state, it is easy to release energy Low energy state. The physical methods used in activated sintering include electric field sintering, magnetic field sintering, sintering under the action of ultrasound or radiation, and so on; the chemical methods used are: chemical reactions based on redox reactions, dissociation of oxides, halides, and hydroxides, and atmospheric sintering. Activated sintering has the advantages of reducing the sintering temperature, shortening the sintering time, and improving the sintering effect.

For some ceramic materials, activated sintering is another effective texturing technique. There is also the use of substances in the phase change, dehydration and other decomposition processes, the atom or ion bond is destroyed, making it in an unstable active state. For example, increase the specific surface area; add substances that can generate new erbium molecules during the sintering process; add substances that can promote the sintering material to form a solid solution; increase the lattice defect substances, all of which are activated sintering. In addition, activated sintering also includes adding a small number of substances that can form an active liquid phase, promote the vitrification of materials, appropriately reduce the viscosity of the liquid phase, wet the solid phase, and promote solid-phase dissolution and recrystallization.

Activated hot sintering

Activated hot-pressing sintering is a new process developed on the basis of activated sintering. It utilizes an activated state with higher energy during the decomposition reaction or phase change of the reactants to perform hot-pressing treatment, which can be performed at lower temperature and lower pressure. It is a high-efficiency hot pressing technology to obtain high-density ceramic materials in a short time. For example, barium titanate, lead zirconium titanate, ferrite, and other electronic ceramics are made by hot pressing by the decomposition reaction of hydroxide and oxide; high-density beryllium oxide, thorium oxide and uranium oxide ceramics were prepared by hot pressing of carbonate decomposition reaction; high-density alumina ceramics are made by hot pressing during phase transition of some materials.

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