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Biomaterials used in orthodontics: brackets, archwires, and clear aligners

May 31,2022 | LOTTEDS-Bob

 

Biomaterials used in orthodontics: brackets, archwires, and clear aligners

Mohamed-Nur Abdallah, ... Sunjay Suri, in Advanced Dental Biomaterials, 2019

20.3.2.1 Gold

Gold alloys were initially used to fabricate archwires because they are inert, biocompatible, and stable. Furthermore, before the widespread use of the SS wire, gold alloys were used due to the lack of the availability of other materials that can tolerate the oral conditions. Gold wires are generally composed by of gold (15–65 wt.%), copper (11–15 wt.%), silver (10–25 wt.%), palladium (5–10 wt.%), platinum (5–10 wt.%), nickel (1–2 wt.%), and traces of zinc. Table 20.7 summarizes the advantage and disadvantages of these wires. To the best of our knowledge, archwires made of gold alloys are not currently used in the clinical orthodontic practice.

Table 20.7. Advantages and disadvantages of gold wires.

Advantages Disadvantages

Low modulus of elasticity

Strength can be increased by heat treatment or cold working

High formability

Excellent biocompatibility

Good environmental stability

Can be soldered or welded

Low springback

Low yield strength

Relatively expensive

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URL: https://www.sciencedirect.com/science/article/pii/B9780081024768000207

Overview of metals and applications

T. Hanawa, in Metals for Biomedical Devices (Second Edition), 2019

1.7 Noble metals and alloys

Au alloys and Ag alloys are used for dental restoratives such as inlays, crowns, bridges, and clasps. Dental casting Au alloys are the Au-Ag-Cu system alloys, which have high corrosion resistance, high ductility, good castability, and cast fitness. Au alloys are categorized into four types, according to their Au content and mechanical properties (Tables 1.7 and 1.8). A type 1 alloy contains the most Au and is used for simple inlays; the type 2 alloy is used for complex inlays and crowns; the type 3 alloy is used for crowns and bridges; and type 4 alloy is used for denture bases, clasps, and bridges that require strength. Type 3 and 4 alloys could be hardened by heat treatment. zinc (Zn) is added to them as a deoxidizer.

Table 1.7. Standardized dental casting gold alloys

Type Heat treatment Vickers hardness (HV) Proof strength (MPa) Elongation (%)
1 Softened 50–90 > 80 > 18
2 Softened 90–120 > 180 > 12
3 Softened 120–150 > 240 > 12
4 Softened > 150 > 300 > 10
Hardened > 220 > 450 > 2

Table 1.8. Composition ranges of dental casting gold alloys

Type Composition (mass%)
Au Ag Cu Pd Pt Zn
1 80.2–95.8 2.4–12.0 1.6–6.2 0–3.6 0–1.0 0–1.2
2 73.0–83.0 6.9–14.5 5.8–10.5 0–5.6 0–4.2 0–1.4
3 71.0–79.8 5.2–13.4 7.1–12.6 0–6.5 0–7.5 0–2.0
4 62.4–71.9 8.0–17.4 8.6–15.4 0–10.1 0.2–8.2 0–2.7

Cross-sectional illustration of porcelain-fused-to-metal is shown in Fig. 1.11A. The composition of Au alloys used for porcelain-fused-to-metal repairs to teeth is listed in Table 1.9. This alloy has a high melting temperature, because the porcelain is sintered during casting. In and Sn are also added to increase the bonding strength with the porcelain. This alloy system contains no Cu and only a small amount of Ag, because these elements stain the alloy after porcelain sintering.

Fig. 1.11. Cross-sectional illustration of porcelain-fused-to-metal (A) and magnetic attachment system (B).

Table 1.9. Representative composition of gold alloy for porcelain-fused-to-metal (mass%)

Au Pt Pd Ag Others
85 8 4 1 2

In the case of bridges connecting several teeth, the cast parts are soldered together. Dental solders have a basic composition of Au-Cu-Ag with 58%–84% Au, and Zn and Sn are added to lower the melting point. Solder for porcelain-fused-to-metal contains Pt and palladium (Pd) to raise the melting point, inhibiting deformation during sintering of the porcelain.

Casting

Mitsuo Niinomi, in Metals for Biomedical Devices (Second Edition), 2019

11.3.1 High precious metals (Au alloys)

Dental casting Au alloys comprise Au, Ag, and copper (Cu), and contain at least 60% Au and 75% [Au + platinum (Pt) group metals (Pt, palladium (Pd))] by mass. They are classified into four types according to the amount of precious metals contained and mechanical properties. Table 11.1 (Takada, 2010) lists their chemical compositions. Types 1, 2, and 3 comprise the Au-Ag-Cu alloys and type 4 the Au-Pt alloy for dental casting. The amount of Cu, which lowers the melting point and improves hardness and strength, increases from type 1 to 4. The addition of Pt and Pd increases the strength and elastic modulus. A small amount of zinc (Zn) or iridium (Ir) is also added to Au alloys. Zn inhibits oxidation, but decreases corrosion resistance. Ir effectively refines the microstructure.

Table 11.1. Composition of Au alloys by type

Gold alloy Composition range (mass%)
Au Ag Cu Pt Pd Zn
Type 1 80.2–95.8 2.4–12.0 1.6–6.2 0–1.0 0–3.6 0–1.2
Type 2 73.0–83.0 6.9–14.5 5.8–10.5 0–4.2 0–5.6 0–1.4
Type 3 71.0–79.8 5.2–13.4 7.1–12.6 0–7.5 0–6.5 0–2.0
Type 4 62.4–71.9 8.0–17.4 8.6–15.4 0.2–8.2 0–10.1 0–2.7

Nanoporous gold for biomedical applications: structure, properties and applications

T.M. Martin, ... R.J. Narayan, in Nanomedicine, 2012

4.5 Dealloying of gold–silver alloy

Dealloying of the gold and silver alloy is usually accomplished in one of two ways: acid etching or electrochemical etching. Acid corrosion is a chemically straightforward process that is used for selective removal of silver. The most common method for dealloying with acid alone involves immersing the sample in concentrated nitric acid (69% HNO3) for a specified period of time. Effective etching of the alloy can be performed in a short amount of time, usually between 2 and 10 min (Quan et al., 2011). Work by Biener et al. (2006) has shown advantages to keeping the alloy in solution for two to five days. After being removed from the acid, the sample is extensively washed with deionized water to remove the remaining acid and reaction byproducts. Nitric acid is commonly used for dealloying because silver is easily oxidized to a nitrate salt whereas gold is not. The result of free corrosion is a porous material with a pore size in the 10–15 nm range (Kertis et al., 2010). One way to modify nitric acidbased dealloying is to increase the temperature. Seker et al. (2007) showed that samples annealed at higher temperatures possessed higher relative density measurements, higher residual stress values and higher elastic modulus values. Another popular type of free corrosion was discussed by Cattarin et al. (2007), which involved using 1 M perchloric acid (HClO4) to remove the silver from the alloy over a period of approximately 3 h. One advantage of the free corrosion process is its simplicity. The disadvantage to this method is the lack of precise control over the pore size and inability to control the porosity beyond the surface of the material. There are two additional variables that can provide control over the dealloying process for acid corrosion. First, the amount of time is related to the size and depth of the pores that are created on the surface. Second, a higher concentration of acid will increase the size of the pores that are created during the etching process. Overall, nitric acid dealloying is the most common process for creating nanoporous gold because it is a straightforward process that provides consistent results.

Dealloying under electrochemical control has become more common in recent years; this approach provides tighter control over the terminal porosity and ligament length. Ligament size refers to the dimensions of the regions that surround the pores on the surface of the structure. Multiple setups and etching solutions can be used to obtain the nanoporous metal structure of interest. The most popular setup for the electrochemical cell involves etching a gold–silver alloy in which the alloy surface acts as the working electrode and a noble metal (platinum, gold, silver) acts as the counter electrode. The setup also has a Ag/Ag + reference electrode. The protocol generally applied to the cell is a constant voltage over a defined period of time. An important parameter indicative of etching is that the charge being passed through the cell is increasing at a generally linear rate. An important parameter for the etching process is the potential that is applied; one method for determining the potential is based on the oxidizing range for the metal that needs to be removed. The metal that is being oxidized is losing electrons to the working electrode and is being deposited onto the counter electrode. The general range for silver/gold dealloying is 1–2 V. Dealloying under electrochemical control provides enhanced control over the ligament size; structures obtained using this technique may possess ligament sizes down to 10 nm (Biener et al., 2006). In comparison, structures obtained using free corrosion may possess ligament sizes down to 25–50 nm (Fig. 4.5).

4.5. Scanning electron microscopy micrograph of sponge-like nanoporous gold material.

Source: Reprinted with permission from Biener J, Hodge A, Hayes J, Volkert C, Zepeda-Ruiz L, Hamza A and Abraham F (2006), ‘Size effects on the mechanical behavior of nanoporous Au’, Nano Letters, 6, 2379–2382. © 2006, American Chemical Society.

Snyder et al. (2008) completed a study involving free corrosion with nitric acid and electrochemical etching. One option for the etching solution is silver nitrate (0.1 M) because silver that is removed from the alloy will deposit on the counter electrode (Snyder et al., 2008). Ideally, the silver concentration does not dramatically change, providing consistent removal of the silver from the alloy. Perchloric acid can be used in place of nitric acid. Jin et al. (2009) used perchloric acid as well as an electrochemical cell setup; a potential of 650 mV was applied for 17 h. Senior and Newman (2006) applied a potential of 1.1 V over a processing time of 30 s to an alloy sample; 0.77 M perchloric acid was used in this study. Senior and Newman (2006) also applied a potential of 1.0 V over a processing time of 120 s and applied a potential of 920 mV over a processing time of 3 h (Senior and Newman, 2006.) The combination of lower potential values and longer processing times was associated with a much more uniform gold structure. Biener et al. (2006) showed complete removal of silver from a gold-silver alloy using a potential of 1 V and an etching solution containing 1 M HNO3 and 0.01 M AgNO3. Similarly, Gittard et al. (2010) processed gold–silver alloy wires using an etching solution containing 1 M HNO3 and 0.01 M AgNO3 for approximately 30 min. The variability of the electrochemical procedures used in previous studies provides many options for future research. There are advantages and disadvantages to each electrochemical etching method; the most uniform pore size porous materials were generally created at lower potentials over longer periods of time (from many minutes to hours). It should be noted that electrochemical methods enable removal of 90-95% of the silver from the alloy. Electrochemical etching also provides an appropriate method to obtain controlled pore sizes as well as deep and long ligaments.

An alternative to using nitric acid or another type of acid for electrochemical processing involves using zinc chloride and dimethyl sulfoxide (DMSO). Dong and Cao (2009) demonstrated use of DMSO/ZnCl2 electrolyte for creating nanoporous gold; the addition of heat to the solution increased the speed of alloying and dealloying reactions. Post-treatment procedures using sulfuric acid were also demonstrated.

Metal injection molding (MIM) of precious metals

J.T. Strauss, in Handbook of Metal Injection Molding (Second Edition), 2019

25.5.1 Silver and gold alloys

MIM processing of silver and gold alloys is straight forward and very similar to MIM processing of conventional materials. The two primary differences are in binder formulation and tooling design.

With respect to binder formulation, of primary importance are (1) the binder must be removed at a relatively low temperature and (2) debinding must not leave any carbonaceous residue. A binder for silver and gold jewelry alloys must accommodate the alloys' low sintering temperatures and zero tolerance for carbon. Sintering of these alloys can initiate as low as 300°C (HJE Company, 1993). It is necessary that the binder be completely eliminated at these low temperatures or else residual binders will be trapped in the part. Related to this is that carbon is essentially insoluble in silver and gold jewelry alloys (McLellan, 1969). Unlike iron-based alloys, where residual carbon from the binder can be dissolved and diffused through the matrix to be subsequently removed in a low carbon potential sintering atmosphere, any residual carbon in a jewelry alloy compact will be trapped within the inter-powder microstructure with essentially no mechanism for its removal. This could impede sintering to full or usable density and the resulting pores or entrapped carbon could affect the cosmetics of the part. Wax-polymer binders suitable for copper MIM will be satisfactory for silver and gold jewelry alloys. Typical solids loading for gold and silver MIM feedstock can be between 57% and 67%, depending on the particle size distribution of the powder.

Thermal debinding can be done in air as long as the alloy does not contain alloying additions whose oxide cannot be reduced during the sintering cycle. In Section 25.3, zinc was stated to be necessary to provide acceptable colors in 14 karat and lower alloys. Zinc oxides will be problematic as the oxide does not reduce until above the sintering temperature range of these alloys. The copper in the silver or gold alloy will oxidize during thermal debinding but copper oxides will be reduced in the subsequent sintering in a reducing atmosphere.

Silver and gold alloys have higher-thermal conductivities than conventional materials. This enhanced heat transfer translates to the powder as well so the feedstock will have a higher-thermal conductivity and can better transfer heat to the tooling leading to faster cooling and solidification. This can be rectified by increasing the cross-sectional area of the runners and sprues and by using warm tooling or hot sprues if found necessary.

Sintering of these alloys is also straight forward. It has been found that a reducing atmosphere of nitrogen and hydrogen in almost any ratio is satisfactory for sintering. The sintering temperatures are alloy-dependent but tend to be within 100 degrees C of the solidus temperature.

Noble metal alloys for load-bearing implant applications

Alireza Nouri, Cuie Wen, in Structural Biomaterials, 2021

5.3.1 Classification of noble metal alloys in dentistry

Some alloys are formed into restorations by casting. After making a wax model of the restoration, an alloy is melted and cast into the shape of the wax, a process called precision or investment casting. Thus these alloys are referred to as dental casting alloys, and the restorations made from these alloys are castings [8]. Although dentures and other dental restorations have been made from metals for centuries, the technology of precision casting was not possible until the 20th century [11]. Historically, it was Taggart who introduced the cast inlay fabrication method in 1907. He practiced the “lost-wax (investment)” casting technique, and since then it has remained as an important technique for the fabrication of restorations [43,44]. For instance, casting is the best and most popular method for making crowns. The impression of the prepared tooth is replicated in a refractory mold, and the required sample is made from wax. Subsequently, the samples are placed into the investment material and burned. In this method, molten metal or alloy is cast under pressure by a centrifugal force [11]. Other alloys are first cast but then formed by mechanical means such as machining into their final shapes. These alloys are referred to as wrought alloys and include wires, files, and dental implants [8].

In dentistry, casting alloys are categorized in several ways. They can be divided into all-metal restorations, porcelain-fused-to-metal (PFM) restorations, and soldering alloys. However, the most typical classification is on the basis of noble metal content. The nobility of an alloy is expressed as a sum of the weight percentages (wt.%) of the noble metals in the alloy. For example, if an alloy contains 60% Au, 10% Pd, 5% Pt, and 25% Cu, it is 75% noble (the sum of Au, Pd, and Pt). Alloys are also defined on the basis of their most common metal. For example, an alloy with 75% Au often is described as an Au-based alloy [45]. In the 1920s the casting alloys were classified by the American Dental Association (ADA) according to their mechanical properties or chemical composition. They were known as Type I (A or soft), Type II (B or medium), Type III (C or hard), and Type IV (D or extra hard).

According to the ISO/DIS 1562 Standard for casting Au alloys, several alloy types are proposed according to their mechanical properties [45,46]:

Type I (low strength)—for castings subject to very low stress levels, with a minimum yield strength of 80 MPa and minimum elongation of 18%. These alloys are relatively weak and used for inlays with little or no occlusal contact. They are highly burnishable.

Type II (moderate strength)—for castings subject to moderate stress, with a minimum yield strength of 180 MPa and minimum elongation of 10%. These alloys are somewhat stronger, with good elongation and sufficient strength to tolerate some occlusal contact, such as with onlays and full crowns.

Type III (high strength)—for castings subject to high strength, with a minimum yield strength of 270 MPa and minimum elongation of 5%. These alloys have lower ductility (percentage elongation) but are nearly twice as strong as type I alloys and are used for onlays, full crowns, bridges, and fixed partial dentures.

Type IV (extra high strength)—for castings subject to very high stress (partial denture framework), with a minimum yield strength of 360 MPa and minimum elongation of 3%.

These specifications are summarized in Table 5.5. Type I and II alloys offer limited resistance to intraoral forces, which lead to the deformation or a loss caused by wear mechanisms over the course of time. Among these four types of alloys, the most common by far is a type III alloy with a typical approximate composition of 75% Au, 10% Ag, 10% Cu, 3% Pd, and 2% Zn. It is important to note that, in this pre-1975 system, the alloy “type” indicated both compositional class and physical properties [33].

Table 5.5. American Dental Association specifications for dental casting gold alloys.

Class Softness/hardness Yield strength (MPa) Elongation (annealed) (%) Vickers hardness (VHN) Application
Type I Soft <140 18 60–90 Inlays: low-stress applications
Type II Moderate 140–240 18 90–120 Inlays and onlays: increased stress applications including cusp replacement
Type III Hard 201–340 12 120–150 Crowns and short-span bridges: high-stress applications
Type IV Very hard >340 10 >150 Long-span bridges, removable partial dentures: high stress, high flexural resistance

In another ADA classification, casting alloys are classified based on their chemical composition as high noble (HN), noble (N), and predominantly base metal (PB) [46]. In general, the N metal alloy compositions are primarily Au- or Pd-based with alloying additions of Ag, Cu, Pt, Zn, and some other trace elements. According to the periodic table, the eight noble metals are Au, Ag, and the PGMs. However, in an oral environment, Ag is not necessarily considered noble. According to the ADA specifications, HN alloys must contain more than 60% noble metal content. The N group should contain more than 25% of noble metals, while PB should contain <25% of noble metals (all in wt.%). PB alloys are either Co–Cr or Ni–Cr (Table 5.6) [45,47].

Table 5.6. Properties of common types of dental casting alloys.

Alloy type Composition Color Elastic modulus (GPa) Hardness (kg/mm2)
High noble alloys (HN) Au–Pt (Zn) Yellow 90 175–195
Au–Pd (Ag) White 100 255–280
Au–Cu–Ag Yellow 100 135–195
Noble alloys (N) Au–Ag–Cu Yellow 100 125–215
Pd–Cu White 120 425
Ag–Pd White 120 140–155
Predominantly base metal alloys (PB) Ni–Cr (Be) White 180–200 340
  Ni–Cr (Be free) White 200 190
  Co–Cr White 220 380

HN alloys are expensive because of their Au, Pd, or Pt constituents. The three common subclasses of HN alloys are Au–Pt, Au–Pd, and Au–Cu–Ag (see Table 5.6). The reason for adding Ag and Cu is to increase hardness or strength. Due to their high density, these alloys have high castability. The high levels of noble metals in these alloys account for their excellent corrosion resistance in the oral environment. Pd and Pt are added to improve the mechanical properties, whereas Zn is added to improve the castability of the alloy [11]. Overall, these alloys do not have high elastic modulus and are too flexible for large cast restorations [8].

On the other hand, the N alloys have at least 25% noble metal content but with no stipulation for Au content. The three common subclasses of this alloy type are Au–Ag–Cu, Pd–Cu, and Ag–Pd. This group of casting alloys is the most compositionally diverse. Au-based alloys in this class contain about 40% Au but contain higher amounts of Cu or Ag than the HN alloys. The ternary Au–Ag–Cu alloy is the oldest alloy used in dentistry. Cu is added to Au to increase the strength, and Ag is added for workability. This alloy with 18-carat (Ct.) Au (75 wt.%) is very resistant to corrosion [11]. Fig. 5.4 depicts the alloy color diagram of the ternary Au–Ag–Cu system. The compositions for 18-, 14- and 10-Ct. Au are marked with horizontal dashed lines. The alloys in red and reddish areas can be made yellowish by additions of Zn. This addition may be up to 15 wt.%, which allows the Au–Ag–Cu–Zn alloys to be softer than the corresponding types of Au–Ag–Cu alloys in both the annealed and precipitation-hardened states [48]. In this class of alloys, Pd-based alloys may contain 77% Pd and almost no Au. The mass balance of these alloys is achieved by the addition of Cu or gallium (Ga). Pd–Cu alloys are extremely hard and strong but have a higher corrosion rate than Au-based alloys and are not easy to use. Ag-based alloys (Ag–Pd) in this class usually contain enough Pd (25%) to be considered as N by the ADA Standard. The cost of N alloys may be lower than that of HN alloys, a fact that has made them popular among dentists. The Pd–Cu and Ag–Pd subclasses may be used for crowns or fixed partial dentures with or without ceramic coverings [8,40,49].

Figure 5.4. Relationship between color and composition in the ternary Au–Ag–Cu system.

PB metal alloys have low levels of noble elements, and their primary constituents are Ni or Co. Although Ti alloys are also in this class, they are typically classified as implants by virtue of their special properties. Composition-wise, these alloys are the most complex class of casting alloys, containing six to eight elements. As a group, PB alloys have extremely high yield strengths and hardness but relatively low densities, making them the most difficult class of casting alloys to cast, polish, and machine. Some PB alloys (especially those based on Ni and Co) have relatively low corrosion resistance and, therefore, low biocompatibility. Nevertheless, their low cost per restoration is the prime reason for their popularity. PB alloys may be used for crowns and fixed partial dentures (with or without ceramics), removable partial dentures, and dental implant substructures [8].

Future Directions for Shape Memory Alloy Development

Tabbi Wilberforce, ... Hussein M. Maghrabie, in Encyclopedia of Smart Materials, 2022

Shape Memory Effect

Shape memory effect can be traced to 1932 in cadmium–gold alloy (Ölander, 1932) but the research into this area actually started in 1932 at the Naval Ordnance Laboratory (Buehler, 1963). The most commonly used type of shape memory alloy is referred to as nitinol and this is predominantly used in fields like civil engineering and the automotive sector. Shape memory element are designed to transition back to its initial shape when exposed to temperature (Gomez, 2011). The ability of a material to recover its original shape after undergoing deformation is called the shape memory effect. This occurs as a result of temperature or stress changes leading to phase changes between martensite and austenite. The transformation temperature is the operating range where phase transformation occurs, and it is sometimes referred to as the phase change temperature in some literature. The transformation temperature is subject to the chemical make-up of the shape memory alloy as well as the thermomechanical training. The parent phase or austenitic phase of shape memory alloy is body centered cubic and this often occurs at elevated temperatures. The martensitic phase on the other hand occurs at lower temperatures as a result of rapid cooling (Schetky, 1979). The phase transition occurs due to rapid mobility of atoms (Duerig et al., 2013). The transformation originates from nucleation and it starts with small seed crystals initiating the crystallization process inside a polymorphous zone after a larger energy barrier has been jumped (Vekilov, 2010). The transition from austenite to martensite occurs in 2 different processes. The first is the lattice deformation that yields new structure through smaller layered dislocations and the next is the lattice invariants shear that includes accommodation of the novel structure via altering the surrounding by twinning or slip (Guo et al., 2013). The process as depicted in Fig. 2 causes hysteresis (Guo et al., 2013).

Fig. 2. Hysteresis loop for shape memory alloys.

Permission to reproduce from Guo, Y., Klink, A., Fu, C., Snyder, J., 2013. Machinability and surface integrity of Nitinol shape memory alloy. CIRP Annals 62 (1), 83–86. https://doi.org/10.1016/j.cirp.2013.03.004.

Other authors also explored the microstructure as well as shape memory effect of some materials (Bhattacharya, 2003). Investigations on shape memory alloy actuators have also been presented in literature as well (Otsuka and Wayman, 1998). In the year 1972, the National Aeronautics and Space Administration reported some data on the outcome of an experimental investigation conducted on shape memory alloys (Jackson et al., 1972). Some authors reported that the phase change for shape memory alloy is very unique because it happens under high strain but the shape of the material can be recovered fully. These are some factors why shape memory alloys are referred to as active materials in some publications (Srinivasan and McFarland, 2000). Shape memory alloys are capable of providing higher forces coupled with wider displacement when activated. Today shape memory alloys are one of the primary materials utilized in the biomedical industry and the aerospace industry (Duerig et al., 1990). Shape memory alloys are categorized under smart materials. They are able to make a process or system simpler. They produce more energy density when subjected to higher activation loads. These are key characteristics required in the aerospace industry. Information on the properties of shape memory alloys is subject to the company, institute as well as the year the data was published. Recent decades have seen several research activities being championed mainly to enhance the behavior of shape memory alloy material as well as to improve their limits. Table 1 depicts some properties of NiTi shape memory alloys.

Table 1. Characteristics of shape memory alloys

  NiTi Unit
One way shape memory effect (OWSME), max 8 %
Two way shape memory effect (TWSME), max 3, 2 %
Phase change temperatures −200 … +110 °C
Temperature hysteresis 30 … 80 K
Overheatable up to 400 °C
Thermal conductivity, martensite 9…10 W/m K
Thermal conductivity, austenite 18 W/m K
Linear thermal expansion coefficient, martensite 6, 6 10–6 1/K
Linear thermal expansion coefficient, austenite 11 10–6 1/K
Specific electrical resistance 0.5…..1.1 10–6 Ωm
Density 6, 45 103 kg/m3
Ultimate tensile strength, martensite 700 … 1100 N/mm2
Ultimate tensile strength, austenite 800…1500 N/mm2
Number of achievable thermal cycles ≥100,000  
Opposition to corrosion Good  
Biocompatibility Good  

Note: Mertmann, M., 2004. Infosheet No. 4 – Selected properties of NiTi shape memory alloys.

 

The effect of electrode-oxide interfaces in gas sensor operation

Sung Pil Lee, Chowdhury Shaestagir, in Semiconductor Gas Sensors (Second Edition), 20203.2.2.4 Palladium–silver

Palladium–silver alloys are perhaps the most widely used conductor compositions. They are less expensive than gold alloys, are compatible with most dielectric and resistor systems, and are suitable for ultrasonic wire bonding. The sheet resistivity is typically in the region 0.01–0.04 Ω/sq and, although this figure is considerably higher than the resistivity of pure metal conductors, it is lower than the figures for gold alloy conductors. The addition of palladium to silver greatly reduces the rate of dissolution of the metal in molten solder. Increasing the palladium content thus provides greater leach resistance, but at the expense of solderability and conductivity. It also increases the cost. It is common practice, therefore, for ink manufacturers to produce a range of palladium–silver alloys of different palladium content so that the best compromise may be chosen for any particular application. Palladium–silver pastes can be fired with excellent initial bond strength to the substrate, but this rapidly degrades if the circuits are stored at elevated temperature (above 70°C) when the conductors are tinned. A further disadvantage is the possibility of silver migration under conditions of high humidity. The rate of migration is, however, considerably reduced by the presence of the palladium.

Metallic Biomaterials

Brian Love, in Biomaterials, 2017

7.3.6 Other Precious Metals: Pt/Rh/Pd

Other precious metals including Au can also be used as electrodes for pacemaker leads and other sensory probes. Typical properties linked with these Au and Au alloys are included in Table 7.5. While platinum, rhodium and palladium in bulk can be prohibitively expensive, there are often instances where, in very small quantities of these precious metals can be processed into unique microstructures can make sensor profiles smaller and smaller. Consider the level of surface area available to a disk as opposed to a porous blob of the same metal. The potential to make 3D sensors and leads has the capacity to shrink the current size of alternative signaling and sensory leads.

Table 7.5. Mechanical properties of metallic orthopedic biomaterials

Metal Alloy Condition Young’s Modulus (GPa) Yield Strength (MPa) Tensile Strength (MPa) Ductility Density (g/cm3)
Steels            
F138 Annealed 200 170–205 480–515 40 7.9
F138 Cold worked 200 310–690 655–860 12–28 7.9
F745 Annealed 200 205 480 30  
Co–Cr            
F75 Cast 210 450 655 8 8–9 nominally
F90 Annealed 210 380 900 30  
F562 Solution annealed 230 240–450 790–1000 50  
F562 Cold worked 230 1590 1790 8  
Titanium            
F67 Cold worked 110–120 485 760 15 4.4–4.5
F136 Forged 110–120 896 965 10 4.4–4.5

Based on [3,7,18–21].

Pt is commonly used as an embolism former in and near aneurysms as deposited by catheters. Platelets, exposed to platinum in a rod or coil form, activate which triggers the coagulation cascade creating local emboli near where the coils are deposited. By locally filling berry aneurysms with well-defined geometries with coils, it is possible that the clot will strengthen and help to reduce wall pressure on the aneurysm region which, if it works, will reduce the risk for aneurysm rupture. Rhodium and Palladium can be coalloyed with these but the raw material costs are sufficiently prohibitive that these materials will not be part of any large-scale effort to displace the use of other metals except in niche areas like sensors.

Production of high copper concentrates—comminution and flotation (Johnson et al., 2019)

Mark E. Schlesinger, ... Gerardo R.F. Alvear Flores, in Extractive Metallurgy of Copper (Sixth Edition), 2022

3.10 Other flotation separations

Copper flotation consists mainly of separating Cu sulfide minerals from nonsulfide rock and Fe-sulfide minerals. Many Cu deposits also contain molybdenite. Others contain pentlandite ((Ni,Fe)9S8), sphalerite (ZnS), or galena (PbS). These can all be separated from Cu minerals by selective flotation. Molybdenite flotation is discussed in Chapter 20. The flotation of sphalerite, galena, and Ni and Cu oxides is discussed by Biswas and Davenport (1994). Pentlandite flotation is described by Crundwell et al. (2011).

3.10.1 Gold flotation

Au is present in many Cu sulfide ores, from just traces to ∼1 g/t Au (10−4 wt.-% Au). Au is mostly present in Cu sulfide ores as metal and Ag–Au alloy (electrum) grains. Some may also be present as tellurides. These grains of Au and Au alloy may be liberated particles, attached to other minerals, encapsulated in sulfide mineral particles, or encapsulated in oxide particles. They seem to be preferentially associated with bornite and pyrite (Agorhom et al., 2015).

The Au-containing mineral grains in Cu–Au ores may be smaller than their companion copper sulfide mineral grains, so higher Au recovery requires finer grinding. Optimum grind size is best determined by controlled in-plant testing.

Fortunately, all of these gold mineral types float efficiently under the same conditions as copper sulfide minerals, using xanthate, dithiophosphate, and thionocarbamate collectors and conventional frothers such as methyl isobutyl carbinol (Agorhom et al., 2015; Woodcock et al., 2007). Generally speaking, liberated Au metal, Au alloy, and Au mineral particles float with chalcopyrite and other sulfide minerals and report to the bulk concentrate (Zahn et al., 2007). Au recoveries to the concentrate of 80%–85% are typical. However, efforts to depress pyrite flotation to reduce sulfur levels in the concentrate can also reduce gold recovery.

Kendrick et al. (2003) suggest that Au recovery to concentrate can be increased ∼10% by decreasing the pH of the ore slurry from 10.7 to 9.5. This also appreciably lowers CaO consumption. Special collectors have also been developed for copper ores with significant gold content. These collectors are a blend of monothiophosphates and dithiophosphates, and may be worth considering for Cu–Au concentrators.

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