Ken K. Ho

Sputter deposition of NiTi thin film shape memory alloy using a heated target

In this paper we present a novel method for depositing NiTi thin film by DC sputtering. The film has transformation temperatures very close to that of the target. The new process involves heating the target and does not require compositional modification of the NiTi target. Results from X-ray diffraction, differential scanning calorimeter, four-point probe, Rutherford backscattering, and transmission electron microscopy are presented. These results indicate that compositional modification can be produced by varying the target temperature. Films produced from hot targets have compositions similar to the target while films produced from cold targets were Ti deficient. Films that were produced by gradual heating of the target have compositional gradation through the film thickness. The gradated films exhibit the two-way shape memory effect.

Examination of the sputtering profile of NiTi under target heating conditions

In this paper we have examined compositional shifts in NiTi film sputter-deposited from a heated and a cooled target. Substrates were radially placed at 18° intervals to capture the semicircular sputtering profile. The composition of the film was measured using Rutherford backscattering spectroscopy (RBS). Film thickness values were measured and the atomic flux captured was calculated. Results indicate that the target temperature significantly influences the composition of the sputtered film. It was found that, for cold targets, Ni and Ti sputter at different angles, producing a compositional shift in the film when compared with the target. For hot targets the sputtering angle is influenced to a lesser degree, such that the compositional shift is negligible. Therefore, target temperature appears to represent an approach to alter the stoichiometry of films deposited.

Static properties of crystallographically aligned Terfenol-D∕polymer composites

Crystallographically aligned [112] magnetostrictive particle (CAMP) composites were characterized quasistatically. Three volume fraction (vf) composites (26%, 37%, and 49%) were manufactured and subjected to compressive stresses ranging from −2to−24MPa, magnetic fields loading up to 400kA∕m, and thermal variations (from 0to30°C). These tests provided property measurement including maximum magnetostrictive strain (emax), elastic modulus (EH), relative permeability (μr), piezomagnetic coefficient (d33), and coupling coefficient (k33). Results show that the 49% volume fraction CAMP composite produced strains of up to 1600ppm saturation magnetostriction, which is 89% of the monolithic value (1800ppm), and a coupling coefficient of 0.65, which is approximately 75% that of the monolithic, and relative thermal stability over the range of 0–30°C.

Sputter deposition and analysis of thin film Nitinol/Terfenol-D multilaminate for vibration damping

Nitinol/silicon laminates and Nitinol/Terfenol-D/nickel laminates were sputter deposited and characterized using x-ray diffraction (XRD), wavelength dispersive spectrometry (WDS), and a SQUID (superconducting quantum interference device). Dynamic testing showed substantial damping (tanδ) measurable in each laminate. An analytical model of a heterogeneous layered thin film damping material was also developed. Parametric studies with the analytical model illustrate the effect of combining Terfenol-D and Nitinol to create a broadband damping laminate with relatively constant tanδ for strain values between 0.05% and 1%. The model also illustrates the importance of relative stiffness on the damping of multilayer laminates, i.e. relatively high stiffness damping materials such as Terfenol-D and Nitinol may be superior to lower stiffness visco-elastics.

Modeling and measuring the response times of thin film TiNi

The response time of TiNi has been the subject of several experimental and theoretical investigations over the past decade. One of the principal concerns with this material is the relatively low cycle speeds or operational bandwidth caused by the considerable length of time required to cool the material. In this paper a finite difference model of heat transfer including the latent heat dissipated during the phase transformation is used to predict the bandwidth of thin film TiNi. The film is modeled as a plate subjected to either forced or free convection along the exposed surfaces and clamped to a large thermal mass representative of silicon wafer at the ends of the specimens. Results indicate that both latent heat as well as the relative ratios of the transformation temperatures to ambient temperature strongly influence the bandwidth of the material. Good correlation between the analytical model and test data obtained on a 38 micron wire indicate the model contains the correct assumptions to predict bandwidths. The bandwidth of TiNi thin film are predicted to be on the order of 100 Hz necessary assuming that the transformation temperatures for the film are the same as the bulk material.

Sputter deposition of NiTi thin film exhibiting the SME at room temperatures

In this paper we will present a novel method for depositing NiTi thin film by DC sputtering that produces films with transformation temperatures very close to that of the target. The new process involves heating the target to temperatures over 400 degrees C and does not require compositional modification of the 50/50atm percent NiTi target. Results from tensile testing, XRD, TEM, and DSC are presented. Conclusions are cold target produces films that were in the Austenite phase at rom temperature while hot target produces films that were Martensite at room temperatures, confirming that compositional modification can be produced by varying the target temperature. Films that were produced by gradual heating of the target, produced a gradation of composition through the film thickness. These gradation films exhibiting the two-way SME. The simplicity of this new process should increase the use of NiTi film sin microactuator devices.

CRYSTALLOGRAPHICALLY ALIGNED TERFENOL-D/POLYMER COMPOSITES FOR A HYBRID SONAR DEVICE

ABSTRACT Crystallographically-aligned magnetostrictive particle (CAMP) composites offer increased bandwidth and durability when compared to monolithic and laminated magnetostrictive systems. CAMP composites were evaluated under combined magnetic, thermal, and mechanical loading to determine fundamental properties used for the design of sonar transducers that incorporate these materials. Here, we examine the minor loop performance of the CAMP composites, i.e. the linear region of the strain field curve. The measurements demonstrate the benefits of the CAMP Composite technology for sonar applications, achieving strain output near that of monolithic Terfenol-D, while increasing bandwidth and durability. All volume fractions exhibited low minor loop hysteresis, and low temperature dependence.

STRESS INDUCED PHASE CHANGING MATERIAL FOR THERMOACOUSTIC REFRIGERATION

ABSTRACT A novel concept to use a stress induced phase transformation in NiMnGa for solid state refrigeration is investigated. The stress needed to induce a phase transformation was measured at temperatures from 24 to 60°C. The stresses were sufficiently low enough such that an acoustic wave can in theory induce the phase transformation. DSC measurements were done on the NiMnGa material to measure the latent heat that can be absorbed and release. Theoretical calculations of the coefficient of performance for a heat pump using this stress induced thermal phenomena showed a possible efficiency of 67%. A solid state heat pump using a piezoelectric or magnetostrictive transducer to generate the acoustic pressure wave and a fluid medium suspended with NiMnGa nanoparticles is suggested as a possible implementation of the concept.

Microscale damping using thin film active materials

This paper focuses on understanding and developing a new approach to dampen MEMS structures using both experiments and analytical techniques. Thin film Nitinol and thin film Terfenol-D are evaluated as a damping solution to the micro scale damping problem. Stress induced twin boundary motion in Nitinol is used to passively dampen potentially damaging vibrations. Magnetic domain wall motion is used to passively dampen vibration in Terfenol-D. The thin films of Nitinol, Nitinol/Silicon laminates and Nitinol/Terfenol-D/Nickel laminates have been produced using a sputter deposition process and damping properties have been evaluated. Dynamic testing shows substantial damping (tan &dgr;) measurable in each case. Nitinol film samples were tested in the Differential Scanning Calorimetry (DSC) to determine phase transformation temperatures. The twin boundary mechanism by which energy absorption occurs is present at all points below the Austenite start temperature (approximately 69°C in our film) and therefore allows damping at cold temperatures where traditional materials fail. Thin film in the NiTi/Si laminate was found to produce substantially higher damping (tan &dgr; = 0.28) due to the change in loading condition. The NiTi/Si laminate sample was tested in bending allowing the twin boundaries to be reset by cyclic tensile and compressive loads. The thin film Terfenol-D in the Nitinol/Terfenol-D/Nickel laminate was shown to produce large damping (tan &dgr; = 0.2). In addition to fabricating and testing, an analytical model of a heterogeneous layered thin film damping material was developed and compared to experimental work.

Passive vibration damping with magnetostrictive composite material

This paper describes evaluation of an autonomous-material system tailored for free-layer vibration damping of structural elements. The magnetostrictive particulate composite (MPC) material described has moderate stiffness and minimal temperature and frequency dependence. The composite is created by curing Terfenol particles {Tb(1-x)Dy(x)Fe(2),0.2 SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring SS 49 2007-03-18|2007-03-22 SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring SS07 725046 San Diego, California, United States Active and Passive Smart Structures and Integrated Systems 2007 6525 Damping and Isolation II 9